diff --git a/.github/workflows/dendrite.yml b/.github/workflows/dendrite.yml
index 2c04005d2..1de39850d 100644
--- a/.github/workflows/dendrite.yml
+++ b/.github/workflows/dendrite.yml
@@ -331,8 +331,7 @@ jobs:
             postgres: postgres
             api: full-http
     container:
-      # Temporary for debugging to see if this image is working better.
-      image: matrixdotorg/sytest-dendrite@sha256:434ad464a9f4ed3f8c3cc47200275b6ccb5c5031a8063daf4acea62be5a23c73
+      image: matrixdotorg/sytest-dendrite
       volumes:
         - ${{ github.workspace }}:/src
         - /root/.cache/go-build:/github/home/.cache/go-build
diff --git a/clientapi/routing/admin.go b/clientapi/routing/admin.go
index 8419622df..dbd913376 100644
--- a/clientapi/routing/admin.go
+++ b/clientapi/routing/admin.go
@@ -137,7 +137,7 @@ func AdminResetPassword(req *http.Request, cfg *config.ClientAPI, device *userap
 	request := struct {
 		Password string `json:"password"`
 	}{}
-	if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+	if err = json.NewDecoder(req.Body).Decode(&request); err != nil {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
 			JSON: jsonerror.Unknown("Failed to decode request body: " + err.Error()),
@@ -150,8 +150,8 @@ func AdminResetPassword(req *http.Request, cfg *config.ClientAPI, device *userap
 		}
 	}
 
-	if resErr := internal.ValidatePassword(request.Password); resErr != nil {
-		return *resErr
+	if err = internal.ValidatePassword(request.Password); err != nil {
+		return *internal.PasswordResponse(err)
 	}
 
 	updateReq := &userapi.PerformPasswordUpdateRequest{
diff --git a/clientapi/routing/auth_fallback.go b/clientapi/routing/auth_fallback.go
index ad870993e..f8d3684fe 100644
--- a/clientapi/routing/auth_fallback.go
+++ b/clientapi/routing/auth_fallback.go
@@ -15,11 +15,11 @@
 package routing
 
 import (
+	"fmt"
 	"html/template"
 	"net/http"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
-	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	"github.com/matrix-org/dendrite/setup/config"
 	"github.com/matrix-org/util"
 )
@@ -101,14 +101,28 @@ func serveTemplate(w http.ResponseWriter, templateHTML string, data map[string]s
 func AuthFallback(
 	w http.ResponseWriter, req *http.Request, authType string,
 	cfg *config.ClientAPI,
-) *util.JSONResponse {
-	sessionID := req.URL.Query().Get("session")
+) {
+	// We currently only support "m.login.recaptcha", so fail early if that's not requested
+	if authType == authtypes.LoginTypeRecaptcha {
+		if !cfg.RecaptchaEnabled {
+			writeHTTPMessage(w, req,
+				"Recaptcha login is disabled on this Homeserver",
+				http.StatusBadRequest,
+			)
+			return
+		}
+	} else {
+		writeHTTPMessage(w, req, fmt.Sprintf("Unknown authtype %q", authType), http.StatusNotImplemented)
+		return
+	}
 
+	sessionID := req.URL.Query().Get("session")
 	if sessionID == "" {
-		return writeHTTPMessage(w, req,
+		writeHTTPMessage(w, req,
 			"Session ID not provided",
 			http.StatusBadRequest,
 		)
+		return
 	}
 
 	serveRecaptcha := func() {
@@ -130,70 +144,44 @@ func AuthFallback(
 
 	if req.Method == http.MethodGet {
 		// Handle Recaptcha
-		if authType == authtypes.LoginTypeRecaptcha {
-			if err := checkRecaptchaEnabled(cfg, w, req); err != nil {
-				return err
-			}
-
-			serveRecaptcha()
-			return nil
-		}
-		return &util.JSONResponse{
-			Code: http.StatusNotFound,
-			JSON: jsonerror.NotFound("Unknown auth stage type"),
-		}
+		serveRecaptcha()
+		return
 	} else if req.Method == http.MethodPost {
 		// Handle Recaptcha
-		if authType == authtypes.LoginTypeRecaptcha {
-			if err := checkRecaptchaEnabled(cfg, w, req); err != nil {
-				return err
-			}
-
-			clientIP := req.RemoteAddr
-			err := req.ParseForm()
-			if err != nil {
-				util.GetLogger(req.Context()).WithError(err).Error("req.ParseForm failed")
-				res := jsonerror.InternalServerError()
-				return &res
-			}
-
-			response := req.Form.Get(cfg.RecaptchaFormField)
-			if err := validateRecaptcha(cfg, response, clientIP); err != nil {
-				util.GetLogger(req.Context()).Error(err)
-				return err
-			}
-
-			// Success. Add recaptcha as a completed login flow
-			sessions.addCompletedSessionStage(sessionID, authtypes.LoginTypeRecaptcha)
-
-			serveSuccess()
-			return nil
+		clientIP := req.RemoteAddr
+		err := req.ParseForm()
+		if err != nil {
+			util.GetLogger(req.Context()).WithError(err).Error("req.ParseForm failed")
+			w.WriteHeader(http.StatusBadRequest)
+			serveRecaptcha()
+			return
 		}
 
-		return &util.JSONResponse{
-			Code: http.StatusNotFound,
-			JSON: jsonerror.NotFound("Unknown auth stage type"),
+		response := req.Form.Get(cfg.RecaptchaFormField)
+		err = validateRecaptcha(cfg, response, clientIP)
+		switch err {
+		case ErrMissingResponse:
+			w.WriteHeader(http.StatusBadRequest)
+			serveRecaptcha() // serve the initial page again, instead of nothing
+			return
+		case ErrInvalidCaptcha:
+			w.WriteHeader(http.StatusUnauthorized)
+			serveRecaptcha()
+			return
+		case nil:
+		default: // something else failed
+			util.GetLogger(req.Context()).WithError(err).Error("failed to validate recaptcha")
+			serveRecaptcha()
+			return
 		}
-	}
-	return &util.JSONResponse{
-		Code: http.StatusMethodNotAllowed,
-		JSON: jsonerror.NotFound("Bad method"),
-	}
-}
 
-// checkRecaptchaEnabled creates an error response if recaptcha is not usable on homeserver.
-func checkRecaptchaEnabled(
-	cfg *config.ClientAPI,
-	w http.ResponseWriter,
-	req *http.Request,
-) *util.JSONResponse {
-	if !cfg.RecaptchaEnabled {
-		return writeHTTPMessage(w, req,
-			"Recaptcha login is disabled on this Homeserver",
-			http.StatusBadRequest,
-		)
+		// Success. Add recaptcha as a completed login flow
+		sessions.addCompletedSessionStage(sessionID, authtypes.LoginTypeRecaptcha)
+
+		serveSuccess()
+		return
 	}
-	return nil
+	writeHTTPMessage(w, req, "Bad method", http.StatusMethodNotAllowed)
 }
 
