# CVE-2026-20796: Time-of-check time-of-use vulnerability in common teams API

> A race condition in the common teams API allows deactivated users to access unauthorized team names, requiring an upgrade to Mattermost.

- **Severity:** Low 3.1 (`CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 13 Feb 2026
- **Updated:** 13 Feb 2026
- **Also known as:** GHSA-2xf7-hmf6-p64j, GO-2026-4495

## What is CVE-2026-20796?

CVE-2026-20796 is a low-severity vulnerability in Mattermost Mattermost Server, published on 13 Feb 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Mattermost versions 10.11.x <= 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /common_teams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549

Weakness: CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition)

## Which versions are affected by CVE-2026-20796?

3 open source packages ship an affected version, including github.com/mattermost/mattermost-server. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/mattermost/mattermost-server` | Go | From 10.11.0 | 10.11.10 |
| `github.com/mattermost/mattermost-server` | Go | From 10.11.0 | 10.11.10 |
| `github.com/mattermost/mattermost-server` | Go | From 10.11.0+incompatible | 10.11.10+incompatible |

Products: Mattermost Mattermost Server.

## How do I fix CVE-2026-20796?

Upgrade github.com/mattermost/mattermost-server to a fixed version for your release line: 10.11.10 or 10.11.10+incompatible.

## Is CVE-2026-20796 being exploited?

There is no known exploitation of CVE-2026-20796 in the wild and no public exploit code.

## Other vulnerabilities in Mattermost Mattermost Server

- [CVE-2025-12419](https://insights.sbomify.com/CVE-2025-12419/): Account takeover on OAuth/OpenID-enabled servers (critical 9.9)
- [CVE-2025-12421](https://insights.sbomify.com/CVE-2025-12421/): Account Takeover via Code Exchange Endpoint (critical 9.9)
- [CVE-2025-4981](https://insights.sbomify.com/CVE-2025-4981/): Path Traversal Leading to RCE by Any Authenticated Mattermost User (critical 9.9)
- [CVE-2025-20051](https://insights.sbomify.com/CVE-2025-20051/): Arbitrary file read via block duplication in Mattermost Boards (critical 9.9)
- [CVE-2025-25279](https://insights.sbomify.com/CVE-2025-25279/): Arbitrary file read in Mattermost Boards via import & export board archive (critical 9.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-20796 and fix it if it is affected.

What it is: CVE-2026-20796 is a low-severity vulnerability in Mattermost Mattermost Server, published on 13 Feb 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-20796 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/mattermost/mattermost-server (Go): affected from 10.11.0; fixed in 10.11.10
- github.com/mattermost/mattermost-server (Go): affected from 10.11.0; fixed in 10.11.10
- github.com/mattermost/mattermost-server (Go): affected from 10.11.0+incompatible; fixed in 10.11.10+incompatible

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-20796/ (Markdown: https://insights.sbomify.com/CVE-2026-20796/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-20796
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-20796
- OSV: https://osv.dev/vulnerability/CVE-2026-20796

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-20796/
Summaries are generated and may contain mistakes; the official description above is the record.