How severe is CVE-2021-3377?

CVE-2021-3377 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-2021-3377?

CVE-2021-3377 affects the following packages: ansi_up (npm). Ecosystems affected: npm.

How do I fix CVE-2021-3377?

Update ansi_up to 5.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3377 is resolved across your whole dependency graph.

How do I detect CVE-2021-3377 in my npm dependencies?

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

How do I mitigate CVE-2021-3377 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-2021-3377 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-3377, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-3377?

CVE-2021-3377 has an EPSS (Exploit Prediction Scoring System) score of 8.0%, placing it in the 95th 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-2021-3377?

CVE-2021-3377 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.

Is there exploit tooling available for CVE-2021-3377?

A Nuclei detection template exists for CVE-2021-3377, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-3377 published, and has it been updated?

CVE-2021-3377 was published on March 5, 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-2021-3377 — ansi_up

MEDIUMFix: drudru/ansi_up@c8c726e

CVE-2021-3377 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in ansi_up. 1 public exploit reference exists, so weaponization risk is real. A fix is available for ansi_up — see the affected versions and patch details below.

ansi_up cross-site scripting vulnerability

Also known asGHSA-2v5f-23xc-v9qr

Published

Mar 5, 2021

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

1 known

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

8.0%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs95th percentile — riskier than 95% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-3377 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

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.

167other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

ansi_upnpm

311Kdownloads / week

Description

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`ansi_up`	all versions	5.0.0`npm install ansi_up@5.0.0`

Affected Products

1 product · 1 configurations

Application

ansi upansi_up_project

< 5.0.0

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

The npm package ansi_up converts ANSI escape codes into HTML. In ansi_up…

doyensec.comNVDMar 2021

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for ansi_up, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update ansi_up to 5.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3377 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.

How to detect CVE-2021-3377

A community-maintained Nuclei template exists for this CVE. You can scan for it directly:

nuclei -id cve-2021-3377 -u https://target

Template: npm ansi_up v4 - Cross-Site Scripting
Severity: medium
Impact: Successful exploitation of this vulnerability could allow an attacker to execute malicious scripts in the context of a user's browser, leading to potential data theft or unauthorized actions.
Remediation: Upgrade to v5.0.0 or later.

Template by ProjectDiscovery nuclei-templates (geeknik), MIT licensed. View the full template. Scan only systems you are authorised to test.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-3377 in your dependencies?

Find it across npm, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

6.1/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

OpenSSF Scorecard

Package health score

ansi_up

2.1/10

Code-Review1/10

Maintained0/10

Branch-Protection0/10

Assessed Oct 5, 2026

Timeline

Disclosed

Mar 5, 2021

First exploit

Mar 9, 2021

4 days after disclosure — rapid exploitation

Last updated

Jul 9, 2026

Identifiers

CVE-2021-3377

GHSA-2v5f-23xc-v9qr

References

Advisorysecurity.netapp.com Advisorynvd.nist.gov Fixgithub.com Websecurity.netapp.com Evidencedoyensec.com

Related vulnerabilities

This CVE is medium severity; 8 of 8 other CVEs with CWE-79 are rated as high or higher (top: high).

CVE-2026-91127CVE-2026-77606CVE-2026-84992CVE-2026-77608CVE-2026-77607GHSA-9rcc-pmj8-ffhrCVE-2026-77616CVE-2025-61682Browse all CWE-79

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedJul 9, 2026