 // writeHTTPMessage writes the given header and message to the HTTP response writer.
@@ -201,13 +189,10 @@ func checkRecaptchaEnabled(
 func writeHTTPMessage(
 	w http.ResponseWriter, req *http.Request,
 	message string, header int,
-) *util.JSONResponse {
+) {
 	w.WriteHeader(header)
 	_, err := w.Write([]byte(message))
 	if err != nil {
 		util.GetLogger(req.Context()).WithError(err).Error("w.Write failed")
-		res := jsonerror.InternalServerError()
-		return &res
 	}
-	return nil
 }
diff --git a/clientapi/routing/auth_fallback_test.go b/clientapi/routing/auth_fallback_test.go
new file mode 100644
index 000000000..0d77f9a01
--- /dev/null
+++ b/clientapi/routing/auth_fallback_test.go
@@ -0,0 +1,149 @@
+package routing
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
+	"github.com/matrix-org/dendrite/setup/config"
+	"github.com/matrix-org/dendrite/test/testrig"
+)
+
+func Test_AuthFallback(t *testing.T) {
+	base, _, _ := testrig.Base(nil)
+	defer base.Close()
+
+	for _, useHCaptcha := range []bool{false, true} {
+		for _, recaptchaEnabled := range []bool{false, true} {
+			for _, wantErr := range []bool{false, true} {
+				t.Run(fmt.Sprintf("useHCaptcha(%v) - recaptchaEnabled(%v) - wantErr(%v)", useHCaptcha, recaptchaEnabled, wantErr), func(t *testing.T) {
+					// Set the defaults for each test
+					base.Cfg.ClientAPI.Defaults(config.DefaultOpts{Generate: true, Monolithic: true})
+					base.Cfg.ClientAPI.RecaptchaEnabled = recaptchaEnabled
+					base.Cfg.ClientAPI.RecaptchaPublicKey = "pub"
+					base.Cfg.ClientAPI.RecaptchaPrivateKey = "priv"
+					if useHCaptcha {
+						base.Cfg.ClientAPI.RecaptchaSiteVerifyAPI = "https://hcaptcha.com/siteverify"
+						base.Cfg.ClientAPI.RecaptchaApiJsUrl = "https://js.hcaptcha.com/1/api.js"
+						base.Cfg.ClientAPI.RecaptchaFormField = "h-captcha-response"
+						base.Cfg.ClientAPI.RecaptchaSitekeyClass = "h-captcha"
+					}
+					cfgErrs := &config.ConfigErrors{}
+					base.Cfg.ClientAPI.Verify(cfgErrs, true)
+					if len(*cfgErrs) > 0 {
+						t.Fatalf("(hCaptcha=%v) unexpected config errors: %s", useHCaptcha, cfgErrs.Error())
+					}
+
+					req := httptest.NewRequest(http.MethodGet, "/?session=1337", nil)
+					rec := httptest.NewRecorder()
+
+					AuthFallback(rec, req, authtypes.LoginTypeRecaptcha, &base.Cfg.ClientAPI)
+					if !recaptchaEnabled {
+						if rec.Code != http.StatusBadRequest {
+							t.Fatalf("unexpected response code: %d, want %d", rec.Code, http.StatusBadRequest)
+						}
+						if rec.Body.String() != "Recaptcha login is disabled on this Homeserver" {
+							t.Fatalf("unexpected response body: %s", rec.Body.String())
+						}
+					} else {
+						if !strings.Contains(rec.Body.String(), base.Cfg.ClientAPI.RecaptchaSitekeyClass) {
+							t.Fatalf("body does not contain %s: %s", base.Cfg.ClientAPI.RecaptchaSitekeyClass, rec.Body.String())
+						}
+					}
+
+					srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+						if wantErr {
+							_, _ = w.Write([]byte(`{"success":false}`))
+							return
+						}
+						_, _ = w.Write([]byte(`{"success":true}`))
+					}))
+					defer srv.Close() // nolint: errcheck
+
+					base.Cfg.ClientAPI.RecaptchaSiteVerifyAPI = srv.URL
+
+					// check the result after sending the captcha
+					req = httptest.NewRequest(http.MethodPost, "/?session=1337", nil)
+					req.Form = url.Values{}
+					req.Form.Add(base.Cfg.ClientAPI.RecaptchaFormField, "someRandomValue")
+					rec = httptest.NewRecorder()
+					AuthFallback(rec, req, authtypes.LoginTypeRecaptcha, &base.Cfg.ClientAPI)
+					if recaptchaEnabled {
+						if !wantErr {
+							if rec.Code != http.StatusOK {
+								t.Fatalf("unexpected response code: %d, want %d", rec.Code, http.StatusOK)
+							}
+							if rec.Body.String() != successTemplate {
+								t.Fatalf("unexpected response: %s, want %s", rec.Body.String(), successTemplate)
+							}
+						} else {
+							if rec.Code != http.StatusUnauthorized {
+								t.Fatalf("unexpected response code: %d, want %d", rec.Code, http.StatusUnauthorized)
+							}
+							wantString := "Authentication"
+							if !strings.Contains(rec.Body.String(), wantString) {
+								t.Fatalf("expected response to contain '%s', but didn't: %s", wantString, rec.Body.String())
+							}
+						}
+					} else {
+						if rec.Code != http.StatusBadRequest {
+							t.Fatalf("unexpected response code: %d, want %d", rec.Code, http.StatusBadRequest)
+						}
+						if rec.Body.String() != "Recaptcha login is disabled on this Homeserver" {
+							t.Fatalf("unexpected response: %s, want %s", rec.Body.String(), "successTemplate")
+						}
+					}
+				})
+			}
+		}
+	}
+
+	t.Run("unknown fallbacks are handled correctly", func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodPost, "/?session=1337", nil)
+		rec := httptest.NewRecorder()
+		AuthFallback(rec, req, "DoesNotExist", &base.Cfg.ClientAPI)
+		if rec.Code != http.StatusNotImplemented {
+			t.Fatalf("unexpected http status: %d, want %d", rec.Code, http.StatusNotImplemented)
+		}
+	})
+
+	t.Run("unknown methods are handled correctly", func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodDelete, "/?session=1337", nil)
+		rec := httptest.NewRecorder()
+		AuthFallback(rec, req, authtypes.LoginTypeRecaptcha, &base.Cfg.ClientAPI)
+		if rec.Code != http.StatusMethodNotAllowed {
+			t.Fatalf("unexpected http status: %d, want %d", rec.Code, http.StatusMethodNotAllowed)
+		}
+	})
+
+	t.Run("missing session parameter is handled correctly", func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodGet, "/", nil)
+		rec := httptest.NewRecorder()
+		AuthFallback(rec, req, authtypes.LoginTypeRecaptcha, &base.Cfg.ClientAPI)
+		if rec.Code != http.StatusBadRequest {
+			t.Fatalf("unexpected http status: %d, want %d", rec.Code, http.StatusBadRequest)
+		}
+	})
+
+	t.Run("missing session parameter is handled correctly", func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodGet, "/", nil)
+		rec := httptest.NewRecorder()
+		AuthFallback(rec, req, authtypes.LoginTypeRecaptcha, &base.Cfg.ClientAPI)
+		if rec.Code != http.StatusBadRequest {
+			t.Fatalf("unexpected http status: %d, want %d", rec.Code, http.StatusBadRequest)
+		}
+	})
+
+	t.Run("missing 'response' is handled correctly", func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodPost, "/?session=1337", nil)
+		rec := httptest.NewRecorder()
+		AuthFallback(rec, req, authtypes.LoginTypeRecaptcha, &base.Cfg.ClientAPI)
+		if rec.Code != http.StatusBadRequest {
+			t.Fatalf("unexpected http status: %d, want %d", rec.Code, http.StatusBadRequest)
+		}
+	})
+}
diff --git a/clientapi/routing/login.go b/clientapi/routing/login.go
index 0de324da1..778c8c0c3 100644
--- a/clientapi/routing/login.go
+++ b/clientapi/routing/login.go
@@ -23,15 +23,13 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/userutil"
 	"github.com/matrix-org/dendrite/setup/config"
 	userapi "github.com/matrix-org/dendrite/userapi/api"
-	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 )
 
