CVE-2021-21274

Name: CVE-2021-21274
Creator: NVD / NIST
Published: 2021-02-26T18:15:12.237+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 2.16%

Last modified Jun 17, 2026

CVE-2021-21274 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. EPSS estimates a 2.16% chance of exploitation in the next 30 days.

Description

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, a malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver. This affects any server which accepts federation requests from untrusted servers. Issue is resolved in version 1.25.0. As a workaround the `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.

Metrics

CVSS 3.1

6.5/10

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

EPSS Probability

2.16%

79.9th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Matrix	Synapse	>= 0.99.0, < 1.25.0
Fedoraproject	Fedora	34

References

https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6Patch, Third Party Advisory
https://github.com/matrix-org/synapse/pull/8950Patch, Third Party Advisory
https://github.com/matrix-org/synapse/releases/tag/v1.25.0Third Party Advisory
https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8Patch, Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6Patch, Third Party Advisory
https://github.com/matrix-org/synapse/pull/8950Patch, Third Party Advisory
https://github.com/matrix-org/synapse/releases/tag/v1.25.0Third Party Advisory
https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8Patch, Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

Timeline

Published: Feb 26, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-21274?

How severe is CVE-2021-21274?

CVE-2021-21274 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 2.16% probability of exploitation in the next 30 days.

How do I fix CVE-2021-21274?

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-2021-21274?

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

Scan your code now

Source: NVD / NIST