CVE-2026-4915: Mattermost doesn't filter nil elements from outgoing webhook attachment payloads before processing

May 26, 2026 (updated June 30, 2026)

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

References

github.com/advisories/GHSA-5gmf-x7hg-97wf
github.com/mattermost/mattermost/commit/faa7d75b4ea041701e97948f8aa1332e3626a39a
mattermost.com/security-updates
nvd.nist.gov/vuln/detail/CVE-2026-4915

Code Behaviors & Features

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