CVE-2026-6758: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-6758 Overview

A use-after-free vulnerability exists in the JavaScript WebAssembly component of Mozilla Firefox and Thunderbird. This memory corruption flaw occurs when the application continues to reference memory after it has been freed, potentially allowing attackers to manipulate program execution. The vulnerability was addressed in Firefox 150 and Thunderbird 150.

Critical Impact

This use-after-free vulnerability in the WebAssembly component could allow remote attackers to cause a denial of service condition through network-based attacks without requiring user interaction or authentication.

Affected Products

• Mozilla Firefox (versions prior to 150)
• Mozilla Thunderbird (versions prior to 150)

Discovery Timeline

• 2026-04-21 - CVE CVE-2026-6758 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-6758

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption vulnerability that occurs when a program continues to use a pointer after the memory it references has been deallocated. In the context of the WebAssembly component within Firefox and Thunderbird, this flaw exists in how the JavaScript engine manages memory during WebAssembly execution.

Use-after-free vulnerabilities in browser engines are particularly dangerous because they can be triggered through malicious web content. When a user visits a crafted webpage or opens a malicious email in Thunderbird, the vulnerable code path in the WebAssembly component can be exercised, leading to memory corruption.

Root Cause

The root cause of this vulnerability lies in improper memory lifecycle management within the WebAssembly component of Mozilla's JavaScript engine. The flaw occurs when memory is freed but subsequent operations continue to access the deallocated memory region. This typically happens due to a missing or incorrect reference count, improper object lifecycle tracking, or race conditions in memory management routines.

Attack Vector

This vulnerability can be exploited remotely over the network without requiring any user interaction or authentication. An attacker could craft a malicious webpage containing specially designed WebAssembly code that triggers the use-after-free condition when processed by the vulnerable browser engine.

The attack scenario involves:

1. Attacker creates a malicious website or email containing crafted WebAssembly content
2. Victim visits the webpage in Firefox or opens the email in Thunderbird
3. The WebAssembly component processes the malicious content
4. The use-after-free condition is triggered, causing a denial of service

For detailed technical information about this vulnerability, see Mozilla Bug Report #2013619.

Detection Methods for CVE-2026-6758

Indicators of Compromise

• Unexpected browser crashes or hangs when loading WebAssembly content
• Abnormal memory allocation patterns in Firefox or Thunderbird processes
• Crash reports indicating memory access violations in WebAssembly-related code paths

Detection Strategies

• Monitor for unusual WebAssembly execution patterns in browser processes
• Implement network-based detection for known malicious WebAssembly payloads targeting this vulnerability
• Deploy endpoint detection solutions to identify exploitation attempts through behavioral analysis
• Review browser crash logs for signatures consistent with use-after-free exploitation

Monitoring Recommendations

• Enable enhanced crash reporting in Firefox and Thunderbird to capture exploitation attempts
• Monitor browser process memory usage for anomalous patterns
• Deploy SentinelOne agents to detect and block exploitation attempts in real-time
• Implement web filtering to block access to known malicious domains serving exploit code

How to Mitigate CVE-2026-6758

Immediate Actions Required

• Update Mozilla Firefox to version 150 or later immediately
• Update Mozilla Thunderbird to version 150 or later immediately
• Consider temporarily disabling WebAssembly in browser settings if immediate patching is not possible
• Ensure automatic updates are enabled for all Mozilla products in your environment

Patch Information

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

• Firefox 150 - Contains the fix for this vulnerability. See Mozilla Security Advisory MFSA-2026-30 for details.
• Thunderbird 150 - Contains the fix for this vulnerability. See Mozilla Security Advisory MFSA-2026-33 for details.

Workarounds

• Disable WebAssembly in Firefox by setting javascript.options.wasm to false in about:config
• Implement network-level blocking of potentially malicious WebAssembly content
• Use browser isolation technologies to contain potential exploitation
• Restrict access to untrusted websites until patches can be applied

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

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