CVE-2025-1942: Mozilla Firefox Use-After-Free Vulnerability

CVE-2025-1942 Overview

CVE-2025-1942 is an uninitialized memory use vulnerability affecting Mozilla Firefox and Thunderbird browsers. When the JavaScript String.toUpperCase() method caused a string to become longer than the original, it was possible for uninitialized memory to be incorporated into the result string. This memory safety issue could potentially expose sensitive data from memory or lead to unpredictable application behavior.

Critical Impact
This vulnerability allows network-based attackers to potentially access uninitialized memory contents through specially crafted JavaScript, which could lead to information disclosure, memory corruption, or arbitrary code execution.

Affected Products

Mozilla Firefox versions prior to 136
Mozilla Thunderbird versions prior to 136

Discovery Timeline

March 4, 2025 - CVE-2025-1942 published to NVD
March 28, 2025 - Last updated in NVD database

Technical Details for CVE-2025-1942

Vulnerability Analysis

This vulnerability stems from improper memory handling in Mozilla's JavaScript engine when processing the String.toUpperCase() function. In certain character encodings and locales, converting a string to uppercase can result in a string that is longer than the input. For example, the German lowercase "ß" (eszett) becomes "SS" when uppercased, doubling in length.

When the JavaScript engine allocates memory for the resulting string, it may not properly initialize all memory in cases where the output string exceeds the expected length. This allows uninitialized heap memory to be appended to the result string, potentially exposing sensitive data that was previously stored in that memory region.

The vulnerability is classified as CWE-908 (Use of Uninitialized Resource), indicating that the application reads or uses memory contents before they have been properly initialized with a known value.

Root Cause

The root cause is a memory allocation issue in the string manipulation code path within Mozilla's SpiderMonkey JavaScript engine. When String.toUpperCase() processes strings that expand during case conversion, the buffer allocated for the result may contain uninitialized memory regions. The engine fails to zero-initialize or properly fill the entire allocated buffer before returning the result to JavaScript code, leading to information leakage.

Attack Vector

An attacker could exploit this vulnerability by crafting malicious JavaScript code that triggers the String.toUpperCase() function with carefully selected input strings known to expand during case conversion. By repeatedly calling this function and analyzing the resulting strings, an attacker could potentially:

Extract fragments of sensitive data from uninitialized heap memory
Leak memory addresses useful for bypassing Address Space Layout Randomization (ASLR)
Combine the information disclosure with other vulnerabilities to achieve code execution

The attack can be delivered through malicious web pages visited in Firefox or malicious email content rendered in Thunderbird. No user interaction beyond visiting a malicious page or viewing a crafted email is required for exploitation.

Detection Methods for CVE-2025-1942

Indicators of Compromise

Unusual JavaScript execution patterns involving repeated String.toUpperCase() calls with specific character sets
Memory access violations or crashes in Firefox or Thunderbird related to string processing
Anomalous network traffic suggesting data exfiltration from browser processes
Browser console errors related to string manipulation or memory allocation

Detection Strategies

Monitor for JavaScript execution involving excessive calls to String.toUpperCase() with non-ASCII characters
Deploy endpoint detection solutions capable of identifying memory access anomalies in browser processes
Implement network monitoring to detect potential data exfiltration from browser sessions
Use browser telemetry to identify crashes related to string operations

Monitoring Recommendations

Enable Mozilla crash reporting to identify potential exploitation attempts
Configure SIEM rules to correlate browser crashes with suspicious web activity
Monitor for access to known malicious domains that may be hosting exploit code
Review browser extension activity for suspicious JavaScript injection patterns

How to Mitigate CVE-2025-1942

Immediate Actions Required

Update Mozilla Firefox to version 136 or later immediately
Update Mozilla Thunderbird to version 136 or later immediately
Consider temporarily disabling JavaScript in browsers if immediate patching is not possible
Block access to untrusted websites until patching is complete

Patch Information

Mozilla has released security patches addressing this vulnerability in Firefox 136 and Thunderbird 136. Organizations should prioritize applying these updates across all systems running affected versions. For detailed patch information, refer to:

Workarounds

Disable JavaScript execution in Firefox via about:config by setting javascript.enabled to false (note: this will break most modern websites)
Use browser extensions like NoScript to selectively disable JavaScript on untrusted sites
Configure enterprise policies to restrict access to untrusted web content
Implement network-level filtering to block known malicious domains

bash

# Firefox enterprise policy example (policies.json)
# Place in Firefox installation directory under /distribution/
{
  "policies": {
    "DisableJavaScript": true,
    "WebsiteFilter": {
      "Block": ["*://*.untrusted-domain.com/*"]
    }
  }
}