CVE-2026-2762: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2026-2762 Overview

CVE-2026-2762 is an integer overflow vulnerability in the JavaScript Standard Library component of Mozilla Firefox and Thunderbird. This memory corruption flaw allows remote attackers to potentially execute arbitrary code by exploiting improper handling of integer values during JavaScript operations, leading to memory corruption that could compromise system security.

Critical Impact

This integer overflow vulnerability in the JavaScript Standard Library can be exploited remotely without authentication, potentially allowing attackers to execute arbitrary code, access sensitive information, or cause denial of service conditions on affected Mozilla products.

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-2026-2762 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-2762

Vulnerability Analysis

CVE-2026-2762 is classified as CWE-190: Integer Overflow or Wraparound. The vulnerability exists within the JavaScript Standard Library component where arithmetic operations on integer values can exceed the maximum representable value, causing the integer to wrap around to a small or negative number. This unexpected behavior can lead to incorrect buffer size calculations, memory allocation errors, and subsequent memory corruption.

The attack surface is accessible over the network without requiring authentication or user interaction. An attacker can craft malicious JavaScript code embedded in a web page or email content that triggers the integer overflow condition when processed by the vulnerable browser or email client. Successful exploitation could result in complete compromise of confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of this vulnerability is improper validation of integer arithmetic operations within the JavaScript Standard Library. When processing certain JavaScript constructs, the affected code fails to check whether arithmetic operations will result in values that exceed the bounds of the integer data type. This lack of bounds checking allows integer overflow conditions to occur, which subsequently corrupts memory structures and can lead to exploitable conditions.

Attack Vector

The vulnerability can be exploited through network-based attacks by delivering malicious content to users running vulnerable versions of Firefox or Thunderbird. Attack scenarios include:

• Hosting a malicious webpage containing crafted JavaScript that triggers the overflow when visited by a victim
• Sending specially crafted HTML emails via Thunderbird that execute JavaScript in the email rendering context
• Injecting malicious JavaScript through compromised or malicious advertisements

The exploitation mechanism leverages the integer overflow to corrupt memory allocations in the JavaScript engine, potentially allowing an attacker to overwrite critical memory regions and gain control of program execution flow.

Detection Methods for CVE-2026-2762

Indicators of Compromise

• Unexpected crashes or abnormal termination of Firefox or Thunderbird processes
• Anomalous memory consumption patterns in browser or email client processes
• Detection of suspicious JavaScript execution in network traffic analysis
• Unusual child process spawning from Firefox or Thunderbird executables

Detection Strategies

• Monitor for process crashes with signatures matching JavaScript engine memory corruption
• Implement network-based detection rules for known exploit patterns targeting CVE-2026-2762
• Deploy endpoint detection and response (EDR) solutions to identify exploitation attempts through behavioral analysis
• Review browser console logs for JavaScript errors that may indicate exploitation attempts

Monitoring Recommendations

• Enable enhanced logging for Firefox and Thunderbird crash reports and analyze for patterns
• Monitor network traffic for connections to known malicious domains hosting exploit code
• Implement browser isolation technologies to contain potential exploitation attempts
• Configure security information and event management (SIEM) rules to alert on suspicious browser behavior

How to Mitigate CVE-2026-2762

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
• Consider temporarily disabling JavaScript in high-risk environments until patches can be applied

Patch Information

Mozilla has released security patches addressing this vulnerability across multiple product lines. The following security advisories provide detailed patch information:

• Mozilla Security Advisory MFSA-2026-13
• Mozilla Security Advisory MFSA-2026-15
• Mozilla Security Advisory MFSA-2026-16
• Mozilla Security Advisory MFSA-2026-17

Technical details about the underlying issue can be found in Mozilla Bug Report #2011649.

Workarounds

• Disable JavaScript execution in Firefox via about:config by setting javascript.enabled to false (note: this will break many websites)
• Use browser extensions such as NoScript to selectively block JavaScript execution on untrusted sites
• Configure Thunderbird to display emails in plain text mode to prevent HTML/JavaScript rendering
• Implement network-level filtering to block known malicious JavaScript patterns
• Deploy browser isolation solutions to sandbox potentially dangerous web content

# Firefox configuration workaround via user.js
# Add to Firefox profile directory user.js file
user_pref("javascript.enabled", false);

# Thunderbird plain text mode configuration
# Add to Thunderbird profile directory user.js file
user_pref("mailnews.display.prefer_plaintext", true);
user_pref("mailnews.display.disallow_mime_handlers", 1);