CVE-2021-3377: The npm package ansi_up converts ANSI escape codes into HTML

This month

MEDIUM 6.1UPGRADE

CVE-2021-3377 is a medium-severity flaw (CVSS 6.1) affecting ansi up. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The npm package ansi_up converts ANSI escape codes into HTML. In ansi_up v4, ANSI escape codes can be used to create HTML hyperlinks. Due to insufficient URL sanitization, this feature is affected by a cross-site scripting (XSS) vulnerability. This issue is fixed in v5.0.0.

Why this priority

EPSS top 5%: very likely to be exploited
Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	6.1 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	8.0% (top 5%)
CISA exploited list	No
Weakness	CWE-79
Published	2021-03-05
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: ansi up project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-03-05 21:15 UTCPublished CVE record published
2026-06-17 04:05 UTCUpdated Record last modified

ansi up project ansi up UPGRADE

Affected: < 5.0.0

Fixed in: 5.0.0

Safe to downgrade to: no unaffected older release

1. Check installed version

npm ls ansi-up

2. UPGRADE to 5.0.0

npm install ansi-up@">=5.0.0"

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Nuclei scan template: npm ansi_up v4 - Cross-Site Scripting

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-3377?

ansi up: upgrade to 5.0.0. Exact commands for each OS are on this page.

Is CVE-2021-3377 being exploited?

CVE-2021-3377 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-3377 affect?

CVE-2021-3377 affects ansi up. Severity is medium (CVSS 6.1). The npm package ansi_up converts ANSI escape codes into HTML. In ansi_up v4, ANSI escape codes can be used to create HTML hyperlinks. Due to insufficient URL sanitization, this feature is affected by a cross-site scripting (XSS) vulnerabili

How urgent is CVE-2021-3377?

Priority 50/100. EPSS top 5%: very likely to be exploited; Public exploit reference

Software pages

npm packages vulnerabilities

References

← Back to latest CVEs