Is CVE-2020-26891 in the CISA KEV catalog?

No. CVE-2020-26891 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2020-26891?

CVE-2020-26891 has a CVSS v3 base score of 6.1 (MEDIUM).

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

CVE-2020-26891 has an EPSS score of 1.91% (77th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2020-26891?

Upgrade matrix-synapse on debian unstable to 1.21.1-1 or later.

CVE & CISA-KEV Catalog

CVE-2020-26891

MEDIUMEPSS 77th pctl

6.1

CVSS v3

NVD

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.

CWE-79Cross-Site Scripting (XSS)

How to fix

Remediation Available

matrix-synapseDebian

Fixed in:1.21.1-1CVE-2020-26891

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionRequired

ScopeChanged

Impact

ConfidentialityLow

IntegrityLow

AvailabilityNone

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Exploit Intelligence

1.91%probability of exploitation in 30 days

77thpercentile

Elevated risk: more likely to be exploited than 77% of all known CVEs.

References

Patch2

Vendor Advisory1

matrix.org

Third-Party Advisory1

github.com

Related Vulnerabilities

Other CWE-79 (Cross-Site Scripting (XSS)) vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2020-11022	Medium	6.9	99%	-	Fix
CVE-2019-3929	Critical	9.8	99%	KEV	Fix
CVE-2020-9496	Medium	6.1	99%	-	-
CVE-2023-28341	Medium	6.1	99%	-	Fix
CVE-2018-12998	Medium	6.1	98%	-	-
CVE-2025-4123	High	7.6	98%	-	Fix

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2024-11-21.