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No exploitation reportedFix available

CVE-2020-26257

No exploitation reported by the sources we read.

  • Published2020-12-09
  • WeaknessCWE-400 · CWE-79

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

Matrix is an ecosystem for open federated Instant Messaging and VoIP. Synapse is a reference "homeserver" implementation of Matrix. A malicious or poorly-implemented homeserver can inject malformed events into a room by specifying a different room id in the path of a `/send_join`, `/send_leave`, `/invite` or `/exchange_third_party_invite` request. This can lead to a denial of service in which future events will not be correctly sent to other servers over federation. This affects any server which accepts federation requests from untrusted servers. The Matrix Synapse reference implementation before version 1.23.1 the implementation is vulnerable to this injection attack. Issue is fixed in version 1.23.1. As a workaround homeserver administrators could limit access to the federation API to trusted servers (for example via `federation_domain_whitelist`).

Timeline

dates in UTC
  1. 2020-12-0919:15CVE publishedNVD

Versions per branch

NVD · OSV
Fedoraproject fedora
BranchAffectedFixed in
32.x= 32no fix published
33.x= 33no fix published
Matrix synapse
BranchAffectedFixed in
1.23< 1.23.11.23.1

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:22.04:LTSall versions · no fixed version listed
  • matrix-synapseUbuntu:Pro:20.04:LTSall versions · no fixed version listed
  • matrix-synapseUbuntu:Pro:18.04:LTSall versions · no fixed version listed
  • matrix-synapseopenSUSE:Tumbleweedall versions · fixed in 1.43.0-1.1
  • synapseCleanStartall versions · fixed in 1.24.0-r0
  • synapseAlpine:v3.24all versions · fixed in 1.24.0-r0
  • synapseAlpine:v3.23all versions · fixed in 1.24.0-r0
  • synapseAlpine:v3.22all versions · fixed in 1.24.0-r0

9 more package ranges on OSV.dev

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 3.16.5known since 2026-10-23
  • NVDCVSS 3.16.5read 2026-10-23
  • NVDCVSS 2.04read 2026-10-23
  • CNAweakness (CWE)CWE-400, CWE-79known since 2026-10-23
  • NVDweakness (CWE)CWE-400read 2026-10-23
  • FIRST EPSSEPSS score2.4 %2026-11-08
  • OSVaffected packages—
  • GitHub AdvisoriesadvisoryHIGH2020-12-09

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Published
2020-12-09
Last Modified
2026-06-17
NVD status
Modified
GitHub Advisory
GHSA-hxmp-pqch-c8mm

Frequently asked questions

What is CVE-2020-26257?

Matrix is an ecosystem for open federated Instant Messaging and VoIP. Synapse is a reference "homeserver" implementation of Matrix. A malicious or poorly-implemented homeserver can inject malformed events into a room by specifying a different room id in the path of a `/send_join`, `/send_leave`, `/invite` or `/exchange_third_party_invite` request. This can lead to a denial of service in which future events will not be correctly sent to other servers over federation. This affects any server which accepts federation requests from untrusted servers. The Matrix Synapse reference implementation before version 1.23.1 the implementation is vulnerable to this injection attack. Issue is fixed in version 1.23.1. As a workaround homeserver administrators could limit access to the federation API to trusted servers (for example via `federation_domain_whitelist`).

Is CVE-2020-26257 actively exploited?

Active exploitation of CVE-2020-26257 has not been confirmed. Its EPSS score was 2.4% on 2026-11-08, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2020-26257?

CVE-2020-26257 has a CVSS v3 base score of 6.5 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.

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