diff --git a/CHANGES.md b/CHANGES.md
index d4cc17948..c1b8673c0 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,17 @@
+For the next release
+====================
+
+Removal warning
+---------------
+
+Some older clients used a
+[disallowed character](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-register-email-requesttoken)
+(`:`) in the `client_secret` parameter of various endpoints. The incorrect
+behaviour was allowed for backwards compatibility, but is now being removed
+from Synapse as most users have updated their client. Further context can be
+found at [\#6766](https://github.com/matrix-org/synapse/issues/6766).
+
+
 Synapse 1.19.0 (2020-08-17)
 ===========================
 
diff --git a/changelog.d/8101.bugfix b/changelog.d/8101.bugfix
new file mode 100644
index 000000000..703bba423
--- /dev/null
+++ b/changelog.d/8101.bugfix
@@ -0,0 +1 @@
+Synapse now correctly enforces the valid characters in the `client_secret` parameter used in various endpoints.
diff --git a/synapse/util/stringutils.py b/synapse/util/stringutils.py
index 2e2b40a42..61d96a6c2 100644
--- a/synapse/util/stringutils.py
+++ b/synapse/util/stringutils.py
@@ -24,9 +24,7 @@ from synapse.api.errors import Codes, SynapseError
 _string_with_symbols = string.digits + string.ascii_letters + ".,;:^&*-_+=#~@"
 
 # https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-register-email-requesttoken
-# Note: The : character is allowed here for older clients, but will be removed in a
-# future release. Context: https://github.com/matrix-org/synapse/issues/6766
-client_secret_regex = re.compile(r"^[0-9a-zA-Z\.\=\_\-\:]+$")
+client_secret_regex = re.compile(r"^[0-9a-zA-Z\.\=\_\-]+$")
 
 # random_string and random_string_with_symbols are used for a range of things,
 # some cryptographically important, some less so. We use SystemRandom to make sure
diff --git a/tests/util/test_stringutils.py b/tests/util/test_stringutils.py
index 4f4da29a9..8491f7cc8 100644
--- a/tests/util/test_stringutils.py
+++ b/tests/util/test_stringutils.py
@@ -28,9 +28,6 @@ class StringUtilsTestCase(unittest.TestCase):
             "_--something==_",
             "...--==-18913",
             "8Dj2odd-e9asd.cd==_--ddas-secret-",
-            # We temporarily allow : characters: https://github.com/matrix-org/synapse/issues/6766
-            # To be removed in a future release
-            "SECRET:1234567890",
         ]
 
         bad = [