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CVE-2026-10103 Mattermost CVE debrief

CVE-2026-10103 is a medium-severity vulnerability affecting Mattermost, a popular communication platform. The issue arises from a failure to verify post ownership in the shared channel inbound sync handler. This weakness allows an authenticated remote cluster to modify or delete posts authored by local users or other remotes via crafted sync messages referencing arbitrary post IDs in channels shared with that remote.

Vendor
Mattermost
Product
Unknown
CVSS
MEDIUM 4.3
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

Administrators and users of Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, and 10.11.x <= 10.11.19 should be aware of this vulnerability. This issue is particularly concerning for organizations that use Mattermost for internal communication, as it could allow an attacker to tamper with sensitive information.

Technical summary

The vulnerability exists in the shared channel inbound sync handler of Mattermost. Specifically, the application fails to properly verify the ownership of posts during the synchronization process. This oversight enables an authenticated remote cluster to send crafted sync messages that reference arbitrary post IDs in shared channels. Consequently, an attacker could modify or delete posts authored by local users or other remote clusters. The affected versions are Mattermost 11.7.x up to 11.7.2, 11.6.x up to 11.6.4, and 10.11.x up to 10.11.19.

Defensive priority

Medium

Recommended defensive actions

  • Apply the patches or updates provided by Mattermost to address the vulnerability.
  • Restrict access to sensitive channels and ensure that only trusted remote clusters can synchronize with the Mattermost instance.
  • Monitor Mattermost instance logs for suspicious activity related to post modifications or deletions.
  • Implement additional security measures, such as two-factor authentication and strict access controls, to reduce the attack surface.
  • 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 2026-07-13T09:16:23.653Z and has not been modified since then. The NVD entry is currently in the 'Received' status. The vulnerability has a CVSS score of 4.3 and a severity rating of MEDIUM. Mattermost has provided an advisory (MMSA-2026-00689) detailing the issue and recommended actions.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-10103 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-10103

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

  • CVE-2026-10103 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-10103

    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.