 type loginResponse struct {
-	UserID      string                       `json:"user_id"`
-	AccessToken string                       `json:"access_token"`
-	HomeServer  gomatrixserverlib.ServerName `json:"home_server"`
-	DeviceID    string                       `json:"device_id"`
+	UserID      string `json:"user_id"`
+	AccessToken string `json:"access_token"`
+	DeviceID    string `json:"device_id"`
 }
 
 type flows struct {
@@ -116,7 +114,6 @@ func completeAuth(
 		JSON: loginResponse{
 			UserID:      performRes.Device.UserID,
 			AccessToken: performRes.Device.AccessToken,
-			HomeServer:  serverName,
 			DeviceID:    performRes.Device.ID,
 		},
 	}
diff --git a/clientapi/routing/password.go b/clientapi/routing/password.go
index cd88b025a..f7f9da622 100644
--- a/clientapi/routing/password.go
+++ b/clientapi/routing/password.go
@@ -82,8 +82,8 @@ func Password(
 	sessions.addCompletedSessionStage(sessionID, authtypes.LoginTypePassword)
 
 	// Check the new password strength.
-	if resErr = internal.ValidatePassword(r.NewPassword); resErr != nil {
-		return *resErr
+	if err := internal.ValidatePassword(r.NewPassword); err != nil {
+		return *internal.PasswordResponse(err)
 	}
 
 	// Get the local part.
diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index 4abbcdf9e..6087bda0c 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -18,12 +18,12 @@ package routing
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net"
 	"net/http"
 	"net/url"
-	"regexp"
 	"sort"
 	"strconv"
 	"strings"
@@ -60,10 +60,7 @@ var (
 	)
 )
 
-const (
-	maxUsernameLength = 254 // http://matrix.org/speculator/spec/HEAD/intro.html#user-identifiers TODO account for domain
-	sessionIDLength   = 24
-)
+const sessionIDLength = 24
 
 // sessionsDict keeps track of completed auth stages for each session.
 // It shouldn't be passed by value because it contains a mutex.
@@ -198,8 +195,7 @@ func (d *sessionsDict) getDeviceToDelete(sessionID string) (string, bool) {
 }
 
 var (
-	sessions           = newSessionsDict()
-	validUsernameRegex = regexp.MustCompile(`^[0-9a-z_\-=./]+$`)
+	sessions = newSessionsDict()
 )
 
 // registerRequest represents the submitted registration request.
@@ -262,10 +258,9 @@ func newUserInteractiveResponse(
 
 // http://matrix.org/speculator/spec/HEAD/client_server/unstable.html#post-matrix-client-unstable-register
 type registerResponse struct {
-	UserID      string                       `json:"user_id"`
-	AccessToken string                       `json:"access_token,omitempty"`
-	HomeServer  gomatrixserverlib.ServerName `json:"home_server"`
-	DeviceID    string                       `json:"device_id,omitempty"`
+	UserID      string `json:"user_id"`
+	AccessToken string `json:"access_token,omitempty"`
+	DeviceID    string `json:"device_id,omitempty"`
 }
 
 // recaptchaResponse represents the HTTP response from a Google Recaptcha server
@@ -276,66 +271,28 @@ type recaptchaResponse struct {
 	ErrorCodes  []int     `json:"error-codes"`
 }
 
-// validateUsername returns an error response if the username is invalid
-func validateUsername(localpart string, domain gomatrixserverlib.ServerName) *util.JSONResponse {
-	// https://github.com/matrix-org/synapse/blob/v0.20.0/synapse/rest/client/v2_alpha/register.py#L161
-	if id := fmt.Sprintf("@%s:%s", localpart, domain); len(id) > maxUsernameLength {
-		return &util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.BadJSON(fmt.Sprintf("%q exceeds the maximum length of %d characters", id, maxUsernameLength)),
-		}
-	} else if !validUsernameRegex.MatchString(localpart) {
-		return &util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.InvalidUsername("Username can only contain characters a-z, 0-9, or '_-./='"),
-		}
-	} else if localpart[0] == '_' { // Regex checks its not a zero length string
-		return &util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.InvalidUsername("Username cannot start with a '_'"),
-		}
-	}
-	return nil
-}
-
-// validateApplicationServiceUsername returns an error response if the username is invalid for an application service
-func validateApplicationServiceUsername(localpart string, domain gomatrixserverlib.ServerName) *util.JSONResponse {
-	if id := fmt.Sprintf("@%s:%s", localpart, domain); len(id) > maxUsernameLength {
-		return &util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.BadJSON(fmt.Sprintf("%q exceeds the maximum length of %d characters", id, maxUsernameLength)),
-		}
-	} else if !validUsernameRegex.MatchString(localpart) {
-		return &util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.InvalidUsername("Username can only contain characters a-z, 0-9, or '_-./='"),
-		}
-	}
-	return nil
-}
+var (
+	ErrInvalidCaptcha  = errors.New("invalid captcha response")
+	ErrMissingResponse = errors.New("captcha response is required")
+	ErrCaptchaDisabled = errors.New("captcha registration is disabled")
+)
 
 // validateRecaptcha returns an error response if the captcha response is invalid
 func validateRecaptcha(
 	cfg *config.ClientAPI,
 	response string,
 	clientip string,
-) *util.JSONResponse {
+) error {
 	ip, _, _ := net.SplitHostPort(clientip)
 	if !cfg.RecaptchaEnabled {
-		return &util.JSONResponse{
-			Code: http.StatusConflict,
-			JSON: jsonerror.Unknown("Captcha registration is disabled"),
-		}
+		return ErrCaptchaDisabled
 	}
 
 	if response == "" {
-		return &util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.BadJSON("Captcha response is required"),
-		}
+		return ErrMissingResponse
 	}
 
-	// Make a POST request to Google's API to check the captcha response
+	// Make a POST request to the captcha provider API to check the captcha response
 	resp, err := http.PostForm(cfg.RecaptchaSiteVerifyAPI,
 		url.Values{
 			"secret":   {cfg.RecaptchaPrivateKey},
@@ -345,10 +302,7 @@ func validateRecaptcha(
 	)
 
 	if err != nil {
-		return &util.JSONResponse{
-			Code: http.StatusInternalServerError,
-			JSON: jsonerror.BadJSON("Error in requesting validation of captcha response"),
-		}
+		return err
 	}
 
 	// Close the request once we're finishing reading from it
@@ -358,25 +312,16 @@ func validateRecaptcha(
 	var r recaptchaResponse
 	body, err := io.ReadAll(resp.Body)
 	if err != nil {
-		return &util.JSONResponse{
-			Code: http.StatusGatewayTimeout,
-			JSON: jsonerror.Unknown("Error in contacting captcha server" + err.Error()),
-		}
+		return err
 	}
 	err = json.Unmarshal(body, &r)
 	if err != nil {
-		return &util.JSONResponse{
-			Code: http.StatusInternalServerError,
-			JSON: jsonerror.BadJSON("Error in unmarshaling captcha server's response: " + err.Error()),
-		}
+		return err
 	}
 
