CVE-2024-0743: Mozilla Firefox DOS Vulnerability

CVE-2024-0743 Overview

CVE-2024-0743 is an Unchecked Return Value vulnerability (CWE-252) affecting Mozilla Firefox, Firefox ESR, and Thunderbird. The vulnerability exists in the TLS handshake code where an unchecked return value could lead to a potentially exploitable crash. This flaw allows remote attackers to trigger a denial of service condition by crafting malicious TLS handshake responses that cause the browser to crash unexpectedly.

Critical Impact

Remote attackers can crash affected Mozilla applications through malformed TLS handshake responses, causing denial of service without requiring user interaction or authentication.

Affected Products

• Mozilla Firefox versions prior to 122
• Mozilla Firefox ESR versions prior to 115.9
• Mozilla Thunderbird versions prior to 115.9

Discovery Timeline

• January 23, 2024 - CVE-2024-0743 published to NVD
• November 3, 2025 - Last updated in NVD database

Technical Details for CVE-2024-0743

Vulnerability Analysis

This vulnerability stems from improper error handling in Mozilla's TLS handshake implementation. During the TLS handshake process, the affected code fails to verify the return value of a critical function call, leading to an unhandled error condition. When an attacker sends specially crafted TLS handshake data that triggers this code path, the application enters an undefined state and crashes.

The vulnerability is classified under CWE-252 (Unchecked Return Value), which occurs when software does not check the return value from a method or function, allowing unexpected states and conditions to be reached. In security-critical code such as TLS handshake processing, failing to validate return values can have severe consequences.

Root Cause

The root cause is the absence of proper return value validation in the TLS handshake code. When certain TLS operations fail or return unexpected values, the code continues execution as if the operation succeeded. This leads to subsequent operations working with invalid data or state, ultimately causing a crash. The vulnerability represents a classic case of defensive programming failure in network protocol handling code.

Attack Vector

The attack vector is network-based and requires no user interaction or authentication. An attacker can exploit this vulnerability by:

1. Setting up a malicious TLS server or performing a man-in-the-middle attack
2. Waiting for a vulnerable Firefox, Firefox ESR, or Thunderbird client to initiate a TLS connection
3. Sending specially crafted TLS handshake responses that trigger the unchecked return value condition
4. Causing the application to crash, resulting in denial of service

The attack can be triggered when users browse to malicious websites or when Thunderbird connects to a compromised email server. The vulnerability allows for reliable crash conditions that could be used to disrupt user sessions or potentially be chained with other vulnerabilities for more severe attacks.

Detection Methods for CVE-2024-0743

Indicators of Compromise

• Unexpected crashes of Firefox, Firefox ESR, or Thunderbird during TLS connection establishment
• Crash reports indicating failures in TLS handshake processing code
• Repeated connection failures to specific external hosts that may be serving malicious TLS responses
• Application crash logs showing segmentation faults or access violations during network operations

Detection Strategies

• Monitor for abnormal crash patterns in Mozilla applications across the organization
• Implement network monitoring to detect anomalous TLS handshake traffic patterns
• Review application crash dumps for indicators of TLS-related failures
• Deploy endpoint detection solutions to identify repeated browser crashes on specific systems

Monitoring Recommendations

• Enable Mozilla crash reporting to track potential exploitation attempts
• Configure SIEM rules to alert on multiple Firefox or Thunderbird crashes within short time windows
• Monitor network traffic for malformed TLS handshake packets targeting internal systems
• Establish baseline crash rates and alert on significant deviations

How to Mitigate CVE-2024-0743

Immediate Actions Required

• Update Firefox to version 122 or later immediately
• Update Firefox ESR to version 115.9 or later
• Update Thunderbird to version 115.9 or later
• Prioritize patching on systems that handle sensitive communications or are exposed to untrusted networks

Patch Information

Mozilla has released security patches addressing this vulnerability. The fixes are included in:

• Firefox 122 - Released with comprehensive TLS handshake fixes
• Firefox ESR 115.9 - Extended Support Release with backported fix
• Thunderbird 115.9 - Email client patch with the same fix

For detailed patch information, refer to the Mozilla Security Advisory MFSA-2024-01, Mozilla Security Advisory MFSA-2024-13, and Mozilla Security Advisory MFSA-2024-14. Linux distribution users should also check the relevant Debian LTS announcements for backported security updates.

Technical details about the vulnerability can be found in Mozilla Bug Report #1867408.

Workarounds

• Avoid browsing to untrusted websites on unpatched systems until updates can be applied
• Consider using alternative browsers temporarily if immediate patching is not possible
• Implement network-level TLS inspection to filter potentially malicious handshake responses
• Restrict Firefox and Thunderbird usage to trusted network environments until patched

# Check current Firefox version on Linux
firefox --version

# Update Firefox on Debian-based systems
sudo apt update && sudo apt upgrade firefox-esr

# Update Thunderbird on Debian-based systems  
sudo apt update && sudo apt upgrade thunderbird