CVE-2020-6819: Mozilla Firefox Use-After-Free Vulnerability

CVE-2020-6819 Overview

CVE-2020-6819 is a use-after-free vulnerability affecting Mozilla Firefox and Thunderbird that occurs due to a race condition in the nsDocShell destructor. Under certain conditions, when the destructor runs, improper synchronization can lead to memory being accessed after it has been freed, potentially allowing attackers to execute arbitrary code in the context of the browser. Mozilla has confirmed that this vulnerability was actively exploited in targeted attacks in the wild, making it a critical security concern for all affected users.

Critical Impact

This vulnerability was actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities catalog. Attackers could potentially achieve remote code execution through malicious web content, compromising user systems without additional user interaction beyond visiting a crafted webpage.

Affected Products

• Mozilla Firefox versions prior to 74.0.1
• Mozilla Firefox ESR versions prior to 68.6.1
• Mozilla Thunderbird versions prior to 68.7.0

Discovery Timeline

• 2020-04-24 - CVE-2020-6819 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2020-6819

Vulnerability Analysis

This vulnerability resides in Mozilla's nsDocShell component, which is responsible for managing document loading and navigation within the browser. The flaw involves a race condition that occurs during the destruction of nsDocShell objects. Race conditions occur when the behavior of software depends on the relative timing of events, such as the order in which threads are scheduled to run. In this case, improper synchronization during the destruction process allows for a use-after-free condition where memory that has already been deallocated can be subsequently accessed.

The use-after-free vulnerability is particularly dangerous because it can lead to memory corruption. When memory is freed but pointers to that memory are still in use, an attacker can potentially manipulate the contents of the freed memory region. If the attacker can control what data is placed in the freed memory location, they may be able to redirect program execution to attacker-controlled code.

Root Cause

The root cause of CVE-2020-6819 lies in the improper handling of concurrent operations during the nsDocShell destructor execution. The race condition (CWE-362) occurs when multiple threads or asynchronous operations interact with the nsDocShell object without proper synchronization primitives. This leads to a situation where the destructor may free memory while another operation still holds a reference to that memory, creating a classic use-after-free (CWE-416) scenario.

The timing-dependent nature of race conditions makes this vulnerability particularly insidious, as it may not manifest consistently during normal testing but can be reliably triggered by a motivated attacker who crafts specific conditions to exploit the timing window.

Attack Vector

This vulnerability is exploitable over the network, requiring no authentication or user privileges beyond visiting a malicious webpage. An attacker could craft a malicious website or inject malicious content into a compromised legitimate site that triggers the race condition in the victim's browser.

The attack flow involves manipulating browser operations to create the precise timing conditions needed to trigger the race condition during nsDocShell destruction. Once the use-after-free is triggered, the attacker can potentially achieve arbitrary code execution within the browser's process context. This could lead to complete compromise of the browser session, theft of sensitive information, or further exploitation of the victim's system.

The vulnerability was exploited in targeted attacks, suggesting sophisticated threat actors developed reliable exploitation techniques for this flaw. For technical details on the vulnerability, see the Mozilla Bug Report #1620818 and Mozilla Security Advisory MFSA-2020-11.

Detection Methods for CVE-2020-6819

Indicators of Compromise

• Unusual browser crashes or instability, particularly when navigating away from pages or closing tabs
• Unexpected browser behavior following visits to suspicious or unknown websites
• Memory corruption artifacts or crash dumps indicating nsDocShell destructor issues
• Browser process spawning unexpected child processes or establishing unusual network connections

Detection Strategies

• Monitor browser crash reports for patterns indicating nsDocShell-related memory corruption
• Deploy endpoint detection and response (EDR) solutions capable of identifying use-after-free exploitation patterns
• Implement network monitoring to detect connections to known malicious infrastructure associated with exploitation campaigns
• Utilize behavioral analysis to identify anomalous browser process activities following web navigation

Monitoring Recommendations

• Enable verbose logging for browser crashes and analyze crash dumps for nsDocShell involvement
• Deploy memory protection technologies that can detect and prevent use-after-free exploitation
• Monitor for indicators of post-exploitation activity such as unusual process creation from browser processes
• Track browser version deployments across the organization to identify vulnerable installations

How to Mitigate CVE-2020-6819

Immediate Actions Required

• Immediately update Mozilla Firefox to version 74.0.1 or later
• Update Mozilla Firefox ESR to version 68.6.1 or later
• Update Mozilla Thunderbird to version 68.7.0 or later
• Verify that automatic updates are enabled for all Mozilla products across the organization

Patch Information

Mozilla released emergency out-of-band patches to address this vulnerability due to its active exploitation in the wild. The fixes are included in:

• Firefox 74.0.1 - Released as an emergency security update
• Firefox ESR 68.6.1 - Extended Support Release security patch
• Thunderbird 68.7.0 - Email client security update

For detailed patch information, refer to Mozilla Security Advisory MFSA-2020-11 for Firefox and Mozilla Security Advisory MFSA-2020-14 for Thunderbird. Linux users can also reference the Ubuntu Security Notice USN-4335-1 for distribution-specific guidance.

Workarounds

• Restrict browsing to trusted sites only until patches can be applied
• Consider using alternative browsers temporarily if immediate patching is not possible
• Implement network-level controls to block access to known malicious domains
• Enable browser sandboxing features to limit the impact of potential exploitation

# Verify Firefox version on Linux/macOS
firefox --version

# Force Firefox update check (Linux)
firefox --check-update

# For enterprise deployments, verify ESR version
firefox-esr --version