 	// Check that we received a "success"
 	if !r.Success {
-		return &util.JSONResponse{
-			Code: http.StatusUnauthorized,
-			JSON: jsonerror.BadJSON("Invalid captcha response. Please try again."),
-		}
+		return ErrInvalidCaptcha
 	}
 	return nil
 }
@@ -508,8 +453,8 @@ func validateApplicationService(
 	}
 
 	// Check username application service is trying to register is valid
-	if err := validateApplicationServiceUsername(username, cfg.Matrix.ServerName); err != nil {
-		return "", err
+	if err := internal.ValidateApplicationServiceUsername(username, cfg.Matrix.ServerName); err != nil {
+		return "", internal.UsernameResponse(err)
 	}
 
 	// No errors, registration valid
@@ -564,15 +509,12 @@ func Register(
 	if resErr := httputil.UnmarshalJSON(reqBody, &r); resErr != nil {
 		return *resErr
 	}
-	if l, d, err := cfg.Matrix.SplitLocalID('@', r.Username); err == nil {
-		r.Username, r.ServerName = l, d
-	}
 	if req.URL.Query().Get("kind") == "guest" {
 		return handleGuestRegistration(req, r, cfg, userAPI)
 	}
 
 	// Don't allow numeric usernames less than MAX_INT64.
-	if _, err := strconv.ParseInt(r.Username, 10, 64); err == nil {
+	if _, err = strconv.ParseInt(r.Username, 10, 64); err == nil {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
 			JSON: jsonerror.InvalidUsername("Numeric user IDs are reserved"),
@@ -584,7 +526,7 @@ func Register(
 			ServerName: r.ServerName,
 		}
 		nres := &userapi.QueryNumericLocalpartResponse{}
-		if err := userAPI.QueryNumericLocalpart(req.Context(), nreq, nres); err != nil {
+		if err = userAPI.QueryNumericLocalpart(req.Context(), nreq, nres); err != nil {
 			util.GetLogger(req.Context()).WithError(err).Error("userAPI.QueryNumericLocalpart failed")
 			return jsonerror.InternalServerError()
 		}
@@ -601,8 +543,8 @@ func Register(
 	case r.Type == authtypes.LoginTypeApplicationService && accessTokenErr == nil:
 		// Spec-compliant case (the access_token is specified and the login type
 		// is correctly set, so it's an appservice registration)
-		if resErr := validateApplicationServiceUsername(r.Username, r.ServerName); resErr != nil {
-			return *resErr
+		if err = internal.ValidateApplicationServiceUsername(r.Username, r.ServerName); err != nil {
+			return *internal.UsernameResponse(err)
 		}
 	case accessTokenErr == nil:
 		// Non-spec-compliant case (the access_token is specified but the login
@@ -614,12 +556,12 @@ func Register(
 	default:
 		// Spec-compliant case (neither the access_token nor the login type are
 		// specified, so it's a normal user registration)
-		if resErr := validateUsername(r.Username, r.ServerName); resErr != nil {
-			return *resErr
+		if err = internal.ValidateUsername(r.Username, r.ServerName); err != nil {
+			return *internal.UsernameResponse(err)
 		}
 	}
-	if resErr := internal.ValidatePassword(r.Password); resErr != nil {
-		return *resErr
+	if err = internal.ValidatePassword(r.Password); err != nil {
+		return *internal.PasswordResponse(err)
 	}
 
 	logger := util.GetLogger(req.Context())
@@ -697,7 +639,6 @@ func handleGuestRegistration(
 		JSON: registerResponse{
 			UserID:      devRes.Device.UserID,
 			AccessToken: devRes.Device.AccessToken,
-			HomeServer:  res.Account.ServerName,
 			DeviceID:    devRes.Device.ID,
 		},
 	}
@@ -761,9 +702,18 @@ func handleRegistrationFlow(
 	switch r.Auth.Type {
 	case authtypes.LoginTypeRecaptcha:
 		// Check given captcha response
-		resErr := validateRecaptcha(cfg, r.Auth.Response, req.RemoteAddr)
-		if resErr != nil {
-			return *resErr
+		err := validateRecaptcha(cfg, r.Auth.Response, req.RemoteAddr)
+		switch err {
+		case ErrCaptchaDisabled:
+			return util.JSONResponse{Code: http.StatusForbidden, JSON: jsonerror.Unknown(err.Error())}
+		case ErrMissingResponse:
+			return util.JSONResponse{Code: http.StatusBadRequest, JSON: jsonerror.BadJSON(err.Error())}
+		case ErrInvalidCaptcha:
+			return util.JSONResponse{Code: http.StatusUnauthorized, JSON: jsonerror.BadJSON(err.Error())}
+		case nil:
+		default:
+			util.GetLogger(req.Context()).WithError(err).Error("failed to validate recaptcha")
+			return util.JSONResponse{Code: http.StatusInternalServerError, JSON: jsonerror.InternalServerError()}
 		}
 
 		// Add Recaptcha to the list of completed registration stages
@@ -924,8 +874,7 @@ func completeRegistration(
 		return util.JSONResponse{
 			Code: http.StatusOK,
 			JSON: registerResponse{
-				UserID:     userutil.MakeUserID(username, accRes.Account.ServerName),
-				HomeServer: accRes.Account.ServerName,
+				UserID: userutil.MakeUserID(username, accRes.Account.ServerName),
 			},
 		}
 	}
@@ -958,7 +907,6 @@ func completeRegistration(
 	result := registerResponse{
 		UserID:      devRes.Device.UserID,
 		AccessToken: devRes.Device.AccessToken,
-		HomeServer:  accRes.Account.ServerName,
 		DeviceID:    devRes.Device.ID,
 	}
 	sessions.addCompletedRegistration(sessionID, result)
@@ -1054,8 +1002,8 @@ func RegisterAvailable(
 		}
 	}
 
-	if err := validateUsername(username, domain); err != nil {
-		return *err
+	if err := internal.ValidateUsername(username, domain); err != nil {
+		return *internal.UsernameResponse(err)
 	}
 
 	// Check if this username is reserved by an application service
@@ -1117,11 +1065,11 @@ func handleSharedSecretRegistration(cfg *config.ClientAPI, userAPI userapi.Clien
 	// downcase capitals
 	ssrr.User = strings.ToLower(ssrr.User)
 
