What is CVE-2021-21274?

CVE-2021-21274 is a medium-severity vulnerability in Matrix Synapse, classified under Uncontrolled Resource Consumption. CVSS score: 4.3/10. Published 2021-02-26.

How severe is CVE-2021-21274?

Medium severity. CVSS v3 base score is 4.3 out of 10.

Resource exhaustion in Matrix Synapse

CVE-2021-21274

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.

Vulnerability class: DoS (Denial of Service)

Published 2021-02-26 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L.

EPSS exploit prediction

EPSS: 0.022 (81.5th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 82nd percentile — 81.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.022 · newest: 0.022 · change: 0.000

Affected products

Matrix Synapse
Matrix-Org Synapse — versions >=0.99.0, < 1.25.0
Fedoraproject Fedora — versions 34

Weakness classification (CWE)

References

github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8 (Patch, Third Party Advisory)
github.com/matrix-org/synapse/releases/tag/v1.25.0 (Third Party Advisory)
github.com/matrix-org/synapse/pull/8950 (Patch, Third Party Advisory)
github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6 (Patch, Third Party Advisory)
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/… (vendor-advisory)

Frequently asked questions

What is CVE-2021-21274?: CVE-2021-21274 is a medium-severity vulnerability in Matrix Synapse, classified under Uncontrolled Resource Consumption. CVSS score: 4.3/10. Published 2021-02-26.
How severe is CVE-2021-21274?: Medium severity. CVSS v3 base score is 4.3 out of 10.