CVE-2026-2765: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-2765 Overview

CVE-2026-2765 is a use-after-free vulnerability affecting the JavaScript Engine component in Mozilla Firefox and Thunderbird. This memory corruption flaw (CWE-416) occurs when the JavaScript engine incorrectly handles memory after it has been freed, potentially allowing attackers to execute arbitrary code within the context of the affected application through maliciously crafted web content or email messages.

Critical Impact
This use-after-free vulnerability in Mozilla's JavaScript engine could allow remote attackers to achieve arbitrary code execution simply by convincing a user to visit a malicious webpage or view a malicious email, requiring no authentication or user interaction beyond normal browsing.

Affected Products

Mozilla Firefox versions prior to 148
Mozilla Firefox ESR versions prior to 140.8
Mozilla Thunderbird versions prior to 148
Mozilla Thunderbird ESR versions prior to 140.8

Discovery Timeline

2026-02-24 - CVE-2026-2765 published to NVD
2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-2765

Vulnerability Analysis

This use-after-free vulnerability resides in Mozilla's SpiderMonkey JavaScript engine, a critical component responsible for executing JavaScript code in both Firefox and Thunderbird. Use-after-free vulnerabilities occur when a program continues to reference memory after it has been deallocated, creating a dangerous condition where the freed memory may be reallocated for different purposes.

In the context of CVE-2026-2765, the JavaScript engine fails to properly track object lifetimes under certain conditions, leading to a scenario where memory that has been freed is subsequently accessed. When an attacker crafts specific JavaScript code that triggers this condition, they can potentially manipulate the freed memory region to gain control over program execution flow.

The network-based attack vector makes this vulnerability particularly dangerous, as exploitation requires no prior authentication and can be triggered through normal user interaction with web content. An attacker could embed malicious JavaScript in a compromised website, malicious advertisement, or phishing email to trigger the vulnerability.

Root Cause

The root cause of CVE-2026-2765 is improper memory management in the JavaScript Engine component. The vulnerability stems from a failure to properly synchronize object lifetime tracking with garbage collection operations, resulting in dangling pointer references. When these dangling pointers are subsequently dereferenced, the attacker-controlled data in the reallocated memory region can be used to hijack execution flow.

Additional technical details are available in the Mozilla Bug Report #2013562.

Attack Vector

The attack vector for CVE-2026-2765 is network-based and requires an attacker to deliver malicious JavaScript content to a victim. Exploitation scenarios include:

Web-based attacks: An attacker hosts malicious JavaScript on a controlled website or injects it into legitimate sites through XSS or compromise
Email-based attacks: For Thunderbird users, attackers can craft malicious emails containing JavaScript that triggers the vulnerability when the email is rendered
Malvertising: Malicious advertisements containing the exploit code can be served through advertising networks to reach victims on otherwise trusted websites

The vulnerability can be exploited remotely without authentication. Once the use-after-free condition is triggered, an attacker can potentially achieve arbitrary code execution with the privileges of the browser or email client process.

Detection Methods for CVE-2026-2765

Indicators of Compromise

Unexpected Firefox or Thunderbird crashes, particularly when visiting specific websites or opening certain emails
Anomalous memory allocation patterns in browser processes, especially rapid allocation/deallocation cycles
Suspicious network connections originating from browser processes to unknown command and control infrastructure
Evidence of shellcode execution or unexpected child processes spawned by Firefox or Thunderbird

Detection Strategies

Deploy endpoint detection solutions capable of monitoring for use-after-free exploitation patterns and memory corruption indicators
Implement network intrusion detection rules to identify known exploit delivery mechanisms targeting Mozilla products
Monitor for suspicious JavaScript behavior patterns including heap spraying techniques commonly used in browser exploitation
Enable crash reporting and analyze crash dumps for signs of exploitation attempts

Monitoring Recommendations

Monitor browser process behavior for anomalous memory access patterns or unexpected code execution
Implement application-level logging to track JavaScript execution anomalies
Configure security information and event management (SIEM) systems to correlate browser crashes with network activity
Review Mozilla security advisories regularly for updated indicators and detection guidance

How to Mitigate CVE-2026-2765

Immediate Actions Required

Update Mozilla Firefox to version 148 or later immediately
Update Mozilla Firefox ESR to version 140.8 or later
Update Mozilla Thunderbird to version 148 or later
Update Mozilla Thunderbird ESR to version 140.8 or later
Prioritize patching for systems exposed to untrusted web content or email

Patch Information

Mozilla has released security patches addressing CVE-2026-2765 across all affected product lines. Organizations should apply the following updates:

Firefox: Update to version 148 or later
Firefox ESR: Update to version 140.8 or later
Thunderbird: Update to version 148 or later
Thunderbird ESR: Update to version 140.8 or later

Detailed patch information is available in the official Mozilla Security Advisories:

Workarounds

Disable JavaScript temporarily in Firefox via about:config by setting javascript.enabled to false (note: this will significantly impact web functionality)
For Thunderbird, ensure remote content is disabled by default and avoid opening emails from untrusted sources
Implement network-level content filtering to block known malicious domains
Use browser isolation technologies to contain potential exploitation attempts

bash

# Firefox/Thunderbird configuration to restrict JavaScript (temporary workaround)
# Navigate to about:config and set:
# javascript.enabled = false

# For enterprise deployments, create a policies.json file:
# Location: /usr/lib/firefox/distribution/policies.json (Linux)
# Location: C:\Program Files\Mozilla Firefox\distribution\policies.json (Windows)

cat << 'EOF' > policies.json
{
  "policies": {
    "DisableJavaScript": true,
    "DisableFirefoxAccounts": true,
    "DisableFormHistory": true
  }
}
EOF