CVE-2020-12420 in Firefox
Summary
by MITRE
When trying to connect to a STUN server, a race condition could have caused a use-after-free of a pointer, leading to memory corruption and a potentially exploitable crash. This vulnerability affects Firefox ESR < 68.10, Firefox < 78, and Thunderbird < 68.10.0.
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Analysis
by VulDB Data Team • 05/05/2025
This vulnerability represents a critical memory safety issue that emerged within the networking stack of Mozilla's browser and email client applications. The flaw occurs during the process of establishing connections to Session Traversal Utilities for NAT (STUN) servers, which are essential components for establishing peer-to-peer connections in VoIP and WebRTC applications. The race condition manifests when multiple threads attempt to access shared memory resources simultaneously, creating a scenario where one thread may free memory while another thread attempts to reference it. This specific use-after-free condition falls under the CWE-416 vulnerability category, which specifically addresses the use of freed memory in software applications. The vulnerability is particularly concerning because it affects the core networking functionality that these applications rely upon for secure communication.
The technical implementation of this flaw involves the improper synchronization of memory management operations within the STUN connection handling code. When a STUN server connection attempt is initiated, the application creates and manages memory pointers that are subject to concurrent access patterns. The race condition occurs during the transition phase where one execution thread completes its operations and frees the memory pointer while another thread is still in the process of accessing that same memory location. This creates a scenario where subsequent memory operations may corrupt adjacent memory regions or cause the application to crash unpredictably. The vulnerability's exploitability is heightened because it can be triggered through normal network operations, making it particularly dangerous in real-world scenarios where users may unknowingly initiate STUN server connections.
The operational impact of this vulnerability extends beyond simple application crashes to potentially enable remote code execution under specific conditions. When the use-after-free occurs, it can corrupt critical memory structures that govern the application's execution flow, potentially allowing attackers to manipulate program execution or inject malicious code. This vulnerability affects multiple Mozilla products including Firefox Extended Support Release versions prior to 68.10, regular Firefox versions before 78, and Thunderbird versions before 68.10.0, indicating a widespread exposure across the Mozilla ecosystem. The vulnerability's presence in these applications means that any user who initiates a connection to a STUN server, which occurs frequently in modern web applications that utilize WebRTC functionality, could be exposed to potential exploitation.
Mitigation strategies for this vulnerability primarily focus on immediate software updates and patches provided by Mozilla. Organizations should prioritize updating all affected versions of Firefox, Firefox ESR, and Thunderbird to their respective patched releases to eliminate the risk of exploitation. Additionally, network administrators should consider implementing monitoring solutions to detect unusual STUN server connection patterns that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper memory management and thread synchronization in security-critical applications, aligning with ATT&CK technique T1059.007 for application execution and T1203 for exploitation for execution. Security teams should also implement process isolation measures to limit the potential impact should exploitation occur, as well as conduct regular vulnerability assessments to identify similar race condition patterns in other network components and libraries that may be susceptible to similar issues.