# CVE-2021-37859: Reflected XSS in OAuth Flow

> Attackers can execute arbitrary scripts in victim browsers via reflected XSS in the OAuth flow of Mattermost instances.

- **Severity:** High 7.1 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 5 Aug 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-r4f6-w245-8wv4

## What is CVE-2021-37859?

CVE-2021-37859 is a high-severity cross-site scripting vulnerability in Mattermost Mattermost, published on 5 Aug 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Fixed a bypass for a reflected cross-site scripting vulnerability affecting OAuth-enabled instances of Mattermost.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2021-37859?

It affects 1 product.


Products: Mattermost Mattermost.

## How do I fix CVE-2021-37859?

No fix has been published for CVE-2021-37859 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-37859 being exploited?

There is no known exploitation of CVE-2021-37859 in the wild and no public exploit code.

## Other vulnerabilities in Mattermost Mattermost

- [CVE-2019-20851](https://insights.sbomify.com/CVE-2019-20851/): Path traversal in n/a (critical 9.1)
- [CVE-2024-39274](https://insights.sbomify.com/CVE-2024-39274/): Malicious remote can add users to arbitrary teams and channels (high 8.7)
- [CVE-2024-39777](https://insights.sbomify.com/CVE-2024-39777/): Malicious remote can invite itself to an arbitrary local channel (high 8.7)
- [CVE-2024-39830](https://insights.sbomify.com/CVE-2024-39830/): Timing attack during remote cluster token comparison when shared channels are enabled (high 8.1)
- [CVE-2023-3615](https://insights.sbomify.com/CVE-2023-3615/): Lack of server certificate validation in websockets connection (high 8.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-37859 and fix it if it is affected.

What it is: CVE-2021-37859 is a high-severity cross-site scripting vulnerability in Mattermost Mattermost, published on 5 Aug 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-37859 in the wild and no public exploit code.

Affected product: Mattermost Mattermost. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2021-37859/ (Markdown: https://insights.sbomify.com/CVE-2021-37859/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-37859
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-37859
- OSV: https://osv.dev/vulnerability/CVE-2021-37859

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-37859/
Summaries are generated and may contain mistakes; the official description above is the record.