HOMEVULNERABILITIESCVE-2021-21274
MEDIUM

CVE-2021-21274

CWE-400Published: February 26, 2021· Updated: Jun 17, 2026

6.5
CVSS v3.1

Official 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.

NVD Source

Technical Analysis

CVE-2021-21274 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

Exploitation does not require any privileges, though user interaction (Required) is needed, which slightly reduces the risk of mass automated attacks.

A successful exploit results in availability disruption (denial of service), with a CVSS base score of 6.5.

CVSS v3.1 Vector Breakdown

Exploitability
Attack VectorNetwork
Attack ComplexityLow
Privileges Req.None
User InteractionRequired
ScopeUnchanged
Impact
ConfidentialityNone
IntegrityNone
AvailabilityHigh
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

Affected Vendors & Products

Fedora1 product(s)
fedora
matrix1 product(s)
synapse
Source: NVD CPE · 2 total CPE entries

Exploit & PoC Resources

NO KNOWN EXPLOITNo public exploit confirmed at this time
Exploit-DB
Offensive Security exploit database
GitHub PoC Search
Community PoC repositories
Rapid7 / Metasploit
Metasploit module search
PacketStorm
Security advisories & exploits
NVD Detail
Official NIST entry
MITRE CVE
CVE Program entry
External links open in a new tab. Always verify in a controlled environment before use.

Official Patches & Advisories

PATCHhttps://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6
PATCHhttps://github.com/matrix-org/synapse/pull/8950
PATCHhttps://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8
PATCHhttps://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6
PATCHhttps://github.com/matrix-org/synapse/pull/8950
PATCHhttps://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8
ADVISORYhttps://github.com/matrix-org/synapse/releases/tag/v1.25.0
ADVISORYhttps://github.com/matrix-org/synapse/releases/tag/v1.25.0

All References (10)

→
https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6Patch · Third Party Advisory
→
https://github.com/matrix-org/synapse/pull/8950Patch · Third 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-v3g8Patch · Third Party Advisory
→
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
→
https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6Patch · Third Party Advisory
→
https://github.com/matrix-org/synapse/pull/8950Patch · Third 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-v3g8Patch · Third Party Advisory
→
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

Quick Facts

CVE IDCVE-2021-21274
CVSS Score6.5 / 10
SeverityMEDIUM
WeaknessCWE-400
CISA KEVNo
Affected2 vendor(s)
PublishedFeb 26, 2021

Related CVEs (CWE-400)

CVE-2024-39462
9.8
CVE-2023-30769
9.8
CVE-2022-48716
9.8
CVE-2024-36543
9.8
CVE-2026-30141
9.8
CVE-2018-11936
9.8

Recommended Actions

  • →Apply vendor patches immediately
  • →Monitor CVE-2021-21274 in threat intel feeds
  • →Review IDS/IPS signatures for exploitation attempts
Data sourced from NVD (NIST), CISA KEV, and EPSS (FIRST). Analysis generated by CTIWatch. CVE data is provided under the NVD usage policy.
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