CVE-2023-38691: matrix-appservice-bridge doesn't verify the sub parameter of an openId token exhange, allowing unauthorized access to provisioning APIs
Impact
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.
Details
The library does not check that the servername part of the sub parameter (containing the user's claimed MXID) is 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.
Workarounds
Disable the provisioning API. If the bridge does not use the provisioning API, you are not vulnerable.
Other sources
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 Software
Remediation
Event History
Frequently Asked Questions
What is the vulnerability ID?
The vulnerability ID is CVE-2023-38691.
What is the severity of CVE-2023-38691?
The severity of CVE-2023-38691 is medium with a CVSS score of 6.5.
Which software versions are affected by CVE-2023-38691?
Versions 4.0.0 to 8.1.2 of matrix-appservice-bridge are affected by CVE-2023-38691. Versions 8.1.2 and 9.0.0 are specifically mentioned.
How can a malicious Matrix server exploit CVE-2023-38691?
A malicious Matrix server can use a foreign user's MXID in an OpenID exchange, allowing them to impersonate users.
How can I fix CVE-2023-38691?
Update to version 9.0.1 of matrix-appservice-bridge.