CVE-2023-38691

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

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.

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 application does not adequately verify the identity of a user, device, or process before granting access. Typical impact: unauthorized access to functions or data reserved for authenticated parties.

CVE-2023-38691 has a CVSS score of 5.0 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (8.1.2, 9.0.1); upgrading removes the vulnerable code path.

Kodem intelligence

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1. What is CVE-2023-38691? CVE-2023-38691 is a medium-severity improper authentication vulnerability in matrix-appservice-bridge (npm), affecting versions >= 4.0.0, < 8.1.2. It is fixed in 8.1.2, 9.0.1. The application does not adequately verify the identity of a user, device, or process before granting access.
2. How severe is CVE-2023-38691? CVE-2023-38691 has a CVSS score of 5.0 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of matrix-appservice-bridge are affected by CVE-2023-38691? matrix-appservice-bridge (npm) versions >= 4.0.0, < 8.1.2 is affected.
4. Is there a fix for CVE-2023-38691? Yes. CVE-2023-38691 is fixed in 8.1.2, 9.0.1. Upgrade to this version or later.
5. Is CVE-2023-38691 exploitable, and should I be worried? Whether CVE-2023-38691 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2023-38691 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2023-38691?
   • Upgrade matrix-appservice-bridge to 8.1.2 or later
   • Upgrade matrix-appservice-bridge to 9.0.1 or later

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