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No exploitation reportedFix available

CVE-2021-21274

No exploitation reported by the sources we read.

  • Published2021-02-26
  • WeaknessCWE-400 · CWE-770

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC
  1. 2021-02-2618:15CVE publishedNVD

Versions per branch

NVD · OSV
Fedoraproject fedora
BranchAffectedFixed in
34.x= 34no fix published
Matrix synapse
BranchAffectedFixed in
1.25≥ 0.99.0, < 1.25.01.25.0

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • matrix-synapseUbuntu:Pro:20.04:LTSall versions · no fixed version listed
  • matrix-synapseUbuntu:Pro:18.04:LTSall versions · no fixed version listed

Full record on NVD

Fix

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof of concept
Metasploit modules
none listed
Metasploit Framework

Sources

  • CNACVSS 3.14.3known since 2026-10-23
  • NVDCVSS 3.16.5read 2026-10-23
  • NVDCVSS 2.04.3read 2026-10-23
  • CNAweakness (CWE)CWE-400known since 2026-10-23
  • NVDweakness (CWE)CWE-770read 2026-10-23
  • FIRST EPSSEPSS score2.2 %2026-11-09
  • OSVaffected packages—
  • GitHub AdvisoriesadvisoryMODERATE2021-03-01

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Published
2021-02-26
Last Modified
2026-06-17
NVD status
Modified
GitHub Advisory
GHSA-2hwx-mjrm-v3g8

Frequently asked questions

What is 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.

Is CVE-2021-21274 actively exploited?

Active exploitation of CVE-2021-21274 has not been confirmed. Its EPSS score was 2.2% on 2026-11-09, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-21274?

CVE-2021-21274 has a CVSS v3 base score of 6.5 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H.

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