CVE-2021-29430

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.

• CWE-20
• CWE-400
• CWE-770

• https://github.com/matrix-org/sydent/commit/0523511d2fb40f2738f8a8549868f44b96e5dab7Patch, Third Party Advisory
• https://github.com/matrix-org/sydent/commit/89071a1a754c69a50deac89e6bb74002d4cda19dPatch, Third Party Advisory
• https://github.com/matrix-org/sydent/commit/f56eee315b6c44fdd9f6aa785cc2ec744a594428Patch, Third Party Advisory
• https://github.com/matrix-org/sydent/releases/tag/v2.3.0Release Notes, Third Party Advisory
• https://github.com/matrix-org/sydent/security/advisories/GHSA-wmg4-8cp2-hpg9Patch, Third Party Advisory
• https://pypi.org/project/matrix-sydent/Product, Third Party Advisory
• https://github.com/matrix-org/sydent/commit/0523511d2fb40f2738f8a8549868f44b96e5dab7Patch, Third Party Advisory
• https://github.com/matrix-org/sydent/commit/89071a1a754c69a50deac89e6bb74002d4cda19dPatch, Third Party Advisory
• https://github.com/matrix-org/sydent/commit/f56eee315b6c44fdd9f6aa785cc2ec744a594428Patch, Third Party Advisory
• https://github.com/matrix-org/sydent/releases/tag/v2.3.0Release Notes, Third Party Advisory
• https://github.com/matrix-org/sydent/security/advisories/GHSA-wmg4-8cp2-hpg9Patch, Third Party Advisory
• https://pypi.org/project/matrix-sydent/Product, Third Party Advisory

Published

Apr 15, 2021

Last Modified

Jun 17, 2026

Status

Modified

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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