-	if resErr := validateUsername(ssrr.User, cfg.Matrix.ServerName); resErr != nil {
-		return *resErr
+	if err = internal.ValidateUsername(ssrr.User, cfg.Matrix.ServerName); err != nil {
+		return *internal.UsernameResponse(err)
 	}
-	if resErr := internal.ValidatePassword(ssrr.Password); resErr != nil {
-		return *resErr
+	if err = internal.ValidatePassword(ssrr.Password); err != nil {
+		return *internal.PasswordResponse(err)
 	}
 	deviceID := "shared_secret_registration"
 
diff --git a/clientapi/routing/register_test.go b/clientapi/routing/register_test.go
index 85846c7d6..b8fd19e90 100644
--- a/clientapi/routing/register_test.go
+++ b/clientapi/routing/register_test.go
@@ -15,12 +15,27 @@
 package routing
 
 import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"reflect"
 	"regexp"
+	"strings"
 	"testing"
 	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/keyserver"
+	"github.com/matrix-org/dendrite/roomserver"
 	"github.com/matrix-org/dendrite/setup/config"
+	"github.com/matrix-org/dendrite/test"
+	"github.com/matrix-org/dendrite/test/testrig"
+	"github.com/matrix-org/dendrite/userapi"
+	"github.com/matrix-org/util"
 )
 
 var (
@@ -264,3 +279,294 @@ func TestSessionCleanUp(t *testing.T) {
 		}
 	})
 }
+
+func Test_register(t *testing.T) {
+	testCases := []struct {
+		name                 string
+		kind                 string
+		password             string
+		username             string
+		loginType            string
+		forceEmpty           bool
+		registrationDisabled bool
+		guestsDisabled       bool
+		enableRecaptcha      bool
+		captchaBody          string
+		wantResponse         util.JSONResponse
+	}{
+		{
+			name:           "disallow guests",
+			kind:           "guest",
+			guestsDisabled: true,
+			wantResponse: util.JSONResponse{
+				Code: http.StatusForbidden,
+				JSON: jsonerror.Forbidden(`Guest registration is disabled on "test"`),
+			},
+		},
+		{
+			name: "allow guests",
+			kind: "guest",
+		},
+		{
+			name:      "unknown login type",
+			loginType: "im.not.known",
+			wantResponse: util.JSONResponse{
+				Code: http.StatusNotImplemented,
+				JSON: jsonerror.Unknown("unknown/unimplemented auth type"),
+			},
+		},
+		{
+			name:                 "disabled registration",
+			registrationDisabled: true,
+			wantResponse: util.JSONResponse{
+				Code: http.StatusForbidden,
+				JSON: jsonerror.Forbidden(`Registration is disabled on "test"`),
+			},
+		},
+		{
+			name:       "successful registration, numeric ID",
+			username:   "",
+			password:   "someRandomPassword",
+			forceEmpty: true,
+		},
+		{
+			name:     "successful registration",
+			username: "success",
+		},
+		{
+			name:     "failing registration - user already exists",
+			username: "success",
+			wantResponse: util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.UserInUse("Desired user ID is already taken."),
+			},
+		},
+		{
+			name:     "successful registration uppercase username",
+			username: "LOWERCASED", // this is going to be lower-cased
+		},
+		{
+			name:         "invalid username",
+			username:     "#totalyNotValid",
+			wantResponse: *internal.UsernameResponse(internal.ErrUsernameInvalid),
+		},
+		{
+			name:     "numeric username is forbidden",
+			username: "1337",
+			wantResponse: util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.InvalidUsername("Numeric user IDs are reserved"),
+			},
+		},
+		{
+			name:      "disabled recaptcha login",
+			loginType: authtypes.LoginTypeRecaptcha,
+			wantResponse: util.JSONResponse{
+				Code: http.StatusForbidden,
+				JSON: jsonerror.Unknown(ErrCaptchaDisabled.Error()),
+			},
+		},
+		{
+			name:            "enabled recaptcha, no response defined",
+			enableRecaptcha: true,
+			loginType:       authtypes.LoginTypeRecaptcha,
+			wantResponse: util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.BadJSON(ErrMissingResponse.Error()),
+			},
+		},
+		{
+			name:            "invalid captcha response",
+			enableRecaptcha: true,
+			loginType:       authtypes.LoginTypeRecaptcha,
+			captchaBody:     `notvalid`,
+			wantResponse: util.JSONResponse{
+				Code: http.StatusUnauthorized,
+				JSON: jsonerror.BadJSON(ErrInvalidCaptcha.Error()),
+			},
+		},
+		{
+			name:            "valid captcha response",
+			enableRecaptcha: true,
+			loginType:       authtypes.LoginTypeRecaptcha,
+			captchaBody:     `success`,
+		},
+		{
+			name:            "captcha invalid from remote",
+			enableRecaptcha: true,
+			loginType:       authtypes.LoginTypeRecaptcha,
+			captchaBody:     `i should fail for other reasons`,
+			wantResponse:    util.JSONResponse{Code: http.StatusInternalServerError, JSON: jsonerror.InternalServerError()},
+		},
+	}
+
+	test.WithAllDatabases(t, func(t *testing.T, dbType test.DBType) {
+		base, baseClose := testrig.CreateBaseDendrite(t, dbType)
+		defer baseClose()
+
+		rsAPI := roomserver.NewInternalAPI(base)
+		keyAPI := keyserver.NewInternalAPI(base, &base.Cfg.KeyServer, nil, rsAPI)
+		userAPI := userapi.NewInternalAPI(base, &base.Cfg.UserAPI, nil, keyAPI, rsAPI, nil)
+		keyAPI.SetUserAPI(userAPI)
+
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				if tc.enableRecaptcha {
+					srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+						if err := r.ParseForm(); err != nil {
+							t.Fatal(err)
+						}
+						response := r.Form.Get("response")
+
+						// Respond with valid JSON or no JSON at all to test happy/error cases
+						switch response {
+						case "success":
+							json.NewEncoder(w).Encode(recaptchaResponse{Success: true})
+						case "notvalid":
+							json.NewEncoder(w).Encode(recaptchaResponse{Success: false})
+						default:
+
+						}
+					}))
+					defer srv.Close()
+					base.Cfg.ClientAPI.RecaptchaSiteVerifyAPI = srv.URL
+				}
+
+				if err := base.Cfg.Derive(); err != nil {
+					t.Fatalf("failed to derive config: %s", err)
+				}
+
+				base.Cfg.ClientAPI.RecaptchaEnabled = tc.enableRecaptcha
+				base.Cfg.ClientAPI.RegistrationDisabled = tc.registrationDisabled
+				base.Cfg.ClientAPI.GuestsDisabled = tc.guestsDisabled
+
+				if tc.kind == "" {
+					tc.kind = "user"
+				}
+				if tc.password == "" && !tc.forceEmpty {
+					tc.password = "someRandomPassword"
+				}
+				if tc.username == "" && !tc.forceEmpty {
+					tc.username = "valid"
+				}
+				if tc.loginType == "" {
+					tc.loginType = "m.login.dummy"
+				}
+
+				reg := registerRequest{
+					Password: tc.password,
+					Username: tc.username,
+				}
+
+				body := &bytes.Buffer{}
+				err := json.NewEncoder(body).Encode(reg)
+				if err != nil {
+					t.Fatal(err)
+				}
+
+				req := httptest.NewRequest(http.MethodPost, fmt.Sprintf("/?kind=%s", tc.kind), body)
+
+				resp := Register(req, userAPI, &base.Cfg.ClientAPI)
+				t.Logf("Resp: %+v", resp)
+
+				// The first request should return a userInteractiveResponse
+				switch r := resp.JSON.(type) {
+				case userInteractiveResponse:
+					// Check that the flows are the ones we configured
+					if !reflect.DeepEqual(r.Flows, base.Cfg.Derived.Registration.Flows) {
+						t.Fatalf("unexpected registration flows: %+v, want %+v", r.Flows, base.Cfg.Derived.Registration.Flows)
+					}
+				case *jsonerror.MatrixError:
+					if !reflect.DeepEqual(tc.wantResponse, resp) {
+						t.Fatalf("(%s), unexpected response: %+v, want: %+v", tc.name, resp, tc.wantResponse)
+					}
+					return
+				case registerResponse:
+					// this should only be possible on guest user registration, never for normal users
+					if tc.kind != "guest" {
+						t.Fatalf("got register response on first request: %+v", r)
+					}
+					// assert we've got a UserID, AccessToken and DeviceID
+					if r.UserID == "" {
+						t.Fatalf("missing userID in response")
+					}
+					if r.AccessToken == "" {
+						t.Fatalf("missing accessToken in response")
+					}
+					if r.DeviceID == "" {
+						t.Fatalf("missing deviceID in response")
+					}
+					return
+				default:
+					t.Logf("Got response: %T", resp.JSON)
+				}
+
+				// If we reached this, we should have received a UIA response
+				uia, ok := resp.JSON.(userInteractiveResponse)
+				if !ok {
+					t.Fatalf("did not receive a userInteractiveResponse: %T", resp.JSON)
+				}
+				t.Logf("%+v", uia)
+
+				// Register the user
+				reg.Auth = authDict{
+					Type:    authtypes.LoginType(tc.loginType),
+					Session: uia.Session,
+				}
+
+				if tc.captchaBody != "" {
+					reg.Auth.Response = tc.captchaBody
+				}
+
+				dummy := "dummy"
+				reg.DeviceID = &dummy
+				reg.InitialDisplayName = &dummy
+				reg.Type = authtypes.LoginType(tc.loginType)
+
+				err = json.NewEncoder(body).Encode(reg)
+				if err != nil {
+					t.Fatal(err)
+				}
+
+				req = httptest.NewRequest(http.MethodPost, "/", body)
+
+				resp = Register(req, userAPI, &base.Cfg.ClientAPI)
+
+				switch resp.JSON.(type) {
+				case *jsonerror.MatrixError:
+					if !reflect.DeepEqual(tc.wantResponse, resp) {
+						t.Fatalf("unexpected response: %+v, want: %+v", resp, tc.wantResponse)
+					}
+					return
+				case util.JSONResponse:
+					if !reflect.DeepEqual(tc.wantResponse, resp) {
+						t.Fatalf("unexpected response: %+v, want: %+v", resp, tc.wantResponse)
+					}
+					return
+				}
+
+				rr, ok := resp.JSON.(registerResponse)
+				if !ok {
+					t.Fatalf("expected a registerresponse, got %T", resp.JSON)
+				}
+
+				// validate the response
+				if tc.forceEmpty {
+					// when not supplying a username, one will be generated. Given this _SHOULD_ be
+					// the second user, set the username accordingly
+					reg.Username = "2"
+				}
+				wantUserID := strings.ToLower(fmt.Sprintf("@%s:%s", reg.Username, "test"))
+				if wantUserID != rr.UserID {
+					t.Fatalf("unexpected userID: %s, want %s", rr.UserID, wantUserID)
+				}
+				if rr.DeviceID != *reg.DeviceID {
+					t.Fatalf("unexpected deviceID: %s, want %s", rr.DeviceID, *reg.DeviceID)
+				}
+				if rr.AccessToken == "" {
+					t.Fatalf("missing accessToken in response")
+				}
+			})
+		}
+	})
+}
diff --git a/clientapi/routing/routing.go b/clientapi/routing/routing.go
index 69b46214c..09c2cd02f 100644
--- a/clientapi/routing/routing.go
+++ b/clientapi/routing/routing.go
@@ -639,9 +639,9 @@ func Setup(
 	).Methods(http.MethodGet, http.MethodPost, http.MethodOptions)
 
