CVE-2023-38691: 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 s...

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.

Metrics

CVSS 3.1

6.5/10

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

EPSS Probability

0.39%

30.4th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Matrix	Matrix-Appservice-Bridge	>= 4.0.0, < 8.1.2
Matrix	Matrix-Appservice-Bridge	9.0.0

References

Timeline

Published: Aug 4, 2023
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2023-38691?

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.

How severe is CVE-2023-38691?

CVE-2023-38691 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 0.39% probability of exploitation in the next 30 days.

How do I fix CVE-2023-38691?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.