Account Takeover via Substring Matching in OpenID Connect Authentication
Priority LOW 28CWE-303Published · Updated
Key facts
- Attack vector
- Network
- Privileges required
- High
- User interaction
- Required
- Attack complexity
- Low
- Impact type
- —
- Confidentiality · integrity · availability
- High · High · None
- Fix status
- Fixed
Attack profile from CVSS 3.1 · NVD. Fix status from CNA and GitHub.
Summary
Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590
Why it matters now
Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00183 7th percentileScored 2026-11-07
Fix Fixed Mattermost (Mattermost): 11.5.0, 11.4.1, 11.3.2 +2github.com/mattermost/mattermost-server (Go): 11.4.1, 11.3.2, 11.2.4 +1+1 more product
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- No
- Technical impact
- Total
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| Mattermost | Mattermost | 11.4.0 – 11.4.0; 11.3.0 – 11.3.1; 11.2.0 – 11.2.3; 10.11.0 – 10.11.11 | 11.5.0, 11.4.1, 11.3.2, 11.2.4, 10.11.12 | Patch available | CNA |
| — | github.com/mattermost/mattermost-server (Go) | 11.4.0-rc1 to before 11.4.1 | 11.4.1 | Patch available | GHSA-fg35-5rf6-qg3g |
| — | github.com/mattermost/mattermost-server (Go) | 11.3.0-rc1 to before 11.3.2 | 11.3.2 | Patch available | GHSA-fg35-5rf6-qg3g |
| — | github.com/mattermost/mattermost-server (Go) | 11.2.0-rc1 to before 11.2.4 | 11.2.4 | Patch available | GHSA-fg35-5rf6-qg3g |
| — | github.com/mattermost/mattermost-server (Go) | 10.11.0-rc1 to before 10.11.12 | 10.11.12 | Patch available | GHSA-fg35-5rf6-qg3g |
| — | github.com/mattermost/mattermost/server/v8 (Go) | 8.0.0-20260105080200-d27a2195068d to before 8.0.0-20260217110922-b7d4a1f1f59b | 8.0.0-20260217110922-b7d4a1f1f59b | Patch available | GHSA-fg35-5rf6-qg3g |
- Mattermost VendorMattermost Affected 11.4.0 – 11.4.0; 11.3.0 – 11.3.1; 11.2.0 – 11.2.3; 10.11.0 – 10.11.11 Fixed in 11.5.0, 11.4.1, 11.3.2, 11.2.4, 10.11.12 Patch available CNA
- github.com/mattermost/mattermost-server (Go) Vendor— Affected 11.4.0-rc1 to before 11.4.1 Fixed in 11.4.1 Patch available GHSA-fg35-5rf6-qg3g
- github.com/mattermost/mattermost-server (Go) Vendor— Affected 11.3.0-rc1 to before 11.3.2 Fixed in 11.3.2 Patch available GHSA-fg35-5rf6-qg3g
- github.com/mattermost/mattermost-server (Go) Vendor— Affected 11.2.0-rc1 to before 11.2.4 Fixed in 11.2.4 Patch available GHSA-fg35-5rf6-qg3g
- github.com/mattermost/mattermost-server (Go) Vendor— Affected 10.11.0-rc1 to before 10.11.12 Fixed in 10.11.12 Patch available GHSA-fg35-5rf6-qg3g
- github.com/mattermost/mattermost/server/v8 (Go) Vendor— Affected 8.0.0-20260105080200-d27a2195068d to before 8.0.0-20260217110922-b7d4a1f1f59b Fixed in 8.0.0-20260217110922-b7d4a1f1f59b Patch available GHSA-fg35-5rf6-qg3g
CPE configurations NVD · 1
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:* | 10.11.0 to before 10.11.12; 11.2.0 to before 11.2.4; 11.3.0 to before 11.3.2; 11.4.0 to before 11.4.1 | Vulnerable | Mattermost |
- cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:* Versions 10.11.0 to before 10.11.12; 11.2.0 to before 11.2.4; 11.3.0 to before 11.3.2; 11.4.0 to before 11.4.1 Vulnerable ·Mattermost
Remediation
- Fix · CNA Mattermost (Mattermost) Fixed in 11.5.0, 11.4.1, 11.3.2, 11.2.4, 10.11.12
- Fix · GHSA-fg35-5rf6-qg3g github.com/mattermost/mattermost-server (Go) Fixed in 11.4.1 Advisory →
- Fix · GHSA-fg35-5rf6-qg3g github.com/mattermost/mattermost-server (Go) Fixed in 11.3.2 Advisory →
- Fix · GHSA-fg35-5rf6-qg3g github.com/mattermost/mattermost-server (Go) Fixed in 11.2.4 Advisory →
- Fix · GHSA-fg35-5rf6-qg3g github.com/mattermost/mattermost-server (Go) Fixed in 10.11.12 Advisory →
- Fix · GHSA-fg35-5rf6-qg3g github.com/mattermost/mattermost/server/v8 (Go) Fixed in 8.0.0-20260217110922-b7d4a1f1f59b Advisory →
Timeline
- 2026-10-02 Threatist record created from CVE-2026-27656
- 2026-10-03 Alias added: GHSA-fg35-5rf6-qg3g
References
More Mattermost Server vulnerabilities
- TI-2025-18933 Arbitrary file read in Mattermost Boards via import & export board archive CVSS 3.1CRITICAL 9.9
- TI-2025-04465 Path Traversal Leading to RCE by Any Authenticated Mattermost User CVSS 3.1CRITICAL 9.9
- TI-2017-14068 An issue was discovered in Mattermost Server before 3.6.2. CVSS 3.1CRITICAL 9.8
- TI-2017-14060 An issue was discovered in Mattermost Server before 3.8.2, 3.7.5, and 3.6.7. CVSS 3.1CRITICAL 9.8
- TI-2017-14048 An issue was discovered in Mattermost Server before 4.1.0, 4.0.4, and 3.10.3. CVSS 3.1CRITICAL 9.8
- TI-2025-14360 Arbitrary file read via block duplication in Mattermost Boards CVSS 3.1CRITICAL 9.9
Sources
- CVE-2026-27656 · CVE Program
- CVE-2026-27656 · NVD
- FIRST EPSS
- GHSA-fg35-5rf6-qg3g · GitHub Advisory Database · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences