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No exploitation reportedFix available

CVE-2021-21273

No exploitation reported by the sources we read.

  • Published2021-02-26
  • WeaknessCWE-601

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, requests to user provided domains were not restricted to external IP addresses when calculating the key validity for third-party invite events and sending push notifications. This could cause Synapse to make requests to internal infrastructure. The type of request was not controlled by the user, although limited modification of request bodies was possible. For the most thorough protection server administrators should remove the deprecated `federation_ip_range_blacklist` from their settings after upgrading to Synapse v1.25.0 which will result in Synapse using the improved default IP address restrictions. See the new `ip_range_blacklist` and `ip_range_whitelist` settings if more specific control is necessary.

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< 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 3.13.1known since 2026-10-23
  • NVDCVSS 3.16.1read 2026-10-23
  • NVDCVSS 2.05.8read 2026-10-23
  • CNAweakness (CWE)CWE-601known since 2026-10-23
  • NVDweakness (CWE)CWE-601read 2026-10-23
  • FIRST EPSSEPSS score1.8 %2026-11-09
  • OSVaffected packages—
  • GitHub AdvisoriesadvisoryMODERATE2021-02-26

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Published
2021-02-26
Last Modified
2026-06-17
NVD status
Modified
GitHub Advisory
GHSA-v936-j8gp-9q3p

Frequently asked questions

What is CVE-2021-21273?

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, requests to user provided domains were not restricted to external IP addresses when calculating the key validity for third-party invite events and sending push notifications. This could cause Synapse to make requests to internal infrastructure. The type of request was not controlled by the user, although limited modification of request bodies was possible. For the most thorough protection server administrators should remove the deprecated `federation_ip_range_blacklist` from their settings after upgrading to Synapse v1.25.0 which will result in Synapse using the improved default IP address restrictions. See the new `ip_range_blacklist` and `ip_range_whitelist` settings if more specific control is necessary.

Is CVE-2021-21273 actively exploited?

Active exploitation of CVE-2021-21273 has not been confirmed. Its EPSS score was 1.8% on 2026-11-09, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-21273?

CVE-2021-21273 has a CVSS v3 base score of 6.1 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

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