PatchSiren cyber security CVE debrief

CVE-2026-9708 Mattermost CVE debrief

CVE-2026-9708 is a medium-severity vulnerability affecting Mattermost, a popular communication platform used by numerous organizations for team collaboration and communication. The issue arises from inadequate access control validation for incoming webhooks, allowing users with webhook management permissions to create posts or direct messages attributed to another user via crafted configurations and payloads. This type of vulnerability can lead to misinformation, impersonation, and potential security breaches if not addressed. Administrators and security teams should be aware of the affected versions and take necessary actions to mitigate the risk. The vulnerability has a CVSS score of 4.9 and is classified as MEDIUM severity.

Vendor: Mattermost
Product: Unknown
CVSS: MEDIUM 4.9
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. Additionally, security teams and IT professionals responsible for maintaining and securing communication platforms should take note of this issue. They should review the vulnerability details, assess the risk to their organization, and take necessary actions to mitigate the vulnerability.

Technical summary

The vulnerability exists due to insufficient validation of assigned incoming webhook users' access to target teams or channels in Mattermost. This oversight enables a requester with webhook management permissions to create posts or direct messages that appear to be from another user by manipulating webhook configurations and payloads. The affected versions are 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. To exploit this vulnerability, an attacker would need to have webhook management permissions and craft specific configurations and payloads to impersonate other users.

Defensive priority

Medium-High

Recommended defensive actions

Update Mattermost to a version that addresses this vulnerability
Review and restrict webhook management permissions
Monitor for suspicious webhook activities
Implement additional access controls for webhook configurations
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 2026-07-13T09:16:25.253Z and has not been modified since then. The NVD entry is currently in the 'Received' status. The vulnerability has a CVSS score of 4.9 and is classified as MEDIUM severity. Evidence is limited to CVE and NVD data. Defenders should verify affected versions (Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, and 10.11.x <= 10.11.19), review webhook configurations, and monitor for suspicious activities. Additional verification tasks include checking system logs for unauthorized webhook activities and ensuring that webhook management permissions are properly restricted.

Official resources

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-07-13T09:16:25.253Z and has not been modified since then. The NVD entry is currently in the 'Received' status.