CVE-2025-4919: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2025-4919 Overview

CVE-2025-4919 is an out-of-bounds read/write vulnerability affecting Mozilla Firefox and Thunderbird. An attacker can exploit this flaw by confusing array index sizes when manipulating JavaScript objects, potentially leading to arbitrary code execution within the context of the browser or email client.

Critical Impact

This vulnerability enables attackers to perform out-of-bounds memory operations on JavaScript objects, which could result in arbitrary code execution, data theft, or complete system compromise when a user visits a malicious webpage or opens a crafted email.

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

• May 17, 2025 - CVE-2025-4919 published to NVD
• November 3, 2025 - Last updated in NVD database

Technical Details for CVE-2025-4919

Vulnerability Analysis

This vulnerability exists within the JavaScript engine used by Mozilla Firefox and Thunderbird. The flaw stems from improper handling of array index sizes during JavaScript object manipulation. When processing certain JavaScript operations, the engine fails to correctly validate and track array index boundaries, leading to a type confusion condition.

The vulnerability allows an attacker to craft malicious JavaScript code that confuses the internal representation of array indices. This confusion causes the JavaScript engine to miscalculate memory boundaries, enabling read or write operations beyond the intended buffer limits. Successful exploitation requires user interaction—specifically, the victim must visit a malicious webpage (Firefox) or view a specially crafted email (Thunderbird).

The impact is significant as exploitation can lead to arbitrary code execution with the privileges of the browser or email client process, potentially enabling attackers to install malware, steal sensitive information, or pivot to further compromise the system.

Root Cause

The root cause is a type confusion vulnerability (CWE-125: Out-of-bounds Read) in the JavaScript engine's array handling logic. When JavaScript arrays are manipulated with certain index operations, the engine incorrectly interprets the size or type of array indices, causing memory access calculations to reference locations outside the allocated buffer boundaries.

Attack Vector

The attack is network-based and requires user interaction. An attacker must convince a victim to:

1. For Firefox: Navigate to a malicious website containing specially crafted JavaScript code
2. For Thunderbird: Open or preview an email containing malicious JavaScript content

Once the victim triggers the malicious JavaScript, the type confusion occurs during array index processing. The attacker's JavaScript causes the engine to perform memory operations at unintended addresses, enabling data exfiltration through out-of-bounds reads or code execution through out-of-bounds writes that corrupt control flow structures.

The vulnerability mechanism involves exploiting the discrepancy between how the JavaScript engine tracks array index sizes internally versus how it actually accesses memory. By crafting specific array operations, an attacker can force the engine into a state where it uses an incorrect size calculation for subsequent memory accesses.

Detection Methods for CVE-2025-4919

Indicators of Compromise

• Unexpected browser or Thunderbird crashes, particularly when loading JavaScript-heavy web content
• Anomalous memory access patterns or segmentation faults in Firefox/Thunderbird processes
• Suspicious JavaScript code with unusual array manipulation patterns in browser cache or email attachments
• Child processes spawned unexpectedly from Firefox or Thunderbird processes

Detection Strategies

• Monitor for abnormal memory access violations or crash reports from Firefox and Thunderbird processes
• Implement network traffic analysis to detect connections to known malicious domains serving JavaScript exploits
• Deploy endpoint detection rules that identify exploitation attempts targeting Mozilla products
• Review browser and email client logs for repeated crashes or unusual JavaScript execution patterns

Monitoring Recommendations

• Enable crash reporting and analyze Mozilla crash dumps for evidence of memory corruption
• Configure SentinelOne to monitor Firefox and Thunderbird processes for anomalous behavior including unexpected child process creation
• Implement web content filtering to block access to known exploit delivery domains
• Monitor for unusual network connections originating from browser or email client processes

How to Mitigate CVE-2025-4919

Immediate Actions Required

• Update Mozilla Firefox to version 138.0.4 or later immediately
• Update Mozilla Firefox ESR to version 128.10.1 or 115.23.1 depending on your ESR channel
• Update Mozilla Thunderbird to version 128.10.2 or 138.0.2 as applicable
• Consider temporarily disabling JavaScript in high-risk environments until patches are applied

Patch Information

Mozilla has released security updates addressing this vulnerability across all affected product lines. Detailed patch information is available in the following security advisories:

• Mozilla Security Advisory MFSA-2025-36
• Mozilla Security Advisory MFSA-2025-37
• Mozilla Security Advisory MFSA-2025-38
• Mozilla Security Advisory MFSA-2025-40
• Mozilla Security Advisory MFSA-2025-41

Additional updates are available for Debian LTS users via Debian LTS Announcement #24 and Debian LTS Announcement #46.

Technical details regarding the vulnerability are tracked in Mozilla Bug Report #1966614.

Workarounds

• Disable JavaScript execution in Firefox via about:config by setting javascript.enabled to false (note: this significantly impacts web functionality)
• Configure Thunderbird to display emails in plain text mode to prevent JavaScript execution
• Use browser extensions that block JavaScript execution on untrusted sites
• Implement network-level filtering to block known malicious JavaScript payloads

# Firefox: Disable JavaScript via user.js configuration
# Add to Firefox profile directory: ~/.mozilla/firefox/<profile>/user.js
echo 'user_pref("javascript.enabled", false);' >> ~/.mozilla/firefox/*.default/user.js

# Thunderbird: Force plain text email display
# Add to Thunderbird profile directory
echo 'user_pref("mailnews.display.html_as", 1);' >> ~/.thunderbird/*.default/user.js