Summary
Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.
Workarounds
Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators.
Administrators that are using a reverse proxy could, with partial loss of group functionality, block the following endpoints:
/_matrix/client/r0/groups/{group_id}/rooms/_matrix/client/unstable/groups/{group_id}/rooms
References
n/a
For more information
If you have any questions or comments about this advisory, e-mail us at [email protected].
Impact
Unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where:
- the vulnerable homeserver is in the room; and
- untrusted users are permitted to create groups (communities).
By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted.
CVE-2021-39163 has a CVSS score of 3.1 (Low). 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 (1.41.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Server administrators should upgrade to 1.41.1 or higher.
Frequently Asked Questions
- What is CVE-2021-39163? CVE-2021-39163 is a low-severity security vulnerability in matrix-synapse (pip), affecting versions < 1.41.1. It is fixed in 1.41.1.
- How severe is CVE-2021-39163? CVE-2021-39163 has a CVSS score of 3.1 (Low). 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.
- Which versions of matrix-synapse are affected by CVE-2021-39163? matrix-synapse (pip) versions < 1.41.1 is affected.
- Is there a fix for CVE-2021-39163? Yes. CVE-2021-39163 is fixed in 1.41.1. Upgrade to this version or later.
- Is CVE-2021-39163 exploitable, and should I be worried? Whether CVE-2021-39163 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
- What actually determines whether CVE-2021-39163 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.
- How do I fix CVE-2021-39163? Upgrade
matrix-synapseto 1.41.1 or later.