CVE-2025-4918: Mozilla Firefox Memory Corruption Vulnerability

CVE-2025-4918 Overview

CVE-2025-4918 is a critical memory corruption vulnerability affecting Mozilla Firefox and Thunderbird that allows an attacker to perform an out-of-bounds read or write on a JavaScript Promise object. This vulnerability exists in the JavaScript engine's handling of Promise objects, enabling potential exploitation through specially crafted web content.

The vulnerability impacts multiple versions of Firefox, Firefox ESR, and Thunderbird, making it a significant security concern for both individual users and enterprise environments that rely on these applications for web browsing and email communication.

Critical Impact

This out-of-bounds memory access vulnerability could allow attackers to read sensitive data from memory, corrupt application state, or potentially achieve remote code execution through malicious web content. Immediate patching is strongly recommended.

Affected Products

• Mozilla Firefox versions prior to 138.0.4
• Mozilla Firefox ESR versions prior to 128.10.1 and 115.23.1
• Mozilla Thunderbird versions prior to 128.10.2 and 138.0.2

Discovery Timeline

• 2025-05-17 - CVE-2025-4918 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-4918

Vulnerability Analysis

This vulnerability resides in Mozilla's SpiderMonkey JavaScript engine, specifically in the code path responsible for handling JavaScript Promise objects. The out-of-bounds read/write condition (CWE-125) occurs during Promise object processing, where improper bounds checking allows memory access beyond allocated buffer boundaries.

When a malicious web page triggers specific Promise operations, the JavaScript engine fails to properly validate memory boundaries, allowing an attacker to read or write data outside the intended memory region. This type of memory corruption can lead to information disclosure when reading beyond bounds, or memory corruption and potential code execution when writing beyond bounds.

The network-based attack vector means that exploitation requires no authentication or special privileges—a user simply needs to visit a malicious website or, in the case of Thunderbird, open a specially crafted email with embedded content.

Root Cause

The root cause of CVE-2025-4918 lies in insufficient bounds validation within the SpiderMonkey JavaScript engine when processing Promise objects. The engine fails to properly verify that memory access operations remain within allocated buffer boundaries during certain Promise-related operations.

This class of vulnerability typically arises from missing or incorrect boundary checks in performance-critical code paths, where the balance between security and speed may have inadvertently favored the latter. Additional technical details are available in Mozilla Bug Report #1966612.

Attack Vector

The attack vector for CVE-2025-4918 is network-based, requiring an attacker to deliver malicious JavaScript code to the victim's browser. Attack scenarios include:

1. Malicious Website: An attacker hosts a webpage containing JavaScript that triggers the out-of-bounds access
2. Compromised Advertisements: Malicious code injected through ad networks
3. Email-based Attack: For Thunderbird users, opening an email with embedded HTML/JavaScript content
4. Man-in-the-Middle: Injecting malicious scripts into unencrypted web traffic

The vulnerability does not require user interaction beyond normal browsing activity, making drive-by attacks feasible. Successful exploitation could allow attackers to read sensitive memory contents, bypass security mechanisms, or achieve arbitrary code execution within the browser sandbox.

Detection Methods for CVE-2025-4918

Indicators of Compromise

• Unexpected browser crashes or instability when visiting certain websites
• Anomalous memory usage patterns in Firefox or Thunderbird processes
• JavaScript errors or exceptions related to Promise object handling
• Unusual child process spawning from browser processes

Detection Strategies

• Monitor for unusual JavaScript execution patterns targeting Promise objects
• Implement endpoint detection rules for memory access violations in Firefox/Thunderbird processes
• Deploy network-based detection for known exploitation patterns
• Use SentinelOne's behavioral AI to detect anomalous browser process behavior indicative of memory corruption attacks

Monitoring Recommendations

• Enable crash reporting to identify potential exploitation attempts
• Monitor browser process memory allocation patterns for anomalies
• Implement application control policies to alert on suspicious browser behavior
• Review web proxy logs for access to known malicious domains serving exploitation code

How to Mitigate CVE-2025-4918

Immediate Actions Required

• Update Mozilla Firefox to version 138.0.4 or later immediately
• Update Firefox ESR to version 128.10.1 or 115.23.1 or later
• Update Thunderbird to version 128.10.2 or 138.0.2 or later
• Enable automatic updates to receive future security patches promptly
• Consider using SentinelOne endpoint protection for defense-in-depth against exploitation attempts

Patch Information

Mozilla has released security updates addressing this vulnerability. Organizations should prioritize deployment of the following patched versions:

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

• MFSA-2025-36
• MFSA-2025-37
• MFSA-2025-38
• MFSA-2025-40
• MFSA-2025-41

Debian users should refer to the Debian LTS Security Announcement for distribution-specific updates.

Workarounds

• Disable JavaScript in Firefox/Thunderbird as a temporary measure (severely impacts functionality)
• Use browser isolation solutions to contain potential exploitation
• Implement network-level content filtering to block known malicious domains
• Configure Thunderbird to display emails in plain text mode to prevent script execution

# Verify Firefox version from command line
firefox --version

# Verify Thunderbird version from command line  
thunderbird --version

# On Linux, check for available security updates
# Debian/Ubuntu
apt update && apt list --upgradable | grep -E 'firefox|thunderbird'

# RHEL/CentOS/Fedora
dnf check-update firefox thunderbird