Mattermost DCR redirect URI allowlist bypass via improper URL component validation

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-12985
StatePUBLISHED
AssignerMattermost
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-14 15:17:04 UTC
Updated2026-10-07 17:58:56 UTC
DescriptionMattermost 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

Risk And Classification

Primary CVSS: v3.1 9.3 CRITICAL from [email protected]

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

EPSS: 0.002800000 probability, percentile 0.205420000 (date 2026-09-16)

Problem Types: CWE-601 | CWE-601 CWE-601: URL Redirection to Untrusted Site (“Open Redirect”)

VersionSourceTypeScoreSeverityVector
3.1[email protected]Primary9.3CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N
3.1[email protected]Secondary6.8MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
3.1CNACVSS6.8MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Changed
Confidentiality
High
Integrity
High
Availability
None

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

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Mattermost Mattermost Server All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Mattermost Mattermost affected 11.9.0 11.9.0 semver Not specified
CNA Mattermost Mattermost affected 11.8.0 11.8.4 semver Not specified
CNA Mattermost Mattermost affected 11.7.0 11.7.7 semver Not specified
CNA Mattermost Mattermost unaffected 11.10.0 Not specified
CNA Mattermost Mattermost unaffected 11.9.1 Not specified
CNA Mattermost Mattermost unaffected 11.8.5 Not specified
CNA Mattermost Mattermost unaffected 11.7.8 Not specified

References

ReferenceSourceLinkTags
mattermost.com/security-updates [email protected] mattermost.com Vendor Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: aryangurjarf401a3 (en)

Additional Advisory Data

Solutions

CNA: Update Mattermost to versions 11.10.0, 11.9.1, 11.8.5, 11.7.8 or higher.

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