# CVE-2026-12985: Mattermost OAuth redirect URI validation bypass

Source: https://junglewise.ai/threats/cve-2026-12985-mattermost-oauth-redirect-uri-validation-bypass  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-14. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-12985 · Severity: medium · CVSS 6.8 · Published 2026-09-14

Mattermost's Dynamic Client Registration feature failed to properly validate redirect URIs, allowing unauthenticated attackers to register malicious OAuth clients with attacker-controlled callback hosts. This bypasses the configured allowlist and could enable phishing attacks or credential theft via OAuth token interception.

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

## Quick answers

**Is CVE-2026-12985 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-12985 being exploited in the wild.

**How severe is CVE-2026-12985?** Medium severity, with a CVSS score of 6.8, published 14 September 2026.

**Which products and versions are affected?** Mattermost Server 11.7.x through 11.7.7, 11.8.x through 11.8.4, 11.9.x through 11.9.0.

**Is there a fix for CVE-2026-12985?** 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-2026-12985 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-12985?** 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 exists in Mattermost's Dynamic Client Registration implementation, which validates redirect URIs using glob pattern matching against the raw URI string rather than parsing individual URL components. An attacker can craft a malicious redirect URI that places an allowlisted host or path as a suffix in the query string parameter, causing the validation check to pass while the actual callback destination remains attacker-controlled. This is a classic URL parsing confusion attack where naive string matching fails to properly distinguish between the URL path and query parameters. The vulnerability requires no authentication and is remotely exploitable via network access to the OAuth registration endpoint.

## Affected products

- Mattermost Mattermost Server 11.7.x through 11.7.7, 11.8.x through 11.8.4, 11.9.x through 11.9.0

## Timeline

- 2026-09-14: disclosed

## References

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

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