CVE-2025-1011: Mozilla Firefox WebAssembly RCE Vulnerability

CVE-2025-1011 Overview

CVE-2025-1011 is a code injection vulnerability affecting Mozilla Firefox and Thunderbird products stemming from a bug in the WebAssembly (Wasm) code generation engine. The flaw occurs during the compilation and execution of WebAssembly bytecode, where improper handling of certain code patterns can lead to memory corruption and application crashes. An attacker could potentially leverage this crash condition to achieve arbitrary code execution within the context of the browser or email client.

WebAssembly is a binary instruction format designed for stack-based virtual machines, widely used for high-performance web applications. The vulnerability exists in how Mozilla's JavaScript engine (SpiderMonkey) generates native machine code from WebAssembly modules, creating an exploitable condition when processing maliciously crafted Wasm content.

Critical Impact

Attackers can potentially achieve remote code execution by exploiting the WebAssembly code generation bug through malicious web content or email attachments, compromising user systems without additional authentication requirements.

Affected Products

• Mozilla Firefox versions prior to 135
• Mozilla Firefox ESR versions prior to 128.7
• Mozilla Thunderbird versions prior to 135 and ESR versions prior to 128.7

Discovery Timeline

• February 4, 2025 - CVE-2025-1011 published to NVD
• November 3, 2025 - Last updated in NVD database

Technical Details for CVE-2025-1011

Vulnerability Analysis

The vulnerability resides within Mozilla's WebAssembly implementation, specifically in the just-in-time (JIT) compilation pathway that transforms Wasm bytecode into executable native code. When the code generator encounters specific instruction sequences or module configurations, it can produce invalid machine code or fail to properly manage memory allocations during the compilation process.

This type of vulnerability in JIT compilers is particularly dangerous because it can corrupt the execution state of the program, potentially allowing an attacker to redirect execution flow. The network-based attack vector means exploitation requires only that a victim visit a malicious website or open a crafted email containing WebAssembly content. No prior authentication or special privileges are required for exploitation.

The impact of successful exploitation includes complete compromise of confidentiality, integrity, and availability of the affected system, as the attacker could execute arbitrary code with the same privileges as the browser or email client process.

Root Cause

The root cause is classified as CWE-94 (Improper Control of Generation of Code), indicating that the WebAssembly code generation component fails to properly validate or handle certain input conditions during the compilation phase. This allows specially crafted WebAssembly modules to trigger undefined behavior in the code generator, leading to memory corruption that can be weaponized for code execution.

Attack Vector

The attack can be executed remotely over a network through the following scenarios:

1. Web-based Attack: An attacker hosts or injects malicious WebAssembly code on a website. When a user visits the page using a vulnerable Firefox browser, the Wasm module is loaded and compiled by SpiderMonkey, triggering the vulnerability.

2. Email-based Attack: For Thunderbird, an attacker sends an HTML email containing embedded WebAssembly content or links to malicious Wasm modules. Opening or previewing the email can trigger the vulnerable code path.

3. Advertisement Networks: Malicious actors could inject weaponized WebAssembly code into advertising networks, potentially affecting users across multiple legitimate websites.

The vulnerability requires user interaction (visiting a page or opening an email) but no additional authentication or privileges, making it suitable for widespread phishing and watering hole attacks.

Detection Methods for CVE-2025-1011

Indicators of Compromise

• Unexpected Firefox or Thunderbird crashes, particularly when visiting unfamiliar websites or opening emails
• Presence of WebAssembly modules with unusual or obfuscated structure in browser cache or temporary files
• Anomalous child process spawning from Firefox or Thunderbird parent processes
• Suspicious network connections originating from browser processes following Wasm content loading

Detection Strategies

• Monitor for crashes and exceptions in libxul.so or xul.dll components related to WebAssembly compilation
• Deploy endpoint detection rules to identify exploitation attempts targeting Wasm JIT compilation
• Analyze web traffic for suspicious WebAssembly module delivery, particularly from untrusted sources
• Review browser telemetry and crash reports for patterns indicating exploitation attempts

Monitoring Recommendations

• Enable enhanced crash reporting in Firefox and Thunderbird deployments to capture exploitation attempts
• Implement network-level inspection for WebAssembly content (application/wasm MIME type) from untrusted domains
• Configure endpoint protection to monitor SpiderMonkey JIT compilation activities
• Establish baseline metrics for normal WebAssembly usage patterns to identify anomalous behavior

How to Mitigate CVE-2025-1011

Immediate Actions Required

• Update Mozilla Firefox to version 135 or later immediately
• Update Mozilla Firefox ESR to version 128.7 or later
• Update Mozilla Thunderbird to version 135 or ESR 128.7 or later
• Prioritize patching on systems with internet-facing browsers or email clients

Patch Information

Mozilla has released security patches addressing CVE-2025-1011 in the following versions:

Additional patches are available for Debian LTS distributions via the Debian LTS Announcements. The technical details of the fix can be tracked in Mozilla Bug Report #1936454.

Workarounds

• Disable WebAssembly in Firefox by navigating to about:config and setting javascript.options.wasm to false
• Block WebAssembly content at the network perimeter for high-security environments
• Use browser security policies to restrict Wasm execution to trusted domains only
• Consider disabling JavaScript entirely on untrusted sites as a temporary measure (this will also disable Wasm)

# Firefox enterprise policy to disable WebAssembly (policies.json)
{
  "policies": {
    "Preferences": {
      "javascript.options.wasm": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}