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CVE-2026-6339

Missing request origin validation on burn-on-read reveal endpoint

MEDIUM
4.3
CVSS Score
Published: May 18, 2026 at 08:05 AM UTC
Updated: October 14, 2026 at 08:10 PM UTC
2 products affected across 1 vendorMattermost ServerMattermost

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Severity
MEDIUM4.3
Exploit prediction
EPSS 0.1%
SSVC
No known exploit

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

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Mattermost Server· Mattermost
95%
11.4.0 ≤ v < 11.4.411.5.0 ≤ v < 11.5.2
Mattermost· Mattermost
90%
11.4.0 ≤ v ≤ 11.4.311.5.0 ≤ v ≤ 11.5.111.4.011.5.0
Vendor states this is fixed in 11.4.4 (unaffected)
Vendor states this is fixed in 11.5.2 (unaffected)
Vendor states this is fixed in 11.6.0 (unaffected)

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

References (1)

Vendor Advisory
Source: cverecord
https://mattermost.com/security-updates

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

4.3MEDIUMMattermost
Attack Vector
NETWORK
Complexity
LOW
Privileges
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
NONE
Integrity
NONE
Availability
LOW
Vector?

Quick Info

StatusANALYZED
CWE
CWE-346

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