CVE-2026-9708 Incoming webhook user attribution via unvalidated webhook owner
Attackers with webhook permissions can impersonate other users to post messages or send direct messages due to missing team/channel access validation.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-9708?
CVE-2026-9708 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 13 Jul 2026. An attacker needs administrator access to exploit it, with no action from the victim.
Official description
Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate that an assigned incoming webhook user has access to the target team or channel, which allows a requester with webhook management permissions to create posts or direct messages attributed to another user via crafted incoming webhook configuration and payloads.. Mattermost Advisory ID: MMSA-2026-00683
Weakness: CWE-639
Which versions are affected by CVE-2026-9708?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-9708?
Upgrade to a fixed version.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-9708 being exploited?
There is no known exploitation of CVE-2026-9708 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-8jmp-2xx6-pjg7, MINI-3hrr-w53p-c4m2, MINI-6vxq-99q8-6c8v.