CVE-2023-38691: matrix-appservice-bridge doesn't verify the sub parameter of an openId token exhange, allowing unauthorized access to provisioning APIs

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2023-08-04 · updated 2026-06-17
Weakness: CWE-287

Description

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-org matrix-appservice-bridge: from 4.0.0 before 8.1.2; 9.0.0 — fixed in 8.1.2

How to fix

Upgrade to a fixed version: matrix-org matrix-appservice-bridge 8.1.2. Check the vendor advisory for mitigations.

References

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