How severe is CVE-2020-15138?

CVE-2020-15138 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2020-15138?

CVE-2020-15138 affects the following packages: prismjs (npm). Ecosystems affected: npm.

How do I fix CVE-2020-15138?

Update prismjs to 1.21.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-15138 is resolved across your whole dependency graph.

How do I detect CVE-2020-15138 in my npm dependencies?

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

How do I mitigate CVE-2020-15138 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-2020-15138 actively exploited in the wild?

No public exploit code has been indexed for CVE-2020-15138 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-2020-15138?

CVE-2020-15138 has an EPSS (Exploit Prediction Scoring System) score of 2.0%, placing it in the 81th 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-2020-15138?

CVE-2020-15138 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-2020-15138 published, and has it been updated?

CVE-2020-15138 was published on August 7, 2020 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-2020-15138 — prismjs

HIGHFix: PrismJS/prism#2506

CVE-2020-15138 is a high-severity (CVSS 7.5) Cross-site Scripting (XSS) vulnerability in prismjs. A fix is available for prismjs — see the affected versions and patch details below.

Cross-Site Scripting in Prism

Also known asGHSA-wvhm-4hhf-97x9

Published

Aug 7, 2020

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 31, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.0%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs81th percentile — riskier than 81% 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.

5Kother npm packages depend on this — each one inherits the vulnerability until it's patched upstream

prismjsnpm

46.1Mdownloads / week

Description

Impact

The easing preview of the Previewers plugin has an XSS vulnerability that allows attackers to execute arbitrary code in Safari and Internet Explorer.

This impacts all Safari and Internet Explorer users of Prism >=v1.1.0 that use the Previewers plugin (>=v1.10.0) or the Previewer: Easing plugin (v1.1.0 to v1.9.0).

Patches

This problem is patched in v1.21.0.

Workarounds

To workaround the issue without upgrading, disable the easing preview on all impacted code blocks. You need Prism v1.10.0 or newer to apply this workaround.

References

The vulnerability was introduced by this commit on Sep 29, 2015 and fixed by Masato Kinugawa (#2506).

For more information

If you have any questions or comments about this advisory, please open an issue.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`prismjs`	≥ 1.1.0&&< 1.21.0	1.21.0`npm install prismjs@1.21.0`

Affected Products

1 product · 1 configurations

Application

previewersprismjs

≥ 1.1.0 && < 1.21.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for prismjs, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update prismjs to 1.21.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-15138 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 easing preview of the Previewers plugin has an XSS vulnerability that allows attackers to execute arbitrary code in Safari and Internet Explorer. This impacts all Safari and Internet Explorer users of Prism >=v1.1.0 that use the _Previewers_ plugin (>=v1.10.0) or the _Previewer: Easing_ plugin (v1.1.0 to v1.9.0). ### Patches This problem is patched in v1.21.0. ### Workarounds To workaround the issue without upgrading, [disable the easing preview](https://prismjs.com/plugins/previewers/#disabling-a-previewer) on all impacted code blocks. You need Prism v1.10.0 or newer to appl