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Medium severity4.3NVD Advisory· Published Feb 26, 2021· Updated Jun 17, 2026

CVE-2021-21274

CVE-2021-21274

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.

AI Insight

LLM-synthesized narrative grounded in this CVE's description and references.

Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
matrix-synapsePyPI
>= 0.99.0, < 1.25.01.25.0

Affected products

4
  • ghsa-coords
    pkg:pypi/matrix-synapse
    Range: >= 0.99.0, < 1.25.0
  • Matrix Org/Synapse2 versions
    cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:*+ 1 more
    • cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:*range: >=0.99.0,<1.25.0
    • (no CPE)range: >=0.99.0, < 1.25.0
  • Fedoraproject/Fedora
    cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*

Patches

Vulnerability mechanics

References

9

News mentions

0

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