How severe is CVE-2021-21306?

CVE-2021-21306 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-21306?

CVE-2021-21306 affects the following packages: marked (npm). Ecosystems affected: npm.

How do I fix CVE-2021-21306?

Update marked to 2.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21306 is resolved across your whole dependency graph.

How do I detect CVE-2021-21306 in my npm dependencies?

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

How do I mitigate CVE-2021-21306 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2021-21306 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21306 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-2021-21306?

CVE-2021-21306 has an EPSS (Exploit Prediction Scoring System) score of 2.5%, placing it in the 85th 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-2021-21306?

CVE-2021-21306 is classified as Uncontrolled Resource Consumption (CWE-400). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21306 published, and has it been updated?

CVE-2021-21306 was published on February 8, 2021 and was last updated on October 20, 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

HIGH severity

CVE-2021-21306 — marked

HIGHFix: markedjs/marked@7293251

CVE-2021-21306 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in marked. A fix is available for marked — see the affected versions and patch details below.

Regular Expression Denial of Service (REDoS) in Marked

Also known asGHSA-4r62-v4vq-hr96

Published

Feb 8, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 4, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.5%probability of exploitation in next 30 days

Lower Risk+0.03%

Lower risk than most CVEs85th percentile — riskier than 85% of all scored CVEsHighest risk

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

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg 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.

15Kother npm packages depend on this — each one inherits the vulnerability until it's patched upstream

markednpm

123.6Mdownloads / week

Description

Impact

What kind of vulnerability is it? Who is impacted?

Regular expression Denial of Service

A Denial of Service attack can affect anyone who runs user generated code through marked.

Patches

Has the problem been patched? What versions should users upgrade to?

patched in v2.0.0

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

None.

References

Are there any links users can visit to find out more?

https://github.com/markedjs/marked/issues/1927 https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS

For more information

If you have any questions or comments about this advisory:

Open an issue in marked

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`marked`	≥ 1.1.1&&< 2.0.0	2.0.0`npm install marked@2.0.0`

Affected Products

1 product · 1 configurations

Application

markedmarked_project

≥ 1.1.1 && < 2.0.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for marked, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update marked to 2.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21306 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### Impact _What kind of vulnerability is it? Who is impacted?_ [Regular expression Denial of Service](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS) A Denial of Service attack can affect anyone who runs user generated code through `marked`. ### Patches _Has the problem been patched? What versions should users upgrade to?_ patched in v2.0.0 ### Workarounds _Is there a way for users to fix or remediate the vulnerability without upgrading?_ None. ### References _Are there any links users can visit to find out more?_ https://github.com/markedjs/marke