CVE-2026-84122 in Firefox
Summary
by MITRE • 09/01/2026
Use-after-free in the Audio/Video component. This vulnerability was fixed in Firefox 155, Firefox ESR 140.15, and Firefox ESR 153.2.
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Analysis
by VulDB Data Team • 09/01/2026
The identified vulnerability represents a critical memory management error classified as an out-of-bounds write resulting from a use-after-free condition within the audio or video processing subsystem of Mozilla Firefox browsers. This specific class of flaw occurs when the application continues to reference a block of memory that has already been deallocated by the system's garbage collector or explicit free operations. In the context of multimedia components, such as those handling WebRTC streams, HTML5 media elements, or plugin-based audio/video rendering engines, rapid allocation and deallocation cycles are common due to the dynamic nature of streaming data buffers. When a pointer retains access to this freed memory region without proper nullification or bounds checking, subsequent read or write operations target arbitrary locations in the process address space rather than valid allocated structures. This discrepancy between logical object lifetime and actual memory availability creates an exploitable race condition that undermines the integrity of the browser's execution environment.
From a technical perspective, use-after-free vulnerabilities are particularly dangerous because they allow attackers to manipulate control flow or leak sensitive information depending on how the freed memory is reallocated. If an attacker can trigger the deallocation of a media buffer and then force the application to write data into that same address space before it is reclaimed by other system processes, they may overwrite critical function pointers, virtual table entries, or security cookies embedded within adjacent heap chunks. This capability effectively bypasses standard memory protection mechanisms such as Data Execution Prevention (DEP) and Address Space Layout Randomization (ASLR), provided the attacker can predict or control the layout of the heap to place malicious shellcode in a location that will be overwritten by their crafted payload. The vulnerability was addressed in Firefox version 155, Firefox Extended Support Release 140.15, and Firefox ESR 153.2 through rigorous code audits that likely involved adding null checks after deallocation events or implementing smart pointer mechanisms to ensure automatic cleanup when object references go out of scope.
The operational impact of this vulnerability is severe, as it enables remote code execution on the victim's machine without requiring any user interaction beyond visiting a maliciously crafted webpage containing specially constructed media content. An attacker could host a website that embeds an audio or video element with malformed headers or corrupted stream data designed to trigger the premature freeing of internal buffers while maintaining active references within JavaScript bindings or native C++ components handling playback state. Upon execution, this leