PatchSiren

PatchSiren cyber security CVE debrief

CVE-2026-6850 Mattermost CVE debrief

CVE-2026-6850 is a denial of service vulnerability in Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19. An authenticated attacker can cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser. This issue affects users of Mattermost who have not applied patches. The vulnerability has a CVSS score of 6.5 and is classified as MEDIUM. Users should apply patches to prevent denial of service attacks.

Vendor
Mattermost
Product
Unknown
CVSS
MEDIUM 6.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-13
Original CVE updated
2026-07-13
Advisory published
2026-07-13
Advisory updated
2026-07-13

Who should care

Users of Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 should apply patches to prevent denial of service attacks. This includes operators, administrators, and security teams managing Mattermost deployments. They should verify their Mattermost versions and apply necessary patches.

Technical summary

The vulnerability exists due to insufficient validation of message attachment field values in Mattermost. An authenticated attacker can craft a malicious payload to trigger catastrophic backtracking in the client-side markdown parser, causing a denial of service for all users in a channel. This issue requires patches to prevent exploitation. Affected users should verify their Mattermost versions and apply necessary patches.

Defensive priority

Medium

Recommended defensive actions

  • Apply patches for Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19
  • Restrict access to untrusted users
  • Monitor for suspicious activity
  • Review compensating controls for exposed systems while remediation is scheduled and verified
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-07-13T09:16:24.890Z and has not been modified since then. The NVD entry is currently 6.5 MEDIUM. Evidence is limited to CVE and NVD details. Defenders should verify affected versions and patch status with Mattermost. Limited source detail suggests validating Mattermost version and patch status.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-6850 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-6850

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-6850 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-6850

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.