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CVE-2026-12985

CRITICALCVSS 9.3/10EPSS 0.28%

Last modified

CVE-2026-12985 is a critical-severity vulnerability rated 9.3/10 on the CVSS scale. 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. EPSS estimates a 0.28% chance of exploitation in the next 30 days.

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

Metrics

CVSS 3.1
9.3/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N

EPSS Probability
0.28%

18.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

VendorProductVersions
MattermostMattermost Server>= 11.7.0, < 11.7.8
MattermostMattermost Server>= 11.8.0, < 11.8.5
MattermostMattermost Server>= 11.9.0, < 11.9.1

References

Timeline

Published
Last Modified
Status
Analyzed

Frequently Asked Questions

What is CVE-2026-12985?
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
How severe is CVE-2026-12985?
CVE-2026-12985 has a CVSS score of 9.3/10 (CRITICAL severity). The EPSS model estimates a 0.28% probability of exploitation in the next 30 days.
How do I fix CVE-2026-12985?
Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

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Source: NVD / NIST