diff --git a/synapse/config/captcha.py b/synapse/config/captcha.py
index 8ebcfc3623..4ed9070b9e 100644
--- a/synapse/config/captcha.py
+++ b/synapse/config/captcha.py
@@ -24,6 +24,7 @@ class CaptchaConfig(Config):
         self.captcha_ip_origin_is_x_forwarded = (
             args.captcha_ip_origin_is_x_forwarded
         )
+        self.captcha_bypass_secret = args.captcha_bypass_secret
 
     @classmethod
     def add_arguments(cls, parser):
@@ -43,4 +44,8 @@ class CaptchaConfig(Config):
             "--captcha_ip_origin_is_x_forwarded", type=bool, default=False,
             help="When checking captchas, use the X-Forwarded-For (XFF) header"
             + " as the client IP and not the actual client IP."
-        )
\ No newline at end of file
+        )
+        group.add_argument(
+            "--captcha_bypass_secret", type=str,
+            help="A secret key used to bypass the captcha test entirely."
+        )
diff --git a/synapse/rest/register.py b/synapse/rest/register.py
index af528a44f6..14d1ab018e 100644
--- a/synapse/rest/register.py
+++ b/synapse/rest/register.py
@@ -21,6 +21,8 @@ from synapse.api.constants import LoginType
 from base import RestServlet, client_path_pattern
 import synapse.util.stringutils as stringutils
 
+from hashlib import sha1
+import hmac
 import json
 import logging
 import urllib
@@ -142,6 +144,38 @@ class RegisterRestServlet(RestServlet):
         if not self.hs.config.enable_registration_captcha:
             raise SynapseError(400, "Captcha not required.")
 
+        yield self._check_recaptcha(request, register_json, session)
+
+        session[LoginType.RECAPTCHA] = True  # mark captcha as done
+        self._save_session(session)
+        defer.returnValue({
+            "next": [LoginType.PASSWORD, LoginType.EMAIL_IDENTITY]
+        })
+
+    @defer.inlineCallbacks
+    def _check_recaptcha(self, request, register_json, session):
+        if ("captcha_bypass_hmac" in register_json and
+                self.hs.config.captcha_bypass_secret):
+            if "user" not in register_json:
+                raise SynapseError(400, "Captcha bypass needs 'user'")
+
+            want = hmac.new(
+                key=self.hs.config.captcha_bypass_secret,
+                msg=register_json["user"],
+                digestmod=sha1,
+            ).hexdigest()
+
+            # str() because otherwise hmac complains that 'unicode' does not
+            # have the buffer interface
+            got = str(register_json["captcha_bypass_hmac"])
+
+            if hmac.compare_digest(want, got):
+                session["user"] = register_json["user"]
+                defer.returnValue(None)
+            else:
+                raise SynapseError(400, "Captcha bypass HMAC incorrect",
+                    errcode=Codes.CAPTCHA_NEEDED)
+
         challenge = None
         user_response = None
         try:
@@ -166,11 +200,6 @@ class RegisterRestServlet(RestServlet):
             challenge,
             user_response
         )
-        session[LoginType.RECAPTCHA] = True  # mark captcha as done
-        self._save_session(session)
-        defer.returnValue({
-            "next": [LoginType.PASSWORD, LoginType.EMAIL_IDENTITY]
-        })
 
     @defer.inlineCallbacks
     def _do_email_identity(self, request, register_json, session):
@@ -195,6 +224,10 @@ class RegisterRestServlet(RestServlet):
             # captcha should've been done by this stage!
             raise SynapseError(400, "Captcha is required.")
 
+        if ("user" in session and "user" in register_json and
+                session["user"] != register_json["user"]):
+            raise SynapseError(400, "Cannot change user ID during registration")
+
         password = register_json["password"].encode("utf-8")
         desired_user_id = (register_json["user"].encode("utf-8") if "user"
                           in register_json else None)