What’s Vulnerable › Vulnerabilities

CVE-2021-21274: Denial of service attack via .well-known lookups

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-02-26 · updated 2026-06-17
• Weakness: CWE-400

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.

Affected products

• Fedoraproject Fedora: 34
• matrix-org synapse: from 0.99.0 before 1.25.0 — fixed in 1.25.0

How to fix

Upgrade to a fixed version: matrix-org synapse 1.25.0. Check the vendor advisory for mitigations.

References

• https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6
• https://github.com/matrix-org/synapse/pull/8950
• https://github.com/matrix-org/synapse/releases/tag/v1.25.0
• https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
• https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6
• https://github.com/matrix-org/synapse/pull/8950
• https://github.com/matrix-org/synapse/releases/tag/v1.25.0
• https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

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