Critical severity · published Sep 14, 2026
CVE-2026-12985
Mattermost DCR redirect URI allowlist bypass via improper URL component validation
9.3
CVSS
0.3%
EPSS
No
On KEV
Description
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7 Mattermost failed to validate Dynamic Client Registration redirect URIs by URL component (matching glob patterns against the raw URI string instead) which allows a remote unauthenticated attacker to register an OAuth client with an attacker-controlled callback host that bypasses the configured redirect URI allowlist via a crafted redirect URI that places an allowlisted host/path suffix inside the query string.. Mattermost Advisory ID: MMSA-2026-00700
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Confidentiality
High
Integrity
High
Availability
None
Scope
Changed
Exploitation
Is it being used?
Exploit prediction (EPSS)0.3% · higher than 19% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
11.9.0 through 11.9.0; 11.8.0 through 11.8.4; 11.7.0 through 11.7.7 · source: vendor
MS
Mattermost Server · Mattermost
11.7.0 before 11.7.8; 11.8.0 before 11.8.5; 11.9.0 before 11.9.1 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-12985
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-12985
Vendor Advisory
https://mattermost.com/security-updates
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