Is CVE-2026-6339 in the CISA KEV catalog?

No. CVE-2026-6339 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2026-6339?

CVE-2026-6339 has a CVSS v3 base score of 4.3 (MEDIUM).

What is the EPSS score for CVE-2026-6339?

CVE-2026-6339 has an EPSS score of 0.11% (2th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2026-6339?

Upgrade mattermost server to 11.5.2 or later.

CVE & CISA-KEV Catalog

CVE-2026-6339

MEDIUM

4.3

CVSS v3

NVD

Description

Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636

CWE-346

How to fix

Remediation Available

mattermost serverNVD

Affected:>= 11.5.0, < 11.5.2Fixed in:11.5.2CVE-2026-6339derived from NVD

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 RequiredLow

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityNone

IntegrityNone

AvailabilityLow

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

Exploit Intelligence

0.11%probability of exploitation in 30 days

2ndpercentile

Low risk: more likely to be exploited than 2% of all known CVEs.

References

Vendor Advisory1

mattermost.com

Related Vulnerabilities

Other CWE-346 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2009-1185	High	7.2	82%	-	Fix
CVE-2025-34291	High	8.8	79%	KEV	-
CVE-2020-16952	High	8.6	71%	-	-
CVE-2023-29711	Critical	9.8	70%	-	-
CVE-2015-4495	High	8.8	70%	KEV	Fix
CVE-2024-23898	High	8.8	67%	-	Fix

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Markdown

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HTML

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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 2026-05-18.