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No exploitation reportedFix available

CVE-2020-26891

No exploitation reported by the sources we read.

  • Published2020-10-19
  • Fixed in1.21.0
  • WeaknessCWE-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

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth/*/fallback/web or /_matrix/client/unstable/auth/*/fallback/web Synapse endpoints.

Timeline

dates in UTC
  1. 2020-10-1917:15CVE publishedNVD

Versions per branch

NVD · OSV
Matrix synapse
BranchAffectedFixed in
1.21< 1.21.01.21.0

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: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 1.21.1-r0
  • synapseAlpine:v3.24all versions · fixed in 1.21.1-r0
  • synapseAlpine:v3.23all versions · fixed in 1.21.1-r0
  • synapseAlpine:v3.22all versions · fixed in 1.21.1-r0

9 more package ranges on OSV.dev

Full record on NVD

Fix

  1. Update Matrix synapse to 1.21.0

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

  • NVDCVSS 3.16.1read 2026-10-23
  • NVDCVSS 2.04.3read 2026-10-23
  • NVDweakness (CWE)CWE-79read 2026-10-23
  • FIRST EPSSEPSS score1.9 %2026-11-09
  • OSVaffected packages—
  • GitHub AdvisoriesadvisoryMODERATE2020-10-16

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
2020-10-19
Last Modified
2026-06-17
NVD status
Modified
GitHub Advisory
GHSA-3x8c-fmpc-5rmq

Frequently asked questions

What is CVE-2020-26891?

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth/*/fallback/web or /_matrix/client/unstable/auth/*/fallback/web Synapse endpoints.

Is CVE-2020-26891 actively exploited?

Active exploitation of CVE-2020-26891 has not been confirmed. Its EPSS score was 1.9% on 2026-11-09, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2020-26891?

CVE-2020-26891 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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