# CVE-2021-39199: Cross site scripting via unsafe defaults in remark-html

> Attackers can inject arbitrary HTML leading to XSS because remark-html did not sanitize user input by default despite documentation claims.

- **Severity:** Critical 10.0 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 7 Sep 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-9q5w-79cv-947m

## What is CVE-2021-39199?

CVE-2021-39199 is a critical cross-site scripting vulnerability in remarkjs remark-html, published on 7 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

remark-html is an open source nodejs library which compiles Markdown to HTML. In affected versions the documentation of remark-html has mentioned that it was safe by default. In practice the default was never safe and had to be opted into. That is, user input was not sanitized. This means arbitrary HTML can be passed through leading to potential XSS attacks. The problem has been patched in 13.0.2 and 14.0.1: `remark-html` is now safe by default, and the implementation matches the documentation. On older affected versions, pass `sanitize: true` if you cannot update.

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

## Which versions are affected by CVE-2021-39199?

4 open source packages ship an affected version, including remark-html. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `remark-html` | npm | All versions | 13.0.2 |
| `remark-html` | npm | All versions | 13.0.2 |
| `remark-html` | npm | From 14.0.0 | 14.0.1 |
| `remark-html` | npm | From 14.0.0 | 14.0.1 |

Products: Remark Remark-Html.

## How do I fix CVE-2021-39199?

Upgrade remark-html to a fixed version for your release line: 13.0.2 or 14.0.1.

## Is CVE-2021-39199 being exploited?

There is no known exploitation of CVE-2021-39199 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-39199 and fix it if it is affected.

What it is: CVE-2021-39199 is a critical cross-site scripting vulnerability in remarkjs remark-html, published on 7 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-39199 in the wild and no public exploit code.

Affected packages and fixed versions:
- remark-html (npm): affected all versions; fixed in 13.0.2
- remark-html (npm): affected all versions; fixed in 13.0.2
- remark-html (npm): affected from 14.0.0; fixed in 14.0.1
- remark-html (npm): affected from 14.0.0; fixed in 14.0.1

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-2021-39199/ (Markdown: https://insights.sbomify.com/CVE-2021-39199/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-39199
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-39199
- OSV: https://osv.dev/vulnerability/CVE-2021-39199

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-39199/
Summaries are generated and may contain mistakes; the official description above is the record.