Summary
Denial of service (via resource exhaustion) due to improper input validation on groups/communities endpoints
Workarounds
The groups feature can be disabled (by setting enable_group_creation to False) to mitigate this issue. Note that it is disabled by default.
Other information
Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them.
Impact
Missing input validation of some parameters on the groups (also known as communities) endpoints could cause excessive use of disk space and memory leading to resource exhaustion. Additionally clients may have issues rendering large fields.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2021-21393 has a CVSS score of 5.3 (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 (1.28.0); 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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This issue is fixed by #9321 and #9393.
Frequently Asked Questions
- What is CVE-2021-21393? CVE-2021-21393 is a medium-severity improper input validation vulnerability in matrix-synapse (pip), affecting versions < 1.28.0. It is fixed in 1.28.0. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- How severe is CVE-2021-21393? CVE-2021-21393 has a CVSS score of 5.3 (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.
- Which versions of matrix-synapse are affected by CVE-2021-21393? matrix-synapse (pip) versions < 1.28.0 is affected.
- Is there a fix for CVE-2021-21393? Yes. CVE-2021-21393 is fixed in 1.28.0. Upgrade to this version or later.
- Is CVE-2021-21393 exploitable, and should I be worried? Whether CVE-2021-21393 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-2021-21393 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-2021-21393? Upgrade
matrix-synapseto 1.28.0 or later.