CVE-2021-21394: Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints
First published: Mon Apr 12 2021(Updated: )
### Impact 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. ### Patches The issue is fixed by #9321. ### 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`
Credit: security-advisories@github.com security-advisories@github.com
| Affected Software | Affected Version | How to fix |
|---|---|---|
| Matrix Synapse | >0.17.0<1.28.0 | |
| Fedoraproject Fedora | =34 | |
| pip/matrix-synapse | <1.28.0 | 1.28.0 |
| >0.17.0<1.28.0 | ||
| =34 |
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Reference Links
- https://github.com/matrix-org/synapse/pull/9321
- https://github.com/matrix-org/synapse/pull/9393
- https://github.com/matrix-org/synapse/security/advisories/GHSA-w9fg-xffh-p362
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
- https://pypi.org/project/matrix-synapse/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
- https://nvd.nist.gov/vuln/detail/CVE-2021-21394
- https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-27.yaml
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
- https://pypi.org/project/matrix-synapse
- https://github.com/advisories/GHSA-w9fg-xffh-p362 (Advisory)
Frequently Asked Questions
What is CVE-2021-21394?
CVE-2021-21394 is a vulnerability found in Synapse, a Matrix reference homeserver, that allows for input validation bypass.
What is the severity of CVE-2021-21394?
The severity of CVE-2021-21394 is medium with a CVSS score of 6.5.
How does CVE-2021-21394 impact Synapse?
CVE-2021-21394 allows attackers to bypass input validation on certain endpoints used to confirm third-party identity.
How do I fix CVE-2021-21394?
To fix CVE-2021-21394, it is recommended to update Synapse to version 1.28.0 or later.
What is the Common Weakness Enumeration (CWE) ID for CVE-2021-21394?
The CWE ID for CVE-2021-21394 is CWE-20, which stands for Improper Input Validation.
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