---
id: "CVE-2020-6820"
kind: "vulnerability"
aliases: ["CVE-2020-6820"]
title: "Mozilla Firefox and Thunderbird: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability"
severity: "high"
cvss: 8.1
epss: 0.07063
exploited: true
status: "open"
category: "runtime"
products: [{"name": "Mozilla Firefox and Thunderbird", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["desktop", "exploited", "firefox", "linux", "macos", "runtime", "thunderbird", "windows"]
not_affected_if: ["unknown"]
published_at: "2020-04-24T16:15:13+00:00"
first_seen_at: "2026-10-23T19:11:53+00:00"
updated_at: "2026-10-23T21:57:34+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2020-6820", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2020-6820", "first_seen_at": "2026-10-23T19:13:43+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2020-6820", "first_seen_at": "2026-10-23T21:57:34+00:00"}]
---

# Mozilla Firefox and Thunderbird: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

Mozilla Firefox and Thunderbird have a race condition in how they handle a ReadableStream, which leads to a use-after-free memory bug. The sources say the impact is unspecified, but the bug is listed as actively exploited in the wild.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Mozilla Firefox and Thunderbird affected ? fixed ?

## How to check

Check the installed Firefox or Thunderbird version under Help > About; unknown for command-line checks.

## Fix

Update Firefox and Thunderbird to the current vendor release, as CISA directs (apply updates per vendor instructions); unknown for exact fixed version.

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2020-6820
- 2026-10-23T19:13:43+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2020-6820
- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2020-6820

## From the sources

Mozilla Firefox and Thunderbird contain a race condition vulnerability when handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.

CISA required action: Apply updates per vendor instructions.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_