Mozilla Firefox and Thunderbird ReadableStream Race Condition Use-After-Free Vulnerability (CVE-2020-6820)
Executive & SecOps Summary
SECOPS ASSESSMENTFirefox < 74.0.1, Firefox ESR < 68.6.1, and Thunderbird < 68.7.0
Remote attackers can execute arbitrary code or escape the browser sandbox via a race condition leading to a Use-After-Free (UAF) state, which has been actively exploited in targeted attacks
Immediately upgrade all installations of Firefox, Firefox ESR, and Thunderbird to the latest patched versions
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
For Debian/Ubuntu-based Linux distributions, update the packages using the APT package manager:
# Update package repositories and upgrade affected applications
sudo apt-get update
sudo apt-get install --only-upgrade firefox thunderbird -yFor RHEL/CentOS/Fedora systems, use the DNF package manager:
sudo dnf upgrade firefox thunderbird -y2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, disable JavaScript execution in Thunderbird to mitigate the primary email-based attack vector:
# Append preference to disable JavaScript in Thunderbird profile
echo "user_pref('javascript.enabled', false);" >> ~/.thunderbird/*.default-release/prefs.js3. Log Audit & IoC Verification Commands
Run the following shell script to audit the installed versions of Firefox and Thunderbird across your endpoints:
#!/bin/bash
# Check installed Firefox version and flag if vulnerable
if command -v firefox >/dev/null 2>&1; then
CURRENT_VER=$(firefox --version | awk '{print $3}')
echo '[*] Current Firefox Version: '$CURRENT_VER
# Compare with patched version 74.0.1
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Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2020-6820) is a critical race condition (CWE-362) discovered in the ReadableStream implementation of Mozilla Firefox and Thunderbird. Due to improper synchronization during asynchronous stream handling, a Use-After-Free (UAF) condition is triggered when the application attempts to access an object that has already been deallocated from memory.
2. Exploitation Mechanics & Attack Scenario
An attacker can exploit this vulnerability by enticing a user to visit a specially crafted malicious website or open a rich-text email in Thunderbird. By precisely timing asynchronous operations on a ReadableStream object, the attacker can manipulate the memory layout, freeing a stream-related object and subsequently reclaiming that memory space with attacker-controlled data. When the browser attempts to reuse the stale pointer, it redirects execution flow to the attacker's payload, bypassing standard security controls.
3. Infrastructure & Business Impact
As a vulnerability listed in the CISA KEV catalog, this flaw represents a high-risk threat vector that has been actively abused in targeted real-world campaigns. Successful exploitation allows remote code execution (RCE) within the context of the browser process. If combined with local privilege escalation (LPE) exploits, it can lead to full endpoint compromise, data exfiltration, and lateral movement within corporate networks.
Official Vendor Advisories & References
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.
Frequently Asked SecOps Questions (CVE-2020-6820)
FAQQ1. How critical is CVE-2020-6820 and is it actively exploited in the wild?
CVE-2020-6820 is rated CVSS 8.1 (HIGH) with a 30-day EPSS exploitation probability of 7.1%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2020-6820?
Primary impacted vendors include mozilla across firefox, firefox_esr, thunderbird. Scope summary: Firefox < 74.0.1, Firefox ESR < 68.6.1, and Thunderbird < 68.7.0
Q3. What temporary mitigations or workarounds exist for CVE-2020-6820 before patching?
[Mitigation] Immediately upgrade all installations of Firefox, Firefox ESR, and Thunderbird to the latest patched versions Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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