 	v3mux.Handle("/auth/{authType}/fallback/web",
-		httputil.MakeHTMLAPI("auth_fallback", base.EnableMetrics, func(w http.ResponseWriter, req *http.Request) *util.JSONResponse {
+		httputil.MakeHTMLAPI("auth_fallback", base.EnableMetrics, func(w http.ResponseWriter, req *http.Request) {
 			vars := mux.Vars(req)
-			return AuthFallback(w, req, vars["authType"], cfg)
+			AuthFallback(w, req, vars["authType"], cfg)
 		}),
 	).Methods(http.MethodGet, http.MethodPost, http.MethodOptions)
 
diff --git a/cmd/create-account/main.go b/cmd/create-account/main.go
index 15b043ed5..772778680 100644
--- a/cmd/create-account/main.go
+++ b/cmd/create-account/main.go
@@ -25,10 +25,10 @@ import (
 	"io"
 	"net/http"
 	"os"
-	"regexp"
 	"strings"
 	"time"
 
+	"github.com/matrix-org/dendrite/internal"
 	"github.com/tidwall/gjson"
 
 	"github.com/sirupsen/logrus"
@@ -58,15 +58,14 @@ Arguments:
 `
 
 var (
-	username           = flag.String("username", "", "The username of the account to register (specify the localpart only, e.g. 'alice' for '@alice:domain.com')")
-	password           = flag.String("password", "", "The password to associate with the account")
-	pwdFile            = flag.String("passwordfile", "", "The file to use for the password (e.g. for automated account creation)")
-	pwdStdin           = flag.Bool("passwordstdin", false, "Reads the password from stdin")
-	isAdmin            = flag.Bool("admin", false, "Create an admin account")
-	resetPassword      = flag.Bool("reset-password", false, "Deprecated")
-	serverURL          = flag.String("url", "http://localhost:8008", "The URL to connect to.")
-	validUsernameRegex = regexp.MustCompile(`^[0-9a-z_\-=./]+$`)
-	timeout            = flag.Duration("timeout", time.Second*30, "Timeout for the http client when connecting to the server")
+	username      = flag.String("username", "", "The username of the account to register (specify the localpart only, e.g. 'alice' for '@alice:domain.com')")
+	password      = flag.String("password", "", "The password to associate with the account")
+	pwdFile       = flag.String("passwordfile", "", "The file to use for the password (e.g. for automated account creation)")
+	pwdStdin      = flag.Bool("passwordstdin", false, "Reads the password from stdin")
+	isAdmin       = flag.Bool("admin", false, "Create an admin account")
+	resetPassword = flag.Bool("reset-password", false, "Deprecated")
+	serverURL     = flag.String("url", "http://localhost:8008", "The URL to connect to.")
+	timeout       = flag.Duration("timeout", time.Second*30, "Timeout for the http client when connecting to the server")
 )
 
 var cl = http.Client{
@@ -95,20 +94,21 @@ func main() {
 		os.Exit(1)
 	}
 
-	if !validUsernameRegex.MatchString(*username) {
-		logrus.Warn("Username can only contain characters a-z, 0-9, or '_-./='")
+	if err := internal.ValidateUsername(*username, cfg.Global.ServerName); err != nil {
+		logrus.WithError(err).Error("Specified username is invalid")
 		os.Exit(1)
 	}
 
-	if len(fmt.Sprintf("@%s:%s", *username, cfg.Global.ServerName)) > 255 {
-		logrus.Fatalf("Username can not be longer than 255 characters: %s", fmt.Sprintf("@%s:%s", *username, cfg.Global.ServerName))
-	}
-
 	pass, err := getPassword(*password, *pwdFile, *pwdStdin, os.Stdin)
 	if err != nil {
 		logrus.Fatalln(err)
 	}
 
+	if err = internal.ValidatePassword(pass); err != nil {
+		logrus.WithError(err).Error("Specified password is invalid")
+		os.Exit(1)
+	}
+
 	cl.Timeout = *timeout
 
 	accessToken, err := sharedSecretRegister(cfg.ClientAPI.RegistrationSharedSecret, *serverURL, *username, pass, *isAdmin)
diff --git a/internal/httputil/httpapi.go b/internal/httputil/httpapi.go
index 383913c60..37d144f4e 100644
--- a/internal/httputil/httpapi.go
+++ b/internal/httputil/httpapi.go
@@ -198,17 +198,12 @@ func MakeExternalAPI(metricsName string, f func(*http.Request) util.JSONResponse
 
