Unauthenticated OAuth client registration via raw URI matching
Published Sep 14, 2026 · Updated Sep 14, 2026
Improper redirect validation in Mattermost allows remote attackers to register OAuth clients with attacker-controlled callbacks. The DCR allowlist applies glob patterns to the complete raw redirect URI instead of parsing and matching URL components, so an allowed host or path placed in the query string satisfies the check. Exploitation requires the non-default Dynamic Client Registration setting and an allowlist; affected branches are 11.7.0 through 11.7.7, 11.8.0 through 11.8.4, and 11.9.0, enabling authorization responses to reach an attacker-controlled host.
Summary
What happened
Improper redirect validation in Mattermost allows remote attackers to register OAuth clients with attacker-controlled callbacks. The DCR allowlist applies glob patterns to the complete raw redirect URI instead of parsing and matching URL components, so an allowed host or path placed in the query string satisfies the check. Exploitation requires the non-default Dynamic Client Registration setting and an allowlist; affected branches are 11.7.0 through 11.7.7, 11.8.0 through 11.8.4, and 11.9.0, enabling authorization responses to reach an attacker-controlled host.
The record
- CVE
- CVE-2026-12985
- Published
- Sep 14, 2026
- Updated
- Sep 14, 2026
- Vendor
- Mattermost
- Product
- Mattermost
- Classifications
- T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Sep 14, 2026CVE publishedPublication date reported by the CVE source.
- Sep 14, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=11.7.0 <=11.7.7
- Affected versionversion=11.8.0 <=11.8.4
- Affected versionversion=11.9.0 <=11.9.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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