PatchSiren cyber security CVE debrief

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.

Official resources

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-07-13T09:16:23.653Z and has not been modified since then. The NVD entry is currently Received.