CVE-2023-38691: Matrix Matrix-Appservice-Bridge

Medium severity, CVSS 6.5. EPSS: 0.5% chance of exploitation in the next 30 days.

matrix-appservice-bridge provides an API for setting up bridges. Starting in version 4.0.0 and prior to versions 8.1.2 and 9.0.1, a malicious Matrix server can use a foreign user's MXID in an OpenID exchange, allowing a bad actor to impersonate users when using the provisioning API. The library does not check that the servername part of the `sub` parameter (containing the user's *claimed* MXID) is the the same as the servername we are talking to. A malicious actor could spin up a server on any given domain, respond with a `sub` parameter according to the user they want to act as and use the resulting token to perform provisioning requests. Versions 8.1.2 and 9.0.1 contain a patch. As a workaround, disable the provisioning API.

Affected products

• Matrix Matrix-Appservice-Bridge: from 4.0.0, before 8.1.2 (fixed in 8.1.2); version 9.0.0 only

Published 2023-08-04. Last modified 2026-06-17.