CVE-2025-6424 Overview
CVE-2025-6424 is a use-after-free vulnerability in Mozilla Firefox's FontFaceSet component that can result in a potentially exploitable crash. This memory corruption vulnerability affects multiple versions of Firefox and Thunderbird, allowing attackers to potentially execute arbitrary code through crafted web content that triggers the use-after-free condition in font handling operations.
Critical Impact
This use-after-free vulnerability in FontFaceSet can lead to memory corruption, potentially enabling remote code execution when users visit malicious websites containing specially crafted font-related content.
Affected Products
- Mozilla Firefox < 140
- Mozilla Firefox ESR < 115.25
- Mozilla Firefox ESR < 128.12
- Mozilla Thunderbird < 140
- Mozilla Thunderbird < 128.12
Discovery Timeline
- June 24, 2025 - CVE-2025-6424 published to NVD
- November 3, 2025 - Last updated in NVD database
Technical Details for CVE-2025-6424
Vulnerability Analysis
This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when a program continues to use a pointer after it has been freed. In the context of CVE-2025-6424, the vulnerability exists within the FontFaceSet API, which is part of the CSS Font Loading specification used by browsers to dynamically load and manage web fonts.
The FontFaceSet interface provides a way to track font loading events and manage collections of FontFace objects. When the browser processes font operations through this interface, improper memory management can lead to a condition where freed memory is accessed, resulting in undefined behavior that attackers can potentially exploit for code execution.
Root Cause
The root cause of CVE-2025-6424 lies in improper memory lifecycle management within Firefox's FontFaceSet implementation. During font loading and rendering operations, object references may be released prematurely while other parts of the codebase still hold dangling pointers to the freed memory regions. This creates a use-after-free condition that can be triggered through specific sequences of font-related JavaScript API calls or CSS font loading operations.
Attack Vector
This vulnerability can be exploited remotely through the network attack vector. An attacker can craft a malicious webpage containing specially designed font loading operations that trigger the use-after-free condition. When a victim visits the malicious page, the browser processes the font-related content, potentially leading to memory corruption. Successful exploitation could allow an attacker to execute arbitrary code in the context of the browser process, potentially compromising user data or gaining control of the affected system.
The exploitation requires no user interaction beyond visiting a malicious website, and no authentication or special privileges are needed by the attacker. For detailed technical information, refer to Mozilla Bug Report #1966423 and the associated security advisories.
Detection Methods for CVE-2025-6424
Indicators of Compromise
- Browser crashes or unexpected terminations when visiting specific websites with complex font loading operations
- Memory access violation errors in browser crash reports referencing font-related components
- Suspicious JavaScript code in web traffic attempting to manipulate FontFaceSet objects in unusual patterns
- Abnormal CPU or memory usage patterns during web page rendering
Detection Strategies
- Monitor browser crash telemetry for patterns indicating memory corruption in font handling subsystems
- Deploy network-based intrusion detection rules to identify suspicious font-related JavaScript patterns in web traffic
- Utilize endpoint detection solutions to identify exploitation attempts targeting browser memory corruption vulnerabilities
- Review browser extension and add-on logs for anomalous font loading behavior
Monitoring Recommendations
- Enable enhanced crash reporting in organizational browser deployments to capture detailed crash analytics
- Implement browser isolation technologies for high-risk browsing activities
- Configure SIEM systems to correlate browser crash events with network traffic to potentially malicious domains
- Monitor for known exploitation patterns through threat intelligence feeds referencing CVE-2025-6424
How to Mitigate CVE-2025-6424
Immediate Actions Required
- Update Mozilla Firefox to version 140 or later immediately
- Update Firefox ESR to version 115.25 or 128.12 or later
- Update Mozilla Thunderbird to version 140 or 128.12 or later
- Prioritize patching on systems with direct internet access and those used for web browsing
Patch Information
Mozilla has released security patches addressing this vulnerability across multiple product lines. Organizations should apply the following updates:
- Firefox: Update to version 140 or later
- Firefox ESR: Update to version 115.25 or 128.12 or later
- Thunderbird: Update to version 140 or 128.12 or later
For complete patch details, refer to Mozilla Security Advisory MFSA-2025-51, Mozilla Security Advisory MFSA-2025-52, and Mozilla Security Advisory MFSA-2025-53. Debian users should consult the Debian LTS Announcement for distribution-specific updates.
Workarounds
- Implement browser isolation solutions to contain potential exploitation attempts
- Consider temporarily using alternative browsers for high-risk browsing until patches can be applied
- Deploy content security policies that restrict font loading from untrusted origins where feasible
- Enable strict site isolation features in browser configurations to limit the impact of memory corruption exploits
# Verify Firefox version on Linux systems
firefox --version
# Update Firefox on Debian-based systems
sudo apt update && sudo apt upgrade firefox-esr
# For enterprise deployments, verify ESR channel version
firefox --version | grep -E "(115\.25|128\.12|140)"
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
