How severe is GHSA-fg35-5rf6-qg3g?

GHSA-fg35-5rf6-qg3g has a CVSS score of 5.7/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by GHSA-fg35-5rf6-qg3g?

GHSA-fg35-5rf6-qg3g affects the following packages: github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost/server/v8 (Go). Ecosystems affected: Go.

How do I fix GHSA-fg35-5rf6-qg3g?

Update github.com/mattermost/mattermost-server to 11.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fg35-5rf6-qg3g is resolved across your whole dependency graph.

How do I detect GHSA-fg35-5rf6-qg3g in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost-server. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate GHSA-fg35-5rf6-qg3g if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against GHSA-fg35-5rf6-qg3g?

O3 pinpoints whether GHSA-fg35-5rf6-qg3g is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is GHSA-fg35-5rf6-qg3g actively exploited in the wild?

No public exploit code has been indexed for GHSA-fg35-5rf6-qg3g yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What type of vulnerability is GHSA-fg35-5rf6-qg3g?

GHSA-fg35-5rf6-qg3g is classified as CWE-303 (CWE-303). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was GHSA-fg35-5rf6-qg3g published, and has it been updated?

GHSA-fg35-5rf6-qg3g was published on March 25, 2026 and was last updated on June 25, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

GHSA-fg35-5rf6-qg3g

MEDIUM

GHSA-fg35-5rf6-qg3g is a medium-severity (CVSS 5.7) CWE-303 vulnerability in github.com/mattermost/mattermost-server. O3 Security confirms whether GHSA-fg35-5rf6-qg3g is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw

Also known asCVE-2026-27656GO-2026-5360

Published

Mar 25, 2026

Updated

Jun 25, 2026

Affected

5 pkgs

Patched

5 / 5

Exploits

None indexed

Blast Radius

5 pkgs affected

🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost/server/v8

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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

Affected Packages

5 total 5 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.4.0-rc1&&< 11.4.1	11.4.1
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.3.0-rc1&&< 11.3.2	11.3.2
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.2.0-rc1&&< 11.2.4	11.2.4
🐹Go	`github.com/mattermost/mattermost-server`	≥ 10.11.0-rc1&&< 10.11.12	10.11.12
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 8.0.0-20260105080200-d27a2195068d&&< 8.0.0-20260217110922-b7d4a1f1f59b	8.0.0-20260217110922-b7d4a1f1f59b

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost-server. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/mattermost/mattermost-server to 11.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fg35-5rf6-qg3g is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-fg35-5rf6-qg3g is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to GHSA-fg35-5rf6-qg3g. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is GHSA-fg35-5rf6-qg3g in your dependencies?

O3 detects GHSA-fg35-5rf6-qg3g across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works