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

Source: https://junglewise.ai/threats/cve-2026-3115-mattermost-allows-authenticated-guest-users-to-enumerate-user-ids  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-25  
License: CC BY 4.0 with a link back

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)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server), [github.com/mattermost/mattermost-server/v5 (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server-v5), [github.com/mattermost/mattermost-server/v6 (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server-v6), [github.com/mattermost/mattermost/server/v8 (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server-v8). Vendors: [Go](https://junglewise.ai/threats/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

  ```sh
  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

  ```sh
  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

## Related threats

- [Mattermost doesn't validate decompressed archive entry sizes during file extraction](https://junglewise.ai/threats/cve-2026-3114-mattermost-doesn-t-validate-decompressed-archive-entry-sizes)
- [GO-2026-5629 - Mattermost doesn't properly validate CSRF tokens in github.com/mattermost/mattermost-server](https://junglewise.ai/threats/cve-2026-27659-mattermost-doesn-t-properly-validate-csrf-tokens)
- [GO-2026-5495 - Mattermost doesn't validate CSRF tokens on an authentication endpoint in github.com/mattermost/mattermost-server](https://junglewise.ai/threats/cve-2026-28741-mattermost-doesn-t-validate-csrf-tokens-on-an-authentication)
- [Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw](https://junglewise.ai/threats/cve-2026-27656-mattermost-allows-attackers-to-take-over-arbitrary-user-accounts)
- [GO-2026-5393 - Mattermost has an Incorrect Authorization issue in github.com/mattermost/mattermost-server](https://junglewise.ai/threats/cve-2026-4274-mattermost-has-an-incorrect-authorization-issue)