CVE-2025-1931: Mozilla Firefox Use-After-Free Vulnerability

CVE-2025-1931 Overview

A use-after-free vulnerability has been identified in the content process side of WebTransport connections in Mozilla Firefox and Thunderbird. This memory corruption flaw occurs when handling WebTransport connections, leading to a potentially exploitable crash condition. WebTransport is a web API that enables low-latency, bidirectional communication between clients and servers, and improper memory management in this component can have serious security implications.

Critical Impact

This use-after-free vulnerability in WebTransport could allow attackers to cause browser crashes or potentially achieve code execution through memory corruption, affecting both Firefox and Thunderbird users.

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

• 2025-03-04 - CVE-2025-1931 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2025-1931

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a class of memory corruption vulnerabilities that occurs when a program continues to reference memory after it has been freed. In the context of CVE-2025-1931, the flaw exists within the content process handling of WebTransport connections in Mozilla's browser engine.

WebTransport connections involve complex state management between the browser's content process and network components. The use-after-free condition can be triggered during the lifecycle management of these connections, where an object may be accessed after its memory has been deallocated. This creates a scenario where the application operates on stale memory references, which could contain arbitrary data or be manipulated by an attacker.

The vulnerability can be exploited remotely via network without requiring user interaction, making it particularly concerning for web browsing scenarios. While the primary impact demonstrated is a denial of service through crashing the browser, use-after-free vulnerabilities are historically known to be exploitable for arbitrary code execution under the right conditions.

Root Cause

The root cause of this vulnerability lies in improper memory lifecycle management within the WebTransport connection handling code in the content process. When a WebTransport connection is being processed, certain memory resources may be freed prematurely while references to those resources still exist elsewhere in the codebase. This leads to a dangling pointer situation where subsequent operations attempt to access memory that has already been released back to the system.

Attack Vector

The attack vector for CVE-2025-1931 is network-based, meaning an attacker could exploit this vulnerability by hosting malicious web content that triggers the vulnerable WebTransport code path. When a victim visits a malicious webpage or opens a crafted email (in the case of Thunderbird), the WebTransport connection handling could be manipulated to trigger the use-after-free condition.

The vulnerability can be exploited without any authentication or special privileges, and does not require user interaction beyond visiting a webpage. This makes drive-by attacks a viable exploitation method, where simply browsing to a malicious site could trigger the vulnerability.

Detection Methods for CVE-2025-1931

Indicators of Compromise

• Unexpected browser or email client crashes, particularly when loading pages with WebTransport functionality
• Crash reports indicating memory access violations in content process components
• Browser process termination with error signatures related to freed memory access
• Increased frequency of about:crashes entries in Firefox installations

Detection Strategies

• Monitor system logs for unexpected Firefox or Thunderbird process crashes
• Implement endpoint detection rules that identify abnormal browser process termination patterns
• Deploy network monitoring to detect connections to known malicious domains that may be serving exploit code
• Utilize crash analysis tools to identify memory corruption signatures consistent with use-after-free exploitation

Monitoring Recommendations

• Enable crash reporting in Mozilla products to collect diagnostic information
• Review network traffic logs for suspicious WebTransport connection patterns
• Implement SentinelOne endpoint protection to detect exploitation attempts and memory corruption attacks
• Monitor for unusual child process behavior from Firefox or Thunderbird processes

How to Mitigate CVE-2025-1931

Immediate Actions Required

• Update Mozilla Firefox to version 136 or later immediately
• Update Mozilla Firefox ESR to version 115.21 or 128.8 or later
• Update Mozilla Thunderbird to version 136 or 128.8 or later
• Prioritize patching on systems that access untrusted web content

Patch Information

Mozilla has released security patches addressing this vulnerability across multiple product versions. The fix has been incorporated into Firefox 136, Firefox ESR 115.21, Firefox ESR 128.8, Thunderbird 136, and Thunderbird 128.8. Organizations should consult the official Mozilla Security Advisories for detailed patch information:

• Mozilla Security Advisory MFSA-2025-14
• Mozilla Security Advisory MFSA-2025-15
• Mozilla Security Advisory MFSA-2025-16
• Mozilla Security Advisory MFSA-2025-17
• Mozilla Security Advisory MFSA-2025-18

Debian users should also review the Debian LTS Security Announcement for distribution-specific guidance.

Workarounds

• Consider disabling WebTransport functionality if not required for business operations until patches can be applied
• Implement network-level filtering to restrict access to untrusted web content on sensitive systems
• Use browser isolation or containerization technologies to limit the impact of potential exploitation
• Deploy application whitelisting to restrict browser access to approved sites only

# Verify Firefox version on Linux/macOS
firefox --version

# Verify Thunderbird version
thunderbird --version

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