CVE-2025-8027: Mozilla Firefox Use-After-Free Vulnerability

CVE-2025-8027 Overview

CVE-2025-8027 is an uninitialized memory use vulnerability affecting Mozilla Firefox and Thunderbird on 64-bit platforms. The flaw resides in the IonMonkey Just-In-Time (JIT) compiler, which incorrectly writes only 32 bits of the 64-bit return value space on the stack. When the Baseline-JIT subsequently reads the entire 64 bits, it may access uninitialized memory content, potentially leading to information disclosure.

Critical Impact

This vulnerability could allow remote attackers to potentially leak sensitive memory contents through crafted JavaScript, compromising user confidentiality on affected 64-bit systems running vulnerable Firefox or Thunderbird versions.

Affected Products

• Mozilla Firefox versions prior to 141
• Mozilla Firefox ESR versions prior to 115.26, 128.13, and 140.1
• Mozilla Thunderbird versions prior to 141, 128.13, and 140.1

Discovery Timeline

• July 22, 2025 - CVE-2025-8027 published to NVD
• November 3, 2025 - Last updated in NVD database

Technical Details for CVE-2025-8027

Vulnerability Analysis

This vulnerability stems from an inconsistency between two JIT compilation tiers in Mozilla's SpiderMonkey JavaScript engine. On 64-bit platforms, the IonMonkey-JIT compiler generates code that only writes 32 bits to the stack when handling return values that should occupy a full 64-bit space. When execution transitions between JIT tiers, the Baseline-JIT reads the complete 64-bit value from the stack, including the 32 bits that were never properly initialized by IonMonkey.

The uninitialized memory read (CWE-457) creates a window for information leakage. Since the upper 32 bits contain whatever residual data was previously stored in that stack location, an attacker could potentially craft JavaScript that triggers this code path repeatedly, sampling uninitialized stack memory contents. This could expose sensitive data such as heap pointers, object addresses, or other memory artifacts that could aid in bypassing security mitigations like ASLR.

The network-based attack vector requires user interaction—typically visiting a malicious webpage or opening a crafted email in Thunderbird. The vulnerability specifically affects 64-bit platforms where the architectural difference between 32-bit and 64-bit value handling creates the exploitable condition.

Root Cause

The root cause is improper handling of 64-bit return values in the IonMonkey-JIT compiler. The compiler incorrectly assumed 32-bit writes were sufficient for certain return value types on the stack, failing to account for the Baseline-JIT's expectation of fully initialized 64-bit values. This architectural mismatch between the two JIT tiers results in uninitialized memory being read when the engine switches between compilation tiers during JavaScript execution.

Attack Vector

The vulnerability is exploitable remotely through network-delivered content. An attacker could craft malicious JavaScript code designed to trigger the vulnerable code path in the JIT compiler. The attack scenario involves:

1. A victim visits a malicious webpage containing specially crafted JavaScript
2. The JavaScript triggers JIT compilation through IonMonkey
3. Subsequent execution causes a tier transition to Baseline-JIT
4. The Baseline-JIT reads uninitialized stack memory
5. The attacker's JavaScript can potentially observe the leaked memory contents

The exploitation mechanism involves carefully constructed JavaScript that maximizes the probability of reading sensitive uninitialized data. For Thunderbird users, the attack surface extends to HTML email content with embedded JavaScript, though many email security policies restrict JavaScript execution by default.

Detection Methods for CVE-2025-8027

Indicators of Compromise

• Unusual JavaScript execution patterns involving repeated JIT tier transitions
• Anomalous memory access patterns in browser processes on 64-bit systems
• Suspicious network connections from Firefox or Thunderbird following exposure to untrusted web content
• JavaScript code attempting to probe or exfiltrate memory contents through timing or side-channel techniques

Detection Strategies

• Deploy endpoint detection rules to monitor for known exploitation patterns targeting browser JIT vulnerabilities
• Implement network-based detection for websites known to host browser exploitation frameworks
• Monitor for suspicious JavaScript payloads that contain JIT-spraying or type confusion patterns
• Use browser telemetry and crash reports to identify potential exploitation attempts

Monitoring Recommendations

• Enable enhanced logging for browser processes to capture JIT compilation events and anomalies
• Implement application behavior monitoring to detect unusual memory access patterns in Firefox and Thunderbird
• Configure SIEM rules to correlate browser process anomalies with network activity to untrusted domains
• Regularly review browser crash reports for patterns indicative of memory corruption or uninitialized memory access

How to Mitigate CVE-2025-8027

Immediate Actions Required

• Update Mozilla Firefox to version 141 or later immediately
• Update Mozilla Firefox ESR to version 115.26, 128.13, or 140.1 depending on your ESR track
• Update Mozilla Thunderbird to version 141, 128.13, or 140.1 depending on your release channel
• Enable automatic updates for all Mozilla products to ensure timely patching of future vulnerabilities

Patch Information

Mozilla has released patches across multiple product versions to address this vulnerability. Security advisories MFSA-2025-56, MFSA-2025-57, MFSA-2025-58, MFSA-2025-59, MFSA-2025-61, MFSA-2025-62, and MFSA-2025-63 provide detailed information about the fixes. The underlying bug is tracked in Mozilla Bug Report #1968423. Debian users should refer to the Debian LTS Security Announcement for distribution-specific patch availability.

Workarounds

• Disable JavaScript execution in Firefox and Thunderbird if immediate patching is not possible (note: this significantly impacts functionality)
• Configure content blockers to prevent execution of untrusted JavaScript from unknown sources
• Use browser isolation solutions to contain potential exploitation attempts in sandboxed environments
• For Thunderbird users, ensure HTML and JavaScript rendering is disabled in email preferences

# Verify Firefox version from command line
firefox --version

# Verify Thunderbird version from command line
thunderbird --version

# Check for available updates on Debian-based systems
sudo apt update && apt list --upgradable | grep -E "(firefox|thunderbird)"