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CVE-2026-6961 Mattermost CVE debrief

CVE-2026-6961 is a HIGH severity vulnerability with a CVSS score of 7.6. Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, and 10.11.x <= 10.11.16 are affected. The vulnerability is caused by Mattermost's failure to sanitize FileInfo.Name received from federated peers during shared channel file sync. This allows an attacker controlling a federated server to write files to arbitrary locations within the target server's filestore via path traversal sequences in the filename field. The Mattermost Advisory ID for this issue is MMSA-2026-00661.

Vendor
Mattermost
Product
Unknown
CVSS
HIGH 7.6
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-12
Original CVE updated
2026-06-18
Advisory published
2026-06-12
Advisory updated
2026-06-18

Who should care

Users of Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, and 10.11.x <= 10.11.16 should apply patches or mitigations.

Technical summary

The vulnerability is caused by inadequate sanitization of FileInfo.Name during shared channel file sync. An attacker can exploit this by sending malicious filenames with path traversal sequences, allowing them to write files to arbitrary locations within the target server's filestore.

Defensive priority

HIGH

Recommended defensive actions

  • Apply patches or updates provided by Mattermost to address the vulnerability.
  • Review and update Mattermost configurations to ensure secure file handling and sanitization.
  • Monitor Mattermost instances for suspicious activity related to file uploads and sync operations.

Evidence notes

The CVE-2026-6961 record was obtained from the official CVE.org database and the NVD detail page.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-6961 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-6961

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

  • CVE-2026-6961 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-6961

    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.