 // MakeHTMLAPI adds Span metrics to the HTML Handler function
 // This is used to serve HTML alongside JSON error messages
-func MakeHTMLAPI(metricsName string, enableMetrics bool, f func(http.ResponseWriter, *http.Request) *util.JSONResponse) http.Handler {
+func MakeHTMLAPI(metricsName string, enableMetrics bool, f func(http.ResponseWriter, *http.Request)) http.Handler {
 	withSpan := func(w http.ResponseWriter, req *http.Request) {
 		span := opentracing.StartSpan(metricsName)
 		defer span.Finish()
 		req = req.WithContext(opentracing.ContextWithSpan(req.Context(), span))
-		if err := f(w, req); err != nil {
-			h := util.MakeJSONAPI(util.NewJSONRequestHandler(func(req *http.Request) util.JSONResponse {
-				return *err
-			}))
-			h.ServeHTTP(w, req)
-		}
+		f(w, req)
 	}
 
 	if !enableMetrics {
diff --git a/internal/validate.go b/internal/validate.go
index fc685ad50..0461b897e 100644
--- a/internal/validate.go
+++ b/internal/validate.go
@@ -15,30 +15,96 @@
 package internal
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
+	"regexp"
 
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 )
 
-const minPasswordLength = 8 // http://matrix.org/docs/spec/client_server/r0.2.0.html#password-based
+const (
+	maxUsernameLength = 254 // http://matrix.org/speculator/spec/HEAD/intro.html#user-identifiers TODO account for domain
 
-const maxPasswordLength = 512 // https://github.com/matrix-org/synapse/blob/v0.20.0/synapse/rest/client/v2_alpha/register.py#L161
+	minPasswordLength = 8   // http://matrix.org/docs/spec/client_server/r0.2.0.html#password-based
+	maxPasswordLength = 512 // https://github.com/matrix-org/synapse/blob/v0.20.0/synapse/rest/client/v2_alpha/register.py#L161
+)
 
-// ValidatePassword returns an error response if the password is invalid
-func ValidatePassword(password string) *util.JSONResponse {
+var (
+	ErrPasswordTooLong    = fmt.Errorf("password too long: max %d characters", maxPasswordLength)
+	ErrPasswordWeak       = fmt.Errorf("password too weak: min %d characters", minPasswordLength)
+	ErrUsernameTooLong    = fmt.Errorf("username exceeds the maximum length of %d characters", maxUsernameLength)
+	ErrUsernameInvalid    = errors.New("username can only contain characters a-z, 0-9, or '_-./='")
+	ErrUsernameUnderscore = errors.New("username cannot start with a '_'")
+	validUsernameRegex    = regexp.MustCompile(`^[0-9a-z_\-=./]+$`)
+)
+
+// ValidatePassword returns an error if the password is invalid
+func ValidatePassword(password string) error {
 	// https://github.com/matrix-org/synapse/blob/v0.20.0/synapse/rest/client/v2_alpha/register.py#L161
 	if len(password) > maxPasswordLength {
-		return &util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.BadJSON(fmt.Sprintf("password too long: max %d characters", maxPasswordLength)),
-		}
+		return ErrPasswordTooLong
 	} else if len(password) > 0 && len(password) < minPasswordLength {
+		return ErrPasswordWeak
+	}
+	return nil
+}
+
+// PasswordResponse returns a util.JSONResponse for a given error, if any.
+func PasswordResponse(err error) *util.JSONResponse {
+	switch err {
+	case ErrPasswordWeak:
 		return &util.JSONResponse{
 			Code: http.StatusBadRequest,
-			JSON: jsonerror.WeakPassword(fmt.Sprintf("password too weak: min %d chars", minPasswordLength)),
+			JSON: jsonerror.WeakPassword(ErrPasswordWeak.Error()),
+		}
+	case ErrPasswordTooLong:
+		return &util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.BadJSON(ErrPasswordTooLong.Error()),
 		}
 	}
 	return nil
 }
+
+// ValidateUsername returns an error if the username is invalid
+func ValidateUsername(localpart string, domain gomatrixserverlib.ServerName) error {
+	// https://github.com/matrix-org/synapse/blob/v0.20.0/synapse/rest/client/v2_alpha/register.py#L161
+	if id := fmt.Sprintf("@%s:%s", localpart, domain); len(id) > maxUsernameLength {
+		return ErrUsernameTooLong
+	} else if !validUsernameRegex.MatchString(localpart) {
+		return ErrUsernameInvalid
+	} else if localpart[0] == '_' { // Regex checks its not a zero length string
+		return ErrUsernameUnderscore
+	}
+	return nil
+}
+
+// UsernameResponse returns a util.JSONResponse for the given error, if any.
+func UsernameResponse(err error) *util.JSONResponse {
+	switch err {
+	case ErrUsernameTooLong:
+		return &util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.BadJSON(err.Error()),
+		}
+	case ErrUsernameInvalid, ErrUsernameUnderscore:
+		return &util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.InvalidUsername(err.Error()),
+		}
+	}
+	return nil
+}
+
+// ValidateApplicationServiceUsername returns an error if the username is invalid for an application service
+func ValidateApplicationServiceUsername(localpart string, domain gomatrixserverlib.ServerName) error {
+	if id := fmt.Sprintf("@%s:%s", localpart, domain); len(id) > maxUsernameLength {
+		return ErrUsernameTooLong
+	} else if !validUsernameRegex.MatchString(localpart) {
+		return ErrUsernameInvalid
+	}
+	return nil
+}
diff --git a/internal/validate_test.go b/internal/validate_test.go
new file mode 100644
index 000000000..d0ad04707
--- /dev/null
+++ b/internal/validate_test.go
@@ -0,0 +1,170 @@
+package internal
+
+import (
+	"net/http"
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+)
+
+func Test_validatePassword(t *testing.T) {
+	tests := []struct {
+		name      string
+		password  string
+		wantError error
+		wantJSON  *util.JSONResponse
+	}{
+		{
+			name:      "password too short",
+			password:  "shortpw",
+			wantError: ErrPasswordWeak,
+			wantJSON:  &util.JSONResponse{Code: http.StatusBadRequest, JSON: jsonerror.WeakPassword(ErrPasswordWeak.Error())},
+		},
+		{
+			name:      "password too long",
+			password:  strings.Repeat("a", maxPasswordLength+1),
+			wantError: ErrPasswordTooLong,
+			wantJSON:  &util.JSONResponse{Code: http.StatusBadRequest, JSON: jsonerror.BadJSON(ErrPasswordTooLong.Error())},
+		},
+		{
+			name:     "password OK",
+			password: util.RandomString(10),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotErr := ValidatePassword(tt.password)
+			if !reflect.DeepEqual(gotErr, tt.wantError) {
+				t.Errorf("validatePassword() = %v, wantJSON %v", gotErr, tt.wantError)
+			}
+
+			if got := PasswordResponse(gotErr); !reflect.DeepEqual(got, tt.wantJSON) {
+				t.Errorf("validatePassword() = %v, wantJSON %v", got, tt.wantJSON)
+			}
+		})
+	}
+}
+
+func Test_validateUsername(t *testing.T) {
+	tooLongUsername := strings.Repeat("a", maxUsernameLength)
+	tests := []struct {
+		name      string
+		localpart string
+		domain    gomatrixserverlib.ServerName
+		wantErr   error
+		wantJSON  *util.JSONResponse
+	}{
+		{
+			name:      "empty username",
+			localpart: "",
+			domain:    "localhost",
+			wantErr:   ErrUsernameInvalid,
+			wantJSON: &util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.InvalidUsername(ErrUsernameInvalid.Error()),
+			},
+		},
+		{
+			name:      "invalid username",
+			localpart: "INVALIDUSERNAME",
+			domain:    "localhost",
+			wantErr:   ErrUsernameInvalid,
+			wantJSON: &util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.InvalidUsername(ErrUsernameInvalid.Error()),
+			},
+		},
+		{
+			name:      "username too long",
+			localpart: tooLongUsername,
+			domain:    "localhost",
+			wantErr:   ErrUsernameTooLong,
+			wantJSON: &util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.BadJSON(ErrUsernameTooLong.Error()),
+			},
+		},
+		{
+			name:      "localpart starting with an underscore",
+			localpart: "_notvalid",
+			domain:    "localhost",
+			wantErr:   ErrUsernameUnderscore,
+			wantJSON: &util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.InvalidUsername(ErrUsernameUnderscore.Error()),
+			},
+		},
+		{
+			name:      "valid username",
+			localpart: "valid",
+			domain:    "localhost",
+		},
+		{
+			name:      "complex username",
+			localpart: "f00_bar-baz.=40/",
+			domain:    "localhost",
+		},
+		{
+			name:      "rejects emoji username 💥",
+			localpart: "💥",
+			domain:    "localhost",
+			wantErr:   ErrUsernameInvalid,
+			wantJSON: &util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.InvalidUsername(ErrUsernameInvalid.Error()),
+			},
+		},
+		{
+			name:      "special characters are allowed",
+			localpart: "/dev/null",
+			domain:    "localhost",
+		},
+		{
+			name:      "special characters are allowed 2",
+			localpart: "i_am_allowed=1",
+			domain:    "localhost",
+		},
+		{
+			name:      "not all special characters are allowed",
+			localpart: "notallowed#", // contains #
+			domain:    "localhost",
+			wantErr:   ErrUsernameInvalid,
+			wantJSON: &util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.InvalidUsername(ErrUsernameInvalid.Error()),
+			},
+		},
+		{
+			name:      "username containing numbers",
+			localpart: "hello1337",
+			domain:    "localhost",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotErr := ValidateUsername(tt.localpart, tt.domain)
+			if !reflect.DeepEqual(gotErr, tt.wantErr) {
+				t.Errorf("ValidateUsername() = %v, wantErr %v", gotErr, tt.wantErr)
+			}
+			if gotJSON := UsernameResponse(gotErr); !reflect.DeepEqual(gotJSON, tt.wantJSON) {
+				t.Errorf("UsernameResponse() = %v, wantJSON %v", gotJSON, tt.wantJSON)
+			}
+
+			// Application services are allowed usernames starting with an underscore
+			if tt.wantErr == ErrUsernameUnderscore {
+				return
+			}
+			gotErr = ValidateApplicationServiceUsername(tt.localpart, tt.domain)
+			if !reflect.DeepEqual(gotErr, tt.wantErr) {
+				t.Errorf("ValidateUsername() = %v, wantErr %v", gotErr, tt.wantErr)
+			}
+			if gotJSON := UsernameResponse(gotErr); !reflect.DeepEqual(gotJSON, tt.wantJSON) {
+				t.Errorf("UsernameResponse() = %v, wantJSON %v", gotJSON, tt.wantJSON)
+			}
+		})
+	}
+}
diff --git a/setup/config/config.go b/setup/config/config.go
index 7e7ed1aa1..6523a2452 100644
--- a/setup/config/config.go
+++ b/setup/config/config.go
@@ -29,7 +29,7 @@ import (
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/crypto/ed25519"
-	yaml "gopkg.in/yaml.v2"
+	"gopkg.in/yaml.v2"
 
 	jaegerconfig "github.com/uber/jaeger-client-go/config"
 	jaegermetrics "github.com/uber/jaeger-lib/metrics"
@@ -314,11 +314,13 @@ func (config *Dendrite) Derive() error {
 
 	if config.ClientAPI.RecaptchaEnabled {
 		config.Derived.Registration.Params[authtypes.LoginTypeRecaptcha] = map[string]string{"public_key": config.ClientAPI.RecaptchaPublicKey}
-		config.Derived.Registration.Flows = append(config.Derived.Registration.Flows,
-			authtypes.Flow{Stages: []authtypes.LoginType{authtypes.LoginTypeRecaptcha}})
+		config.Derived.Registration.Flows = []authtypes.Flow{
+			{Stages: []authtypes.LoginType{authtypes.LoginTypeRecaptcha}},
+		}
 	} else {
-		config.Derived.Registration.Flows = append(config.Derived.Registration.Flows,
-			authtypes.Flow{Stages: []authtypes.LoginType{authtypes.LoginTypeDummy}})
+		config.Derived.Registration.Flows = []authtypes.Flow{
+			{Stages: []authtypes.LoginType{authtypes.LoginTypeDummy}},
+		}
 	}
 
 	// Load application service configuration files
diff --git a/setup/config/config_clientapi.go b/setup/config/config_clientapi.go
index 0a871da18..11628b1b0 100644
--- a/setup/config/config_clientapi.go
+++ b/setup/config/config_clientapi.go
@@ -78,9 +78,6 @@ func (c *ClientAPI) Verify(configErrs *ConfigErrors, isMonolith bool) {
 	c.TURN.Verify(configErrs)
 	c.RateLimiting.Verify(configErrs)
 	if c.RecaptchaEnabled {
-		checkNotEmpty(configErrs, "client_api.recaptcha_public_key", c.RecaptchaPublicKey)
-		checkNotEmpty(configErrs, "client_api.recaptcha_private_key", c.RecaptchaPrivateKey)
-		checkNotEmpty(configErrs, "client_api.recaptcha_siteverify_api", c.RecaptchaSiteVerifyAPI)
 		if c.RecaptchaSiteVerifyAPI == "" {
 			c.RecaptchaSiteVerifyAPI = "https://www.google.com/recaptcha/api/siteverify"
 		}
@@ -93,6 +90,10 @@ func (c *ClientAPI) Verify(configErrs *ConfigErrors, isMonolith bool) {
 		if c.RecaptchaSitekeyClass == "" {
 			c.RecaptchaSitekeyClass = "g-recaptcha-response"
 		}
+		checkNotEmpty(configErrs, "client_api.recaptcha_public_key", c.RecaptchaPublicKey)
+		checkNotEmpty(configErrs, "client_api.recaptcha_private_key", c.RecaptchaPrivateKey)
+		checkNotEmpty(configErrs, "client_api.recaptcha_siteverify_api", c.RecaptchaSiteVerifyAPI)
+		checkNotEmpty(configErrs, "client_api.recaptcha_sitekey_class", c.RecaptchaSitekeyClass)
 	}
 	// Ensure there is any spam counter measure when enabling registration
 	if !c.RegistrationDisabled && !c.OpenRegistrationWithoutVerificationEnabled {