CVE-2026-25783: Mattermost fails to properly validate User-Agent header tokens

March 16, 2026 (updated March 17, 2026)

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to properly validate User-Agent header tokens which allows an authenticated attacker to cause a request panic via a specially crafted User-Agent header. Mattermost Advisory ID: MMSA-2026-00586

References

github.com/advisories/GHSA-2v3w-6g35-5f9v
github.com/mattermost/mattermost
github.com/mattermost/mattermost/commit/1346cf529aef0672c39a56ec10d1b8a9c8fb387d
mattermost.com/security-updates
nvd.nist.gov/vuln/detail/CVE-2026-25783

Code Behaviors & Features

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