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Fix bcrypt errors preventing users from being able to log in (#19101)

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This commit is contained in:
Andrew Morgan
2025-10-28 11:16:02 +01:00
committed by GitHub
parent 1271e896b5
commit db9a61c30f
5 changed files with 19 additions and 6 deletions
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1
changelog.d/19101.bugfix Normal file
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@@ -0,0 +1 @@
Fix users being unable to log in if their password, or the server's configured pepper, was too long.

2
docs/usage/configuration/config_documentation.md
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@@ -3815,7 +3815,7 @@ This setting has the following sub-options:
* `localdb_enabled` (boolean): Set to false to disable authentication against the local password database. This is ignored if `enabled` is false, and is only useful if you have other `password_providers`. Defaults to `true`.
* `pepper` (string|null): Set the value here to a secret random string for extra security. DO NOT CHANGE THIS AFTER INITIAL SETUP! Defaults to `null`.
* `pepper` (string|null): A secret random string that will be appended to user's passwords before they are hashed. This improves the security of short passwords. DO NOT CHANGE THIS AFTER INITIAL SETUP! Defaults to `null`.
* `policy` (object): Define and enforce a password policy, such as minimum lengths for passwords, etc. This is an implementation of MSC2000.

5
schema/synapse-config.schema.yaml
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@@ -4695,8 +4695,9 @@ properties:
pepper:
type: ["string", "null"]
description: >-
Set the value here to a secret random string for extra security. DO
NOT CHANGE THIS AFTER INITIAL SETUP!
A secret random string that will be appended to user's passwords
before they are hashed. This improves the security of short passwords.
DO NOT CHANGE THIS AFTER INITIAL SETUP!
default: null
policy:
type: object

2
synapse/_scripts/hash_password.py
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@@ -77,7 +77,7 @@ def main() -> None:
if len(bytes_to_hash) > 72:
# bcrypt only looks at the first 72 bytes
print(
f"Password is too long ({len(bytes_to_hash)} bytes); truncating to 72 bytes for bcrypt. "
f"Password + pepper is too long ({len(bytes_to_hash)} bytes); truncating to 72 bytes for bcrypt. "
"This is expected behaviour and will not affect a user's ability to log in. 72 bytes is "
"sufficient entropy for a password."
)

15
synapse/handlers/auth.py
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@@ -1691,7 +1691,7 @@ class AuthHandler:
#
# Note: we explicitly DO NOT log the length of the user's password here.
logger.debug(
"Password is too long; truncating to 72 bytes for bcrypt. "
"Password + pepper is too long; truncating to 72 bytes for bcrypt. "
"This is expected behaviour and will not affect a user's ability to log in. 72 bytes is "
"sufficient entropy for a password."
)
@@ -1720,9 +1720,20 @@ class AuthHandler:
def _do_validate_hash(checked_hash: bytes) -> bool:
# Normalise the Unicode in the password
pw = unicodedata.normalize("NFKC", password)
password_pepper = self.hs.config.auth.password_pepper
bytes_to_hash = pw.encode("utf8") + password_pepper.encode("utf8")
if len(bytes_to_hash) > 72:
# bcrypt only looks at the first 72 bytes
logger.debug(
"Password + pepper is too long; truncating to 72 bytes for bcrypt. "
"This is expected behaviour and will not affect a user's ability to log in. 72 bytes is "
"sufficient entropy for a password."
)
bytes_to_hash = bytes_to_hash[:72]
return bcrypt.checkpw(
pw.encode("utf8") + self.hs.config.auth.password_pepper.encode("utf8"),
bytes_to_hash,
checked_hash,
)