# CVE-2017-18906: Mattermost Server vulnerable to user account takeover when Single Sign-On OAuth2 is used

> Attackers can claim other users' accounts via OAuth2 SSO flaws, requiring immediate upgrades to Mattermost Server.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/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:** 19 Jun 2020
- **Updated:** 5 Aug 2024
- **Also known as:** GHSA-fpcr-4rr5-hpcp, GO-2026-4477

## What is CVE-2017-18906?

CVE-2017-18906 is a high-severity authentication bypass vulnerability in Mattermost Mattermost Server, published on 19 Jun 2020. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

An issue was discovered in Mattermost Server before 4.0.0, 3.10.2, and 3.9.2, when Single Sign-On OAuth2 is used. An attacker could claim somebody else's account.

Weakness: CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2017-18906?

5 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 | All versions | 3.9.2-0.20170714134023-b17fca0d5ee7 |
| `github.com/mattermost/mattermost-server` | Go | All versions | 3.9.2-0.20170714134023-b17fca0d5ee7 |
| `github.com/mattermost/mattermost-server` | Go | From 3.10.0 | 3.10.2 |
| `github.com/mattermost/mattermost-server` | Go | From 3.10.0 | 3.10.2 |
| `github.com/mattermost/mattermost-server` | Go | From 3.10.0+incompatible | 3.10.2+incompatible |

Products: Mattermost Mattermost Server.

## How do I fix CVE-2017-18906?

Upgrade github.com/mattermost/mattermost-server to a fixed version for your release line: 3.9.2-0.20170714134023-b17fca0d5ee7, 3.10.2 or 3.10.2+incompatible.

## Is CVE-2017-18906 being exploited?

There is no known exploitation of CVE-2017-18906 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-2017-18906 and fix it if it is affected.

What it is: CVE-2017-18906 is a high-severity authentication bypass vulnerability in Mattermost Mattermost Server, published on 19 Jun 2020. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2017-18906 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/mattermost/mattermost-server (Go): affected all versions; fixed in 3.9.2-0.20170714134023-b17fca0d5ee7
- github.com/mattermost/mattermost-server (Go): affected all versions; fixed in 3.9.2-0.20170714134023-b17fca0d5ee7
- github.com/mattermost/mattermost-server (Go): affected from 3.10.0; fixed in 3.10.2
- github.com/mattermost/mattermost-server (Go): affected from 3.10.0; fixed in 3.10.2
- github.com/mattermost/mattermost-server (Go): affected from 3.10.0+incompatible; fixed in 3.10.2+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-2017-18906/ (Markdown: https://insights.sbomify.com/CVE-2017-18906/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2017-18906
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2017-18906
- OSV: https://osv.dev/vulnerability/CVE-2017-18906

---
From sbomify Insights: https://insights.sbomify.com/CVE-2017-18906/
Summaries are generated and may contain mistakes; the official description above is the record.