CVE-2026-6334 OAuth authorization code client binding not enforced during token redemption in Mattermost
An authenticated OAuth client can redeem authorization codes issued to other clients, potentially leading to unauthorized access or account takeover.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-6334?
CVE-2026-6334 is a low-severity authentication bypass vulnerability in Mattermost Mattermost Server, published on 18 May 2026. An attacker needs administrator access to exploit it, and only if a user opens a file or follows a link.
Official description
Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to enforce client identity binding during the OAuth authorization code redemption flow which allows an authenticated OAuth client to redeem authorization codes issued to a different client via a crafted token exchange request.. Mattermost Advisory ID: MMSA-2026-00570
Weakness: CWE-305
Which versions are affected by CVE-2026-6334?
12 open source packages ship an affected version, including github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server and github.com/mattermost/mattermost-server/v5. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260318173148-e9ae890a013b |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260318173148-e9ae890a013b |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260318173148-e9ae890a013b |
github.com/mattermost/mattermost/server/v8 Go | From 10.11.0 | 10.11.14 |
github.com/mattermost/mattermost/server/v8 Go | From 10.11.0 | 10.11.14 |
github.com/mattermost/mattermost/server/v8 Go | From 11.5.0 | 11.5.2 |
6 more packages
github.com/mattermost/mattermost/server/v8 Go | From 11.5.0 | 11.5.2 |
github.com/mattermost/mattermost-server Go | All versions | 5.3.2-0.20260318173148-e9ae890a013b |
github.com/mattermost/mattermost-server Go | All versions | 5.3.2-0.20260318173148-e9ae890a013b |
github.com/mattermost/mattermost-server Go | From 10.11.0+incompatible | 10.11.14+incompatible, 11.5.2+incompatible |
github.com/mattermost/mattermost-server/v5 Go | All versions | None |
github.com/mattermost/mattermost-server/v6 Go | All versions | None |
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-6334?
Upgrade github.com/mattermost/mattermost/server/v8 to a fixed version for your release line: 8.0.0-20260318173148-e9ae890a013b, 10.11.14 or 11.5.2.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-6334 being exploited?
There is no known exploitation of CVE-2026-6334 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-jp3f-x449-4q75, GO-2026-5474.