CVE-2020-6820: Mozilla Firefox Use-After-Free Vulnerability

CVE-2020-6820 Overview

CVE-2020-6820 is a critical use-after-free vulnerability affecting Mozilla Firefox, Firefox ESR, and Thunderbird. The vulnerability exists in the handling of ReadableStream objects, where a race condition can lead to memory corruption. Mozilla has confirmed that this vulnerability has been actively exploited in targeted attacks in the wild, making immediate patching essential.

Critical Impact

This use-after-free vulnerability has been actively exploited in targeted attacks and is listed in CISA's Known Exploited Vulnerabilities Catalog. Successful exploitation could allow remote attackers to execute arbitrary code with the privileges of the user running the browser.

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-2020-6820 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2020-6820

Vulnerability Analysis

This vulnerability is classified as CWE-362 (Race Condition). The flaw exists in Mozilla's implementation of the Streams API, specifically in how ReadableStream objects are processed. Under certain conditions, concurrent operations on a ReadableStream can trigger a race condition that results in a use-after-free scenario.

A use-after-free vulnerability occurs when memory is accessed after it has been deallocated. In this case, the race condition creates a window where one thread may free memory associated with a ReadableStream while another thread continues to reference that memory. This can lead to memory corruption, application crashes, or arbitrary code execution if an attacker can control the contents of the freed memory region.

The network-based attack vector with no user interaction requirement makes this vulnerability particularly dangerous, though the high attack complexity requirement indicates that successful exploitation requires precise timing to trigger the race condition.

Root Cause

The root cause of CVE-2020-6820 lies in improper synchronization when handling ReadableStream objects within Mozilla's JavaScript engine. The Streams API allows for asynchronous data processing, and the implementation failed to properly serialize access to shared resources during stream operations. This lack of proper locking mechanisms or atomic operations created a race condition that could be exploited under specific timing conditions.

Attack Vector

The vulnerability can be exploited remotely through network-based attacks. An attacker could craft a malicious web page containing JavaScript code designed to manipulate ReadableStream objects in a way that triggers the race condition. When a victim visits the malicious page using a vulnerable version of Firefox, Firefox ESR, or Thunderbird (when rendering HTML email content), the attacker could potentially achieve arbitrary code execution.

The exploitation scenario involves:

1. Crafting JavaScript that creates and manipulates ReadableStream objects
2. Triggering concurrent operations that exploit the race condition timing window
3. Achieving use-after-free by accessing memory that has been prematurely deallocated
4. Potentially controlling the freed memory to achieve code execution

Due to the sensitive nature of this vulnerability and its active exploitation in the wild, detailed technical exploitation specifics are not being disclosed. For additional technical details, refer to Mozilla Bug Report #1626728.

Detection Methods for CVE-2020-6820

Indicators of Compromise

• Unexpected browser crashes or memory corruption errors when visiting specific web pages
• JavaScript errors related to ReadableStream or Streams API in browser console logs
• Anomalous memory access patterns detected by endpoint protection solutions
• Evidence of exploit code delivery through malicious websites or phishing emails

Detection Strategies

• Monitor endpoint telemetry for Firefox, Firefox ESR, and Thunderbird processes exhibiting unusual memory behavior or crashes
• Deploy network intrusion detection rules to identify potential exploit delivery via malicious JavaScript payloads
• Review browser crash reports for patterns consistent with use-after-free exploitation
• Correlate browser version information with known vulnerable versions across the enterprise

Monitoring Recommendations

• Implement browser version auditing to identify instances of vulnerable Firefox, Firefox ESR, and Thunderbird versions
• Enable JavaScript execution logging where feasible to capture potential exploitation attempts
• Configure endpoint detection and response (EDR) solutions to monitor for memory corruption indicators in Mozilla applications
• Establish alerting for connections to known malicious domains associated with targeted attacks

How to Mitigate CVE-2020-6820

Immediate Actions Required

• Update Mozilla Firefox to version 74.0.1 or later immediately
• Update Mozilla Firefox ESR to version 68.6.1 or later
• Update Mozilla Thunderbird to version 68.7.0 or later
• Prioritize patching as this vulnerability is listed in CISA's Known Exploited Vulnerabilities Catalog and has confirmed active exploitation

Patch Information

Mozilla has released security patches addressing this vulnerability. Detailed patch information is available in the official Mozilla Security Advisories:

• Mozilla Security Advisory MFSA-2020-11 - Firefox 74.0.1 and Firefox ESR 68.6.1
• Mozilla Security Advisory MFSA-2020-14 - Thunderbird 68.7.0

Linux distributions have also released corresponding security updates. For Ubuntu systems, see Ubuntu Security Notice USN-4335-1.

Workarounds

• If immediate patching is not possible, consider temporarily restricting JavaScript execution through browser settings or extensions
• Implement network-level controls to block access to known malicious domains
• Configure content security policies on internal web applications to limit exposure
• Consider using alternative browsers until patching can be completed in high-risk environments

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

# Verify Thunderbird version on Linux/macOS
thunderbird --version

# Update Firefox on Ubuntu/Debian
sudo apt update && sudo apt install firefox

# Update Thunderbird on Ubuntu/Debian
sudo apt update && sudo apt install thunderbird