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CVE-2026-5740 Mattermost CVE debrief

CVE-2026-5740 is a high-severity vulnerability affecting Mattermost Server versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, and 10.11.x <= 10.11.14. The vulnerability allows an unauthenticated remote attacker to crash the server process by sending a crafted binary WebSocket message to the public WebSocket endpoint, resulting in a full service outage for all users. The vulnerability is caused by the failure of Mattermost Server to properly validate msgpack-encoded WebSocket frames before memory allocation. Administrators and users of Mattermost Server versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, and 10.11.x <= 10.11.14 should be aware of this vulnerability and take immediate action to mitigate the risk.

Vendor
Mattermost
Product
Mattermost Server
CVSS
HIGH 7.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-22
Original CVE updated
2026-07-23
Advisory published
2026-05-22
Advisory updated
2026-07-23

Who should care

Administrators and users of Mattermost Server versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, and 10.11.x <= 10.11.14 should be aware of this vulnerability and take immediate action to mitigate the risk. This includes reviewing and applying patches or updates provided by Mattermost, restricting access to the public WebSocket endpoint, and monitoring server logs for suspicious activity.

Technical summary

The vulnerability is caused by the failure of Mattermost Server to properly validate msgpack-encoded WebSocket frames before memory allocation. This allows an unauthenticated remote attacker to crash the server process by sending a crafted binary WebSocket message to the public WebSocket endpoint, resulting in a full service outage for all users. The vulnerability affects Mattermost Server versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, and 10.11.x <= 10.11.14.

Defensive priority

High

Recommended defensive actions

  • Apply the patches or updates provided by Mattermost to vulnerable versions
  • Restrict access to the public WebSocket endpoint
  • Monitor server logs for suspicious activity
  • Implement additional security measures to prevent similar attacks
  • 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

Evidence notes

The CVE record was published on 2026-05-22T11:16:23.163Z and last modified on 2026-07-23T16:10:00.137Z. The NVD entry is currently Analyzed. The vulnerability affects Mattermost Server versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, and 10.11.x <= 10.11.14. The vulnerability allows an unauthenticated remote attacker to crash the server process by sending a crafted binary WebSocket message to the public WebSocket endpoint. The vulnerability is caused by the failure of Mattermost Server to properly validate msgpack-encoded WebSocket frames before memory allocation.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-5740 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-5740

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

  • CVE-2026-5740 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-5740

    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.