CVE-2021-21394

Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints

Workarounds

Depending on the needs and configuration of the homeserver a few options are available:

1. Using email as third-party identifiers be disabled by not configuring the email setting.

2. Using phone numbers as third-party identifiers can be disabled by ensuring that account_threepid_delegates.msisdn is not configured.

3. Additionally, the affected endpoint patterns can be blocked at a reverse proxy:

   • ^/_matrix/client/(r0|unstable)/register/email
   • ^/_matrix/client/(r0|unstable)/register/msisdn
   • ^/_matrix/client/(r0|unstable)/account/password
   • ^/_matrix/client/(r0|unstable)/account/3pid

Missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion.

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-2021-21394 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.

Kodem intelligence

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The issue is fixed by #9321.

1. What is CVE-2021-21394? CVE-2021-21394 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.
2. How severe is CVE-2021-21394? CVE-2021-21394 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.
3. Which versions of matrix-synapse are affected by CVE-2021-21394? matrix-synapse (pip) versions < 1.28.0 is affected.
4. Is there a fix for CVE-2021-21394? Yes. CVE-2021-21394 is fixed in 1.28.0. Upgrade to this version or later.
5. Is CVE-2021-21394 exploitable, and should I be worried? Whether CVE-2021-21394 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-2021-21394 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-2021-21394? Upgrade matrix-synapse to 1.28.0 or later.

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