CVE-2024-7520: Mozilla Firefox WebAssembly RCE Vulnerability

CVE-2024-7520 Overview

CVE-2024-7520 is a type confusion vulnerability in the WebAssembly (Wasm) implementation of Mozilla Firefox, Firefox ESR, and Thunderbird. This memory corruption flaw allows an attacker to potentially achieve arbitrary code execution by crafting malicious WebAssembly content that triggers the type confusion condition. When successfully exploited, this vulnerability could enable complete compromise of the affected browser or email client.

Critical Impact

Remote code execution through malicious WebAssembly content could allow attackers to execute arbitrary code with the privileges of the user running the vulnerable application.

Affected Products

• Mozilla Firefox versions prior to 129
• Mozilla Firefox ESR versions prior to 128.1
• Mozilla Thunderbird versions prior to 128.1

Discovery Timeline

• 2024-08-06 - CVE-2024-7520 published to NVD
• 2025-03-24 - Last updated in NVD database

Technical Details for CVE-2024-7520

Vulnerability Analysis

This vulnerability stems from a type confusion bug (CWE-843) in Mozilla's WebAssembly implementation that can lead to code injection (CWE-94). Type confusion vulnerabilities occur when a program accesses a resource using an incompatible type, causing undefined behavior and potentially allowing attackers to corrupt memory or execute arbitrary code.

In this case, the WebAssembly engine incorrectly handles type information during execution, allowing an attacker to manipulate how the engine interprets data in memory. The vulnerability requires user interaction—specifically, a victim must navigate to a malicious webpage or open malicious content containing the weaponized WebAssembly code.

The network-accessible attack vector with low complexity makes this vulnerability particularly dangerous for browser-based attacks, as exploitation can occur simply by visiting a compromised or malicious website.

Root Cause

The root cause is improper type handling within the WebAssembly execution engine. When processing WebAssembly modules, the engine fails to properly validate type consistency, leading to a type confusion condition. This allows memory to be interpreted as a different type than intended, potentially leading to memory corruption and code execution.

Technical details are tracked in Mozilla Bug Report #1903041.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker could exploit this vulnerability by:

1. Hosting a malicious webpage containing crafted WebAssembly code
2. Luring a victim to visit the page through phishing or other social engineering
3. The malicious WebAssembly triggers the type confusion during execution
4. Successful exploitation results in arbitrary code execution within the browser context

The vulnerability mechanism involves crafting WebAssembly bytecode that exploits the type handling flaw. When the victim's browser processes this malicious content, the type confusion allows the attacker to corrupt memory structures and gain code execution. For detailed technical information, refer to the Mozilla Security Advisory MFSA-2024-33.

Detection Methods for CVE-2024-7520

Indicators of Compromise

• Unusual WebAssembly module loading patterns from untrusted or suspicious domains
• Browser crashes or unexpected terminations potentially indicating exploitation attempts
• Anomalous memory access patterns in Firefox, Firefox ESR, or Thunderbird processes
• Network connections to known malicious infrastructure following browser activity

Detection Strategies

• Monitor browser version deployments across the organization to identify unpatched instances
• Implement web filtering to block access to known malicious domains hosting exploit code
• Deploy endpoint detection and response (EDR) solutions capable of detecting memory corruption exploitation techniques
• Enable browser telemetry and crash reporting to identify potential exploitation attempts

Monitoring Recommendations

• Review browser crash reports for patterns indicating type confusion exploitation
• Monitor for child process spawning from browser processes that may indicate successful code execution
• Track WebAssembly-related events and module loads from external sources
• Implement network traffic analysis to detect post-exploitation communication patterns

How to Mitigate CVE-2024-7520

Immediate Actions Required

• Update Mozilla Firefox to version 129 or later immediately
• Update Mozilla Firefox ESR to version 128.1 or later
• Update Mozilla Thunderbird to version 128.1 or later
• Verify updates have been applied across all managed endpoints

Patch Information

Mozilla has released security updates to address this vulnerability. Organizations should apply the following patches:

• Firefox 129: Addresses CVE-2024-7520 and other security issues documented in MFSA-2024-33
• Firefox ESR 128.1: Security update detailed in MFSA-2024-35
• Thunderbird 128.1: Security update detailed in MFSA-2024-37

Workarounds

• Disable WebAssembly in Firefox by setting javascript.options.wasm to false in about:config (note: this may break some web applications)
• Implement web content filtering to block untrusted WebAssembly content
• Use browser isolation solutions to contain potential exploitation attempts
• Restrict browsing to trusted sites until patches can be applied

# Firefox WebAssembly disable configuration (workaround)
# Navigate to about:config and set:
# javascript.options.wasm = false

# For enterprise deployment via policies.json:
{
  "policies": {
    "Preferences": {
      "javascript.options.wasm": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}