CVE-2026-4915 Server panic via outgoing webhook responses
An authenticated user can crash the Mattermost server by sending a crafted webhook response with null attachment entries.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-4915?
CVE-2026-4915 is a medium-severity denial of service vulnerability in Mattermost Mattermost Server, published on 25 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to filter nil elements from outgoing webhook attachment payloads before processing, which allows an authenticated user to cause a denial of service (server process termination) via a crafted webhook callback response containing a null attachment entry.. Mattermost Advisory ID: MMSA-2026-00641
Weakness: CWE-754
Which versions are affected by CVE-2026-4915?
14 open source packages ship an affected version, including github.com/mattermost/mattermost-server, github.com/mattermost/mattermost-server/v5 and github.com/mattermost/mattermost-server/v6. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/mattermost/mattermost-server Go | From 10.11.0 | 10.11.15 |
github.com/mattermost/mattermost-server Go | From 10.11.0 | 10.11.15 |
github.com/mattermost/mattermost-server Go | From 10.11.0+incompatible | 10.11.15+incompatible, 11.4.5+incompatible, 11.5.4+incompatible, 11.6.1+incompatible |
github.com/mattermost/mattermost-server Go | From 11.4.0 | 11.4.5 |
github.com/mattermost/mattermost-server Go | From 11.4.0 | 11.4.5 |
github.com/mattermost/mattermost-server Go | From 11.5.0 | 11.5.4 |
8 more packages
github.com/mattermost/mattermost-server Go | From 11.5.0 | 11.5.4 |
github.com/mattermost/mattermost-server Go | From 11.6.0 | 11.6.1 |
github.com/mattermost/mattermost-server Go | From 11.6.0 | 11.6.1 |
github.com/mattermost/mattermost-server/v5 Go | All versions | None |
github.com/mattermost/mattermost-server/v6 Go | All versions | None |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260407102538-faa7d75b4ea0 |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260407102538-faa7d75b4ea0 |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260407102538-faa7d75b4ea0 |
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-4915?
Upgrade github.com/mattermost/mattermost-server to a fixed version for your release line: 10.11.15, 10.11.15+incompatible, 11.4.5+incompatible or 11.5.4+incompatible.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-4915 being exploited?
There is no known exploitation of CVE-2026-4915 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-5gmf-x7hg-97wf, GO-2026-5845.