CVE-2021-29430

Name: CVE-2021-29430
Creator: NVD / NIST
Published: 2021-04-15T21:15:17.413+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 7.5/10EPSS 1.83%

Last modified Jun 17, 2026

CVE-2021-29430 is a high-severity vulnerability rated 7.5/10 on the CVSS scale. Sydent is a reference Matrix identity server. Sydent does not limit the size of requests it receives from HTTP clients. EPSS estimates a 1.83% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

1.83%

76.2th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Matrix	Sydent	< 2.3.0

References

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

Timeline

Published: Apr 15, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-29430?

How severe is CVE-2021-29430?

CVE-2021-29430 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 1.83% probability of exploitation in the next 30 days.

How do I fix CVE-2021-29430?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-29430?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST