CVE-2020-26950: Mozilla Firefox Use-After-Free Flaw

CVE-2020-26950 Overview

CVE-2020-26950 is a use-after-free vulnerability in Mozilla Firefox, Firefox ESR, and Thunderbird that occurs when the MCallGetProperty opcode is emitted with unmet assumptions during JavaScript JIT compilation. This memory corruption flaw can be exploited by an attacker to achieve arbitrary code execution within the context of the browser process.

Critical Impact
This use-after-free vulnerability allows remote attackers to execute arbitrary code through specially crafted web content, potentially leading to complete system compromise when a user visits a malicious webpage.

Affected Products

Mozilla Firefox versions prior to 82.0.3
Mozilla Firefox ESR versions prior to 78.4.1
Mozilla Thunderbird versions prior to 78.4.2

Discovery Timeline

2020-12-09 - CVE-2020-26950 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-26950

Vulnerability Analysis

This vulnerability resides in Firefox's SpiderMonkey JavaScript engine, specifically within the JIT (Just-In-Time) compilation process. The flaw occurs when the MCallGetProperty opcode is emitted under certain circumstances where its underlying assumptions are not met. When these assumptions fail during runtime, the engine continues to operate on memory that has already been freed, creating a classic use-after-free condition.

The vulnerability is particularly dangerous because it can be triggered remotely through malicious JavaScript code embedded in a webpage or email (in the case of Thunderbird). An attacker can craft JavaScript that triggers the specific code path where the JIT compiler makes incorrect assumptions about object properties, leading to the premature freeing of memory that is subsequently accessed.

Root Cause

The root cause lies in the SpiderMonkey JIT compiler's handling of property access operations. The MCallGetProperty opcode is designed to optimize property getter calls, but under specific circumstances, the compiler emits this opcode without properly verifying that all required conditions are met. When the getter has write side effects, it can invalidate the assumptions made by the JIT compiler, causing the engine to access memory that has already been deallocated.

Attack Vector

The attack vector for CVE-2020-26950 is network-based and requires user interaction. An attacker must convince a victim to visit a malicious website or open a crafted email (for Thunderbird users). The malicious content contains JavaScript code specifically designed to trigger the vulnerable code path in the JIT compiler.

The exploitation process involves:

Crafting JavaScript that triggers the MCallGetProperty opcode emission with unmet assumptions
Manipulating memory layout to control the contents of the freed memory region
Triggering the use-after-free to gain control over execution flow
Leveraging the corrupted state to execute arbitrary code

Technical details and exploitation mechanisms are documented in the Packet Storm Security exploit published for this vulnerability.

Detection Methods for CVE-2020-26950

Indicators of Compromise

Unexpected browser crashes or abnormal termination of Firefox, Firefox ESR, or Thunderbird processes
Unusual memory consumption patterns in browser processes indicating heap spray attempts
Evidence of code execution originating from browser processes accessing unauthorized system resources
Suspicious JavaScript files or web content in browser cache directories

Detection Strategies

Monitor for anomalous behavior in Firefox, Firefox ESR, and Thunderbird processes including unexpected child process spawning
Implement network-based detection for known exploitation patterns targeting this vulnerability
Deploy endpoint detection rules to identify memory corruption exploitation attempts in Mozilla products
Review web proxy logs for access to known malicious domains serving exploit code

Monitoring Recommendations

Enable crash reporting and analyze crash dumps for patterns consistent with use-after-free exploitation
Implement application whitelisting to detect unauthorized code execution from browser processes
Monitor for unusual network connections initiated by browser processes that may indicate successful exploitation
Configure SIEM alerts for security events related to Mozilla browser crash patterns

How to Mitigate CVE-2020-26950

Immediate Actions Required

Update Firefox to version 82.0.3 or later immediately
Update Firefox ESR to version 78.4.1 or later
Update Thunderbird to version 78.4.2 or later
Consider temporarily disabling JavaScript in high-risk environments until patching is complete

Patch Information

Mozilla has released security patches addressing CVE-2020-26950 through their standard update channels. The patches are documented in Mozilla Security Advisory MFSA-2020-49. Additional technical details about the vulnerability can be found in Mozilla Bug Report #1675905.

Organizations should ensure automatic updates are enabled for all Mozilla products or deploy the patched versions through enterprise software distribution systems.

Workarounds

Disable JavaScript execution in Firefox by navigating to about:config and setting javascript.enabled to false (note: this will break most modern websites)
Use browser isolation technologies to contain potential exploitation attempts
Implement network-level filtering to block known malicious content targeting this vulnerability
Consider using alternative browsers for sensitive operations until patching is complete

bash

# Verify Firefox version from command line
firefox --version
# Expected output should show version 82.0.3 or higher

# For Firefox ESR
firefox-esr --version
# Expected output should show version 78.4.1 or higher

# For Thunderbird
thunderbird --version
# Expected output should show version 78.4.2 or higher