How severe is CVE-2021-39199?

CVE-2021-39199 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-39199?

CVE-2021-39199 affects the following packages: remark-html (npm), remark-html (npm). Ecosystems affected: npm.

How do I fix CVE-2021-39199?

Update remark-html to 13.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39199 is resolved across your whole dependency graph.

How do I detect CVE-2021-39199 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for remark-html, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-39199 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2021-39199 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-39199 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-39199?

CVE-2021-39199 has an EPSS (Exploit Prediction Scoring System) score of 1.1%, placing it in the 64th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-39199?

CVE-2021-39199 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-39199 published, and has it been updated?

CVE-2021-39199 was published on September 7, 2021 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 npm

Not in CISA KEV

MEDIUM severity

CVE-2021-39199 — remark-html

MEDIUMFix: remarkjs/remark-html@b75c9dd

CVE-2021-39199 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in remark-html. A fix is available for remark-html — see the affected versions and patch details below.

Unsafe defaults in `remark-html`

Also known asGHSA-9q5w-79cv-947m

Published

Sep 7, 2021

Updated

Jul 9, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Oct 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.1%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs64th percentile — riskier than 64% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-39199 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

2 pkgs affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

426other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

remark-htmlnpm

693Kdownloads / week

Description

Impact

The documentation of remark-html has mentioned that it was safe by default. In practise the default was never safe and had to be opted into. This means arbitrary HTML can be passed through leading to potential XSS attacks.

Patches

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.

Workarounds

On older affected versions, pass sanitize: true, like so:

-  .use(remarkHtml)
+  .use(remarkHtml, {sanitize: true})

References

n/a

For more information

If you have any questions or comments about this advisory:

Open an issue in remark-html
Email us at [email protected]

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`remark-html`	all versions	13.0.2`npm install remark-html@13.0.2`
📦npm	`remark-html`	≥ 14.0.0&&< 14.0.1	14.0.1`npm install remark-html@14.0.1`

Affected Products

1 product · 2 configurations

Application

remark-htmlremark

≥ 14.0.0 && < 14.0.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for remark-html, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update remark-html to 13.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39199 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### Impact The documentation of `remark-html` has mentioned that it was safe by default. In practise the default was never safe and had to be opted into. This means arbitrary HTML can be passed through leading to potential XSS attacks. ### Patches 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. ### Workarounds On older affected versions, pass `sanitize: true`, like so: ```diff - .use(remarkHtml) + .use(remarkHtml, {sanitize: true}) ``` ### References n/a ### For more information If you have a