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CVE-2026-12985

Mattermost DCR redirect URI allowlist bypass via improper URL component validation

CVSS 3.1 6.8 MEDIUMCheck source for fixed versionsKEV status unknown

Original advisory

cve-program · CVE-2026-12985

Published

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

Read the publisher's advisory

Affected versions, risk and source history

cve-program · CVE-2026-12985

Published

Mattermost

Vendor product

Publisher's version information

Shown as published. These rules are not interpreted as NuGet package version ranges.

{
                    "defaultStatus": "unaffected",
                    "product": "Mattermost",
                    "vendor": "Mattermost",
                    "versions": [
                        {
                            "lessThanOrEqual": "11.9.0",
                            "status": "affected",
                            "version": "11.9.0",
                            "versionType": "semver"
                        },
                        {
                            "lessThanOrEqual": "11.8.4",
                            "status": "affected",
                            "version": "11.8.0",
                            "versionType": "semver"
                        },
                        {
                            "lessThanOrEqual": "11.7.7",
                            "status": "affected",
                            "version": "11.7.0",
                            "versionType": "semver"
                        },
                        {
                            "version": "11.10.0",
                            "status": "unaffected"
                        },
                        {
                            "version": "11.9.1",
                            "status": "unaffected"
                        },
                        {
                            "version": "11.8.5",
                            "status": "unaffected"
                        },
                        {
                            "version": "11.7.8",
                            "status": "unaffected"
                        }
                    ]
                }

Original source record · Cite this exact observation · Complete original publisher JSON