CVE-2026-3115: Mattermost allows authenticated guest users to enumerate user IDs outside their allowed visibility scope

CVE-2026-3115 · Severity: low · CVSS 3.1 · Published 2026-06-25

Mattermost allows authenticated guest users to enumerate user IDs outside their allowed visibility scope in github.com/mattermost/mattermost-server

Technologies: github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server/v5 (Go), github.com/mattermost/mattermost-server/v6 (Go), github.com/mattermost/mattermost/server/v8 (Go). Vendors: Go.

Quick answers

Is CVE-2026-3115 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-3115 being exploited in the wild.
How severe is CVE-2026-3115?
Low severity, with a CVSS score of 3.1, published 25 June 2026.
Which products and versions are affected?
github.com/mattermost/mattermost-server (Go) SEMVER:introduced=10.11.0-rc1+incompatible,fixed=10.11.11+incompatible,introduced=11.2.0-rc1+incompatible,fixed=11.2.3+incompatible,introduced=11.3.0-rc1+incompatible,fixed=11.3.2+incompatible,introduced=11.4.0+incompatible,fixed=11.4.1+incompatible; github.com/mattermost/mattermost-server/v5 (Go) SEMVER:introduced=0; github.com/mattermost/mattermost-server/v6 (Go) SEMVER:introduced=0; github.com/mattermost/mattermost/server/v8 (Go) SEMVER:introduced=11.4.0,fixed=11.4.1, SEMVER:introduced=8.0.0-20260105080200-d27a2195068d,fixed=8.0.0-20260217110922-b7d4a1f1f59b.
Is there a fix for CVE-2026-3115?
Yes. github.com/mattermost/mattermost-server (Go): upgrade to 11.4.1+incompatible or later; github.com/mattermost/mattermost/server/v8 (Go): upgrade to 11.4.1, 8.0.0-20260217110922-b7d4a1f1f59b or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
Am I affected by CVE-2026-3115?
Check whether you run github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server/v5 (Go), github.com/mattermost/mattermost-server/v6 (Go) or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • github.com/mattermost/mattermost-server (Go): upgrade to 11.4.1+incompatible or later
    go get github.com/mattermost/mattermost-server@v11.4.1+incompatible
  • github.com/mattermost/mattermost/server/v8 (Go): upgrade to 11.4.1, 8.0.0-20260217110922-b7d4a1f1f59b or later
    go get github.com/mattermost/mattermost/server/v8@v11.4.1

Affected products

  • github.com/mattermost/mattermost-server (Go) SEMVER:introduced=10.11.0-rc1+incompatible,fixed=10.11.11+incompatible,introduced=11.2.0-rc1+incompatible,fixed=11.2.3+incompatible,introduced=11.3.0-rc1+incompatible,fixed=11.3.2+incompatible,introduced=11.4.0+incompatible,fixed=11.4.1+incompatible
  • github.com/mattermost/mattermost-server/v5 (Go) SEMVER:introduced=0
  • github.com/mattermost/mattermost-server/v6 (Go) SEMVER:introduced=0
  • github.com/mattermost/mattermost/server/v8 (Go) SEMVER:introduced=11.4.0,fixed=11.4.1, SEMVER:introduced=8.0.0-20260105080200-d27a2195068d,fixed=8.0.0-20260217110922-b7d4a1f1f59b

Identifiers

  • CVE-2026-3115
  • GHSA-mpc7-mm28-f6wq
  • GO-2026-5512

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