CVE-2025-1012: Mozilla Firefox Use-After-Free Vulnerability

CVE-2025-1012 Overview

CVE-2025-1012 is a use-after-free vulnerability in Mozilla Firefox and Thunderbird caused by a race condition during concurrent delazification of JavaScript source. The flaw resides in the SpiderMonkey JavaScript engine, where parallel parsing threads can access memory that has already been freed by another thread. Mozilla addressed the issue in Firefox 135, Firefox ESR 115.20, Firefox ESR 128.7, Thunderbird 128.7, and Thunderbird 135. The vulnerability is tracked under [CWE-416] (Use After Free) and requires user interaction, such as visiting a crafted web page.

Critical Impact
Successful exploitation can lead to memory corruption and potential arbitrary code execution within the browser process, enabling attackers to compromise victim systems through malicious web content.

Affected Products

Mozilla Firefox versions prior to 135
Mozilla Firefox ESR versions prior to 115.20 and 128.7
Mozilla Thunderbird versions prior to 128.7 and 135

Discovery Timeline

2025-02-04 - CVE-2025-1012 published to NVD
2026-04-13 - Last updated in NVD database

Technical Details for CVE-2025-1012

Vulnerability Analysis

The vulnerability stems from a thread-safety defect in SpiderMonkey's delazification logic. Delazification is the process where lazily parsed JavaScript functions are fully compiled the first time they execute. SpiderMonkey supports concurrent delazification, allowing helper threads to parse functions in parallel with the main thread. The race condition occurs when two threads attempt to delazify or access the same script structure without proper synchronization.

One thread can free memory backing script data while another thread still holds a reference to it. The dangling pointer is then dereferenced, producing a use-after-free condition. Memory corruption of this class can lead to arbitrary code execution within the content process.

Root Cause

The root cause is missing or insufficient locking around shared script state during concurrent parsing operations. The defect is described in Mozilla Bug Report #1939710. Concurrent access to the same lazy script object lacks the atomic guarantees needed to prevent one thread from freeing structures still in use by another.

Attack Vector

Exploitation requires a victim to load attacker-controlled JavaScript, typically by visiting a malicious or compromised website. The attacker crafts JavaScript designed to trigger the race window between concurrent delazification operations. Because the timing window is narrow, the attack complexity is high, but the network reachability and lack of required privileges make it broadly applicable to any user of an unpatched browser or mail client that renders remote content.

No verified public proof-of-concept code is available for this vulnerability. Refer to the Mozilla Security Advisory MFSA-2025-07 for vendor technical details.

Detection Methods for CVE-2025-1012

Indicators of Compromise

Unexpected crashes of firefox.exe or thunderbird.exe content processes with signatures indicating heap corruption or invalid memory access in SpiderMonkey functions.
Crash reports referencing js:: namespace frames, DelazifyTask, or related script compilation routines.
Outbound connections from browser processes to unfamiliar hosts immediately following a crash event.

Detection Strategies

Inventory installed Firefox and Thunderbird versions across managed endpoints and flag hosts running builds older than Firefox 135, Firefox ESR 115.20, Firefox ESR 128.7, Thunderbird 128.7, or Thunderbird 135.
Monitor for anomalous child process creation from browser processes, which is a common post-exploitation signal for browser memory corruption flaws.
Correlate browser crash telemetry with subsequent process injection, suspicious file writes, or persistence creation on the same host.

Monitoring Recommendations

Forward Windows Error Reporting and macOS CrashReporter data to a central log platform for analysis of repeated SpiderMonkey faults.
Enable browser telemetry and crash submission to identify clusters of crashes that may indicate exploitation attempts.
Track network traffic from browser processes for connections to newly registered or low-reputation domains hosting JavaScript payloads.

How to Mitigate CVE-2025-1012

Immediate Actions Required

Update Firefox to version 135 or later, Firefox ESR to 115.20 or 128.7 or later, and Thunderbird to 128.7 or 135 or later on all endpoints.
Prioritize patching for users who routinely browse untrusted content or open HTML email with remote content enabled.
Audit third-party Firefox or Thunderbird redistributions, including Debian LTS packages, against the fixed versions.

Patch Information

Mozilla released fixes in Firefox 135, Firefox ESR 115.20, Firefox ESR 128.7, Thunderbird 128.7, and Thunderbird 135. Vendor advisories are published as MFSA-2025-07, MFSA-2025-08, MFSA-2025-09, MFSA-2025-10, and MFSA-2025-11. Debian users should apply updates referenced in Debian LTS Announcement #5 and Debian LTS Announcement #6.

Workarounds

Disable JavaScript for untrusted sites using browser configuration policies until patches are deployed.
Disable remote content loading in Thunderbird to reduce exposure from HTML email rendering.
Enforce strict update policies through enterprise management tools to prevent users from running outdated browser builds.

bash

# Configuration example: enforce Firefox auto-update via policies.json
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "JavascriptBlockedForUrls": ["http://*/*", "https://untrusted.example/*"]
  }
}