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🛡️ CVE-2026-6339 — mattermost-server

🟡 CVSS 4.3 — Medium ✅ No Known Exploit CWE-346 NVD
4.3
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

Mattermost doesn't validate the X-Requested-With header on the burn-on-read reveal endpoint in github.com/mattermost/mattermost-server

Mattermost doesn't validate the X-Requested-With header on the burn-on-read reveal endpoint in github.com/mattermost/mattermost-server.

NOTE: The source advisory for this report contains additional versions that could not be automatically mapped to standard Go module versions.

(If this is causing false-positive reports from vulnerability scanners, please suggest an edit to the report.)

The additional affected modules and versions are: github.com/mattermost/mattermost-server/v5 before v5.3.2-0.20260327001745-7a339a6438f5.

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs low-level privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality none, integrity none, availability low.

CVSS metrics in full

The score comes from this vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Privileges required: Low — an ordinary user account is enough.
  • User interaction: None — nobody has to be tricked into anything.
  • Scope: Unchanged — the damage stays inside the vulnerable component.
  • Confidentiality impact: None.
  • Integrity impact: None.
  • Availability impact: Low — limited, and the attacker does not choose what is affected.

Weakness class

CVE-2026-6339 is classified as CWE-346: Origin Validation Error. The origin of a message or request is not verified properly, so one source can pose as another.

Affected software

CVE-2026-6339 is recorded against 5 packages.

  • github.com/mattermost/mattermost-server
  • github.com/mattermost/mattermost-server/v5
  • github.com/mattermost/mattermost-server/v6
  • github.com/mattermost/mattermost/server/v8
  • mattermost-server (from 11.5.0 up to 11.5.2)

Timeline and source

Published on 18 May 2026 and last revised on 25 June 2026. No public exploit is currently recorded for this entry. Record sourced from NVD.

References

nvd.nist.gov (Advisory)
github.com (Web)
github.com (Package)
mattermost.com (Web)

Other advisories for this package

github.com/mattermost/mattermost-server has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-346: Origin Validation Error) in other software:

Details

Severity Medium
CVSS Score 4.3
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
CWE CWE-346
Public Exploit ✅ No
Source NVD
Published 2026-05-18
Updated 2026-08-20
Modified 2026-06-25
Fix URL N/A

Affected Packages

Software From version Fixed in
github.com/mattermost/mattermost-server
github.com/mattermost/mattermost-server/v5
github.com/mattermost/mattermost-server/v6
github.com/mattermost/mattermost/server/v8
mattermost-server 11.5.0 11.5.2

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