We became aware today of a flaw in sydent’s validation of email addresses which can lead to a failure to correctly limit registration to a given email domain. This only affects people who run their own sydent, and are relying on allowed_local_3pid in their synapse config. We’d like to thank @fs0c131y for bringing it to our attention on Twitter this morning. We are not aware of this being exploited in the wild other than the initial report.

If you are running your own sydent, and limiting signup for your server using the allowed_local_3pids configuration option, then you need to upgrade your sydent immediately to Sydent 1.0.2.

Meanwhile, if you have been relying on the allowed_local_3pids configuration option to restrict access to your homeserver, you may wish to check your homeserver’s user_threepids table for malformed email addresses and your sydent’s database as follows:

$ sqlite3 sydent.db 
sqlite> select count(*) from global_threepid_associations where address like '%@%@%';

$ psql matrix
matrix=> select count(*) from user_threepids where address like '%@%@%';

If the queries return more than 0 results, please let us know at [email protected] - otherwise you are fine.


A flaw existed in sydent whereby it was possible to bypass the requirement specified in synapse’s allowed_local_3pids option, which restricts that users may only register with an email address matching a specific format.

This relied on two things:

  1. sydent uses python's email.utils.parseaddr function to parse the input email address before sending validation mail to it, but it turns out that if you hand parseaddr an malformed email address of form [email protected]@c.com, it silently discards the @c.com prefix without error. The result of this is that if one requested a validation token for '[email protected]@important.com', the token would be sent to '[email protected]', but the address '[email protected]@important.com' would be marked as validated. This release fixes this behaviour by asserting that the parsed email address is the same as the input email address.
  2. synapse's checking of email addresses relies on regular expressions in the home server configuration file. synapse does not validate email addresses before checking them against these regular expressions, so naive regular expressions will detect the second domain in email addresses such as the above, causing them to pass the check.

You can get sydent 1.0.2 from https://github.com/matrix-org/sydent/releases/tag/v1.0.2.

The Foundation needs you

The Matrix.org Foundation is a non-profit and only relies on donations to operate. Its core mission is to maintain the Matrix Specification, but it does much more than that.

It maintains the matrix.org homeserver and hosts several bridges for free. It fights for our collective rights to digital privacy and dignity.

Support us