# CVE-2026-96259: Mattermost internal-connection filter bypass in OAuth endpoints

Source: https://junglewise.ai/threats/cve-2026-96259-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-22. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-96259 · Severity: medium · CVSS 5.5 · Published 2026-09-22

Mattermost is a team collaboration platform that organizations use for internal communication. A flaw in certain versions allows system administrators to bypass network security controls and make the server request data from internal network resources, potentially exposing sensitive internal services or information.

Technologies: [Mattermost Server](https://junglewise.ai/threats/technologies/server-public). Vendors: [Mattermost](https://junglewise.ai/threats/vendors/mattermost).

## Quick answers

**Is CVE-2026-96259 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-96259 being exploited in the wild.

**How severe is CVE-2026-96259?** Medium severity, with a CVSS score of 5.5, published 22 September 2026.

**Which products and versions are affected?** Mattermost Server 11.7.x through 11.7.10, 11.8.x through 11.8.5, 11.9.x through 11.9.1, 11.10.x through 11.10.1.

**Is there a fix for CVE-2026-96259?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-96259 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-96259?** Check whether you run Mattermost Server 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.

## Technical details

The vulnerability is a failure to apply the internal-connection filter to OAuth endpoint requests, allowing authenticated system administrators to make the Mattermost server issue requests to internal network addresses. An attacker with system administrator privileges can read responses from configured OAuth token and userinfo endpoints that would normally be restricted from accessing internal resources.

## Affected products

- Mattermost Mattermost Server 11.7.x through 11.7.10, 11.8.x through 11.8.5, 11.9.x through 11.9.1, 11.10.x through 11.10.1

## Timeline

- 2026-09-22: disclosed

## References

- [https://mattermost.com/security-updates](https://mattermost.com/security-updates)

## Related threats

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