Home/CVE/AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET para
CVE

CVE-2020-26891

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET para

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.

MEDIUM · CVSS 6.1 EPSS 0.00439
Monitor
  • No active-exploitation, high-EPSS, or public-exploit signals - routine patching cadence
Sigma rules0 YARA rules0
⬡

Weakness Classification

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
▤

Affected Products & Versions

1
matrix synapse< 1.21.0
▤

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 1.21.0
▣

Scoring & Timeline

6.1
MEDIUM · CVSS v3.1 · cve@mitre.org
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 NVD19 Oct 2020 · 05:15 PM
CVSS VectorCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
⚑

Vendor Advisories

1
suse-csafopenSUSE-SU-2024:11041-1
🔗

References & Sources

4
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/pull/8444Issue TrackingPatchThird Party Advisory
https://github.com/matrix-org/synapse/releases/tag/v1.21.2Release NotesThird Party Advisory
https://github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmqPatchThird Party Advisory
https://matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisoryRelease NotesVendor Advisory
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