CVE-2026-2804: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-2804 Overview

A use-after-free vulnerability has been identified in the JavaScript WebAssembly component of Mozilla Firefox and Thunderbird. This memory corruption flaw occurs when the application accesses memory that has already been freed, potentially leading to unexpected behavior including data corruption or exploitation by malicious actors. The vulnerability requires user interaction through a network-based attack vector, typically by enticing a user to visit a malicious website or open crafted content.

Critical Impact

Attackers can exploit this use-after-free condition in the WebAssembly component to potentially gain unauthorized read or write access to memory, which could lead to information disclosure or partial integrity compromise in affected Firefox and Thunderbird versions.

Affected Products

• Mozilla Firefox versions prior to 148
• Mozilla Thunderbird versions prior to 148

Discovery Timeline

• 2026-02-24 - CVE CVE-2026-2804 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-2804

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue where a program continues to use a pointer after the memory it references has been deallocated. In the context of Mozilla's WebAssembly implementation, this flaw exists within the JavaScript engine's handling of WebAssembly objects.

WebAssembly (Wasm) is a binary instruction format designed for efficient execution in web browsers. The complexity of managing memory between JavaScript and WebAssembly contexts creates opportunities for dangling pointer conditions. When WebAssembly modules interact with JavaScript garbage collection, improper synchronization of object lifetimes can result in references to freed memory being retained and subsequently dereferenced.

The network-based attack vector with required user interaction indicates that exploitation would typically occur when a victim navigates to a malicious webpage containing crafted WebAssembly code or opens a malicious email in Thunderbird with embedded web content.

Root Cause

The root cause is improper memory management in the JavaScript WebAssembly component. Specifically, a reference to a WebAssembly-related object is retained after the underlying memory has been freed by the garbage collector or explicit deallocation routines. When this stale reference is subsequently accessed, the application operates on memory that may have been reallocated for other purposes, leading to undefined behavior.

This type of vulnerability often arises from:

• Race conditions between JavaScript execution and WebAssembly memory operations
• Incorrect reference counting of shared WebAssembly objects
• Improper handling of WebAssembly module instantiation and destruction cycles

Attack Vector

An attacker could exploit this vulnerability by creating a specially crafted webpage or email containing malicious WebAssembly code designed to trigger the use-after-free condition. The attack requires user interaction—the victim must navigate to the malicious site or open the crafted content in Thunderbird.

The exploitation mechanism involves manipulating the timing of WebAssembly object allocation and deallocation to create a dangling pointer, then triggering a read or write operation through that invalid reference. Successful exploitation could allow an attacker to leak sensitive information from browser memory or potentially corrupt application state.

For detailed technical information, refer to the Mozilla Bug Report #2013584.

Detection Methods for CVE-2026-2804

Indicators of Compromise

• Unexpected crashes or instability in Firefox or Thunderbird, particularly when loading WebAssembly content
• Anomalous memory access patterns or crash dumps indicating heap corruption
• Unusual WebAssembly module loading activity from untrusted sources

Detection Strategies

• Monitor for Firefox or Thunderbird crash reports with stack traces involving WebAssembly or JavaScript JIT components
• Implement browser content security policies to restrict WebAssembly execution from untrusted origins
• Deploy endpoint detection solutions capable of identifying heap spray or memory corruption exploitation attempts
• Review browser logs for repeated crashes when accessing specific websites

Monitoring Recommendations

• Enable crash reporting and analyze dumps for patterns consistent with use-after-free exploitation
• Monitor network traffic for connections to known malicious domains serving WebAssembly exploit content
• Implement application allowlisting to ensure only approved Firefox and Thunderbird versions are deployed

How to Mitigate CVE-2026-2804

Immediate Actions Required

• Update Mozilla Firefox to version 148 or later immediately
• Update Mozilla Thunderbird to version 148 or later immediately
• Ensure automatic updates are enabled for both applications across all managed endpoints
• Consider temporarily restricting access to untrusted websites until patching is complete

Patch Information

Mozilla has addressed this vulnerability in Firefox 148 and Thunderbird 148. Detailed patch information and security advisories are available from Mozilla:

• Mozilla Security Advisory MFSA-2026-13 - Firefox security update
• Mozilla Security Advisory MFSA-2026-16 - Thunderbird security update

Organizations should prioritize deployment of these updates through their software management systems. The updates include fixes for the use-after-free condition in the WebAssembly component.

Workarounds

• Disable WebAssembly in Firefox by setting javascript.options.wasm to false in about:config (may break legitimate web applications)
• Implement network-level filtering to block known malicious WebAssembly content
• Use browser isolation solutions to contain potential exploitation attempts
• Enable enhanced tracking protection and strict content blocking in Firefox

# Firefox configuration workaround (about:config)
# Navigate to about:config in Firefox address bar
# Search for: javascript.options.wasm
# Set value to: false
# Note: This will disable WebAssembly entirely and may break legitimate web applications