CVE-2025-1933: Mozilla Firefox Use-After-Free Vulnerability

CVE-2025-1933 Overview

CVE-2025-1933 is a type confusion vulnerability affecting Mozilla Firefox and Thunderbird browsers. On 64-bit CPUs, when the Just-In-Time (JIT) compiler processes WebAssembly (WASM) i32 return values, the compiled code can inadvertently pick up bits from residual memory. This memory contamination can cause the return values to be interpreted as a different type than intended, potentially leading to security issues including memory corruption or arbitrary code execution.

Critical Impact

This type confusion vulnerability in the JIT compilation of WASM code could allow attackers to exploit memory handling issues on 64-bit systems, potentially leading to code execution or denial of service through maliciously crafted WebAssembly content.

Affected Products

• Mozilla Firefox versions prior to 136
• Mozilla Firefox ESR versions prior to 115.21 and 128.8
• Mozilla Thunderbird versions prior to 136 and 128.8

Discovery Timeline

• March 4, 2025 - CVE-2025-1933 published to NVD
• April 13, 2026 - Last updated in NVD database

Technical Details for CVE-2025-1933

Vulnerability Analysis

The vulnerability resides in the JIT compiler's handling of WebAssembly i32 (32-bit integer) return values on 64-bit CPU architectures. When the JIT compiler generates native code for WASM functions that return i32 values, it fails to properly clear or mask the upper 32 bits of the 64-bit register used to hold the return value. As a result, residual data from previous operations can remain in the upper bits of the register.

When subsequent code interprets this 64-bit register value, the leftover bits can cause the value to be misinterpreted as a different type entirely. This type confusion can lead to incorrect memory access patterns, corrupted pointer calculations, or bypassing of security checks that depend on type integrity.

Root Cause

The root cause is improper handling of 32-bit integer return values in 64-bit register contexts within the JIT compiler. The JIT compiler does not properly zero-extend or sign-extend i32 return values to fill the entire 64-bit register, leaving stale data in the upper 32 bits. This violates the assumption that return values are properly sanitized, creating a type confusion condition when the value is subsequently used.

Attack Vector

An attacker can exploit this vulnerability by crafting malicious WebAssembly code that is loaded and executed in the browser context. The attack is network-based and requires user interaction, such as visiting a malicious website or opening a crafted email in Thunderbird. The malicious WASM code would be designed to manipulate the JIT compilation process in a way that triggers the type confusion, potentially allowing:

• Memory corruption through misinterpreted pointer values
• Bypass of bounds checking through incorrect integer comparisons
• Potential arbitrary code execution if memory corruption can be leveraged

The vulnerability can be triggered through any web content that loads and executes WebAssembly, making it accessible through standard browsing activity.

Detection Methods for CVE-2025-1933

Indicators of Compromise

• Unusual browser crashes or instability when loading WebAssembly content
• Unexpected memory access violations in Firefox or Thunderbird processes
• Detection of malicious WebAssembly modules designed to exploit JIT compilation
• Abnormal JIT compilation behavior or performance anomalies

Detection Strategies

• Monitor browser crash reports for patterns indicating memory corruption in WASM JIT code paths
• Deploy endpoint detection rules that identify suspicious WebAssembly modules with unusual return value patterns
• Implement network-level inspection to detect known malicious WASM payloads
• Review browser telemetry for anomalous JIT compilation errors or failures

Monitoring Recommendations

• Enable enhanced logging for browser processes to capture JIT-related errors
• Implement memory protection monitoring to detect type confusion exploitation attempts
• Deploy SentinelOne Singularity Platform to monitor for behavioral anomalies in browser processes
• Configure alerting for unusual WebAssembly execution patterns or memory access violations

How to Mitigate CVE-2025-1933

Immediate Actions Required

• Update Mozilla Firefox to version 136 or later immediately
• Update Mozilla Firefox ESR to version 115.21 or 128.8 depending on your release channel
• Update Mozilla Thunderbird to version 136 or 128.8
• Verify all managed endpoints have received the security updates
• Consider temporarily disabling WebAssembly in Firefox via about:config if immediate patching is not possible

Patch Information

Mozilla has released security patches addressing this vulnerability in the following versions:

• Firefox 136 - Full patch for standard release
• Firefox ESR 115.21 - Patch for extended support release 115.x branch
• Firefox ESR 128.8 - Patch for extended support release 128.x branch
• Thunderbird 136 - Patch for standard Thunderbird release
• Thunderbird 128.8 - Patch for Thunderbird ESR

Detailed patch information is available in Mozilla Security Advisory MFSA-2025-14, MFSA-2025-15, MFSA-2025-16, MFSA-2025-17, and MFSA-2025-18. The underlying bug is tracked in Mozilla Bug Report #1946004.

Debian users should also reference the Debian LTS Announcement for distribution-specific updates.

Workarounds

• Disable WebAssembly execution in Firefox by setting javascript.options.wasm to false in about:config
• Use browser isolation solutions to contain potential exploitation
• Implement strict Content Security Policy headers to control WebAssembly loading sources
• Consider using browser profiles with restricted permissions for untrusted web content

# Firefox configuration to disable WebAssembly (workaround)
# Navigate to about:config and set:
# javascript.options.wasm = false
# javascript.options.wasm_baselinejit = false
# javascript.options.wasm_ionjit = false

# For enterprise deployment via policies.json:
{
  "policies": {
    "Preferences": {
      "javascript.options.wasm": false
    }
  }
}