CVE-2020-26891: matrix synapse Cross-site Scripting
Vulnerability summary
CVE-2020-26891 is a medium (CVSS 6.1) Cross-site Scripting vulnerability in matrix synapse. EPSS estimates a 1.92% chance of exploitation in the next 30 days, higher than 79.6% of scored vulnerabilities.
Advisory lineage
Upstream 0 · Downstream 1 · Related 1Timeline
- 19-10-2020PublishedVulnerability first disclosed.
- 04-08-2024Last modifiedVulnerability information updated.
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.
CVSS metrics
6 scores · 4 sources · 3 versions- OSV · PypiEcosystem advisory · calculated6.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- OSV · UbuntuEcosystem advisory · calculated6.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- OSV · DebianEcosystem advisory · calculated6.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
EPSS trend
Probability
1.92%
Chance of exploitation in the next 30 days
Percentile
80%
Versus all other CVEs
30-day change
-0.01 pp
From 1.93% on 19-09-2026
Known exploited
Compare KEV catalogsNot in any of the 3 tracked catalogs.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeAffected systems
Debian1 product
- matrix-synapse< 1.21.1-1
Matrix1 product
- synapse< 1.21.0
PyPI1 product
- matrix-synapse< 1.21.0
Ubuntu1 product
- matrix-synapseAll versions (No fix recorded)
4 vendors · 4 products
References
11 links- github.com/matrix-org/synapse/pull/8444X Refsource MISCWEBREPORTIssue TrackingPatchThird Party Advisory
- matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisoryX Refsource MISCWEBAdvisoryREPORTRelease NotesVendor Advisory
- github.com/matrix-org/synapse/releases/tag/v1.21.2X Refsource MISCWEBRelease NotesThird Party Advisory
- github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmqX Refsource CONFIRMWEBAdvisoryPatchThird Party Advisory
