Home/CVE/Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for ope
CVE

CVE-2021-21274

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for ope

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.

MEDIUM · CVSS 4.3 EPSS 0.00446
Monitor
  • No active-exploitation, high-EPSS, or public-exploit signals - routine patching cadence
Sigma rules0 YARA rules0
⬡

Weakness Classification

CWE-400Uncontrolled Resource Consumption
CWE-770Allocation of Resources Without Limits or Throttling
▤

Affected Products & Versions

2
matrix synapse>= 0.99.0 and < 1.25.0
fedoraproject fedoraall versions
▤

Affected Packages

1
Language-ecosystem packages (from OSV) tied to this CVE, with the version that fixes it - the dependency-level detail NVD doesn’t carry.
PyPI matrix-synapse MODERATE fixed in 1.25.0
▣

Scoring & Timeline

4.3
MEDIUM · CVSS v3.1 · security-advisories@github.com
View on NVD
Attack Vector
Network Adjacent Local Physical
Attack Complexity
Low High
Privileges Required
None Low High
User Interaction
None Required
Scope
Unchanged Changed
Confidentiality
None Low High
Integrity
None Low High
Availability
None Low High
Published to NVD26 Feb 2021 · 06:15 PM
CVSS VectorCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
🔗

References & Sources

5
Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6PatchThird Party Advisory
https://github.com/matrix-org/synapse/pull/8950PatchThird 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-v3g8PatchThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
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