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Advisory record · GHSA-vqp5-2mrp-qqxg

Mattermost doesn't validate the Host header when constructing response URLs for custom slash command

CVSS 3.5 LowEPSS 0.0014 · 3rd percentile

Published by the GitHub Advisory Database at 18 May 2026, 09:31 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

PackageEcosystemAffectedFixed in
github.com/mattermost/mattermost/server/v8Go>=11.5.0, <11.5.2 || >=10.11.0, <10.11.14 || <8.0.0-20260325160634-e738016c592011.5.2, 10.11.14, 8.0.0-20260325160634-e738016c5920
github.com/mattermost/mattermost-serverGo<5.3.2-0.20260325160634-e738016c59205.3.2-0.20260325160634-e738016c5920
Identifiers
GHSA-vqp5-2mrp-qqxg · CVE-2026-6333 · GO-2026-5671
Weakness (CWE)
CWE-918
Tags
Published
18 May 2026
Updated
25 Jun 2026

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

Mattermost server versions before 10.11.14 (in the 10.11 line), 11.5.0 through 11.5.1 and 8.x and related Go module versions are affected, because custom slash command response URLs are built from an unvalidated Host header. An authenticated attacker can use a spoofed Host header to redirect slash command responses to a server they control. Upgrade to 10.11.14, 11.5.2, or the fixed module versions listed for your release line.

Written 7 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

ScoreRatingCVSSVectorSource
3.5 Low 3.1 CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:N/I:L/A:N CNA (Mattermost)
  • CISA KEV Not listed.
  • EPSS 0.0014, the 3rd percentile, on 14 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

  1. 31 Oct 2026, 15:06 UTC First seen by agentprime
  2. 31 Oct 2026, 15:06 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.