Mozilla Firefox and Thunderbird nsDocShell Use-After-Free Remote Code Execution Vulnerability (CVE-2020-6819)
Executive & SecOps Summary
SECOPS ASSESSMENTFirefox < 74.0.1, Firefox ESR < 68.6.1, and Thunderbird < 68.7.0
A race condition leading to a Use-After-Free (UAF) allows remote attackers to execute arbitrary code or cause a Denial of Service (DoS) via targeted web-based exploits
Immediately update all affected Mozilla browser and mail client installations 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, execute the following commands to update the packages to the latest secure versions.
# Update package repositories and upgrade Firefox and Thunderbird
sudo apt-get update && sudo apt-get install --only-upgrade -y firefox thunderbird2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, restrict outbound access to untrusted websites and enforce application control policies to prevent unauthorized browser execution.
# Temporarily restrict execution permissions of Firefox to administrators only
sudo chmod 700 /usr/bin/firefox3. Log Audit & IoC Verification Commands
Use the following script to audit the installed versions of Firefox and verify if the system is running a vulnerable version.
# Bash script to check installed Firefox version against the vulnerability threshold
INST_VER=$(firefox --version 2>/dev/null | awk '{print $3}')
if [ -n '$INST_VER' ]; then
echo 'Detected Firefox Version: '$INST_VER
# Compare version with the patched release (74.0.1)
dpkg --compare-versions '$INST_VER' 'lt' '74.0.1' && echo '[ALERT] Vulnerable Firefox version detected!' || echo '[OK] Firefox is patched.'
else
echo 'Firefox is not detected in the standard PATH.'
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Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability is a Use-After-Free (UAF) flaw caused by a race condition within the nsDocShell destructor in the Mozilla browser engine. Under specific conditions, when the destructor is executed, the memory allocated for the nsDocShell object is freed, but dangling pointers to this memory space are not properly cleared, allowing subsequent invalid memory access.
2. Exploitation Mechanics & Attack Scenario
An attacker can exploit this vulnerability by hosting a specially crafted web page containing malicious JavaScript. By manipulating the lifecycle of nsDocShell objects, the script triggers a race condition during asynchronous operations. Once the object is freed, the attacker reclaims the freed memory space with controlled payloads (heap spraying), hijacking the control flow to execute arbitrary shellcode within the context of the browser process.
3. Infrastructure & Business Impact
This vulnerability has been actively exploited in the wild in targeted attacks (Zero-Day). Successful exploitation can lead to complete compromise of the user's endpoint. In an enterprise environment, this could serve as an initial entry point for lateral movement, credential theft, and subsequent ransomware deployment or data exfiltration.
Official Vendor Advisories & References
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.
Frequently Asked SecOps Questions (CVE-2020-6819)
FAQQ1. How critical is CVE-2020-6819 and is it actively exploited in the wild?
CVE-2020-6819 is rated CVSS 8.1 (HIGH) with a 30-day EPSS exploitation probability of 3.0%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2020-6819?
Primary impacted vendors include mozilla across firefox, firefox_esr, thunderbird. Scope summary: Firefox prior to 74.0.1, Firefox ESR prior to 68.6.1, and Thunderbird prior to 68.7.0 across all operating systems
Q3. What temporary mitigations or workarounds exist for CVE-2020-6819 before patching?
[Mitigation] Immediately update all affected Mozilla browser and mail client installations 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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