What’s Vulnerable › Vulnerabilities

CVE-2021-29430: Denial of service attack via memory exhaustion

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-04-15 · updated 2026-06-17
• Weakness: CWE-20

Description

Sydent is a reference Matrix identity server. Sydent does not limit the size of requests it receives from HTTP clients. A malicious user could send an HTTP request with a very large body, leading to memory exhaustion and denial of service. Sydent also does not limit response size for requests it makes to remote Matrix homeservers. A malicious homeserver could return a very large response, again leading to memory exhaustion and denial of service. This affects any server which accepts registration requests from untrusted clients. This issue has been patched by releases 89071a1, 0523511, f56eee3. As a workaround request sizes can be limited in an HTTP reverse-proxy. There are no known workarounds for the problem with overlarge responses.

Affected products

• matrix-org sydent: before 2.3.0 — fixed in 2.3.0

How to fix

Upgrade to a fixed version: matrix-org sydent 2.3.0. Check the vendor advisory for mitigations.

References

• https://github.com/matrix-org/sydent/commit/0523511d2fb40f2738f8a8549868f44b96e5dab7
• https://github.com/matrix-org/sydent/commit/89071a1a754c69a50deac89e6bb74002d4cda19d
• https://github.com/matrix-org/sydent/commit/f56eee315b6c44fdd9f6aa785cc2ec744a594428
• https://github.com/matrix-org/sydent/releases/tag/v2.3.0
• https://github.com/matrix-org/sydent/security/advisories/GHSA-wmg4-8cp2-hpg9
• https://pypi.org/project/matrix-sydent/
• https://github.com/matrix-org/sydent/commit/0523511d2fb40f2738f8a8549868f44b96e5dab7
• https://github.com/matrix-org/sydent/commit/89071a1a754c69a50deac89e6bb74002d4cda19d
• https://github.com/matrix-org/sydent/commit/f56eee315b6c44fdd9f6aa785cc2ec744a594428
• https://github.com/matrix-org/sydent/releases/tag/v2.3.0
• https://github.com/matrix-org/sydent/security/advisories/GHSA-wmg4-8cp2-hpg9
• https://pypi.org/project/matrix-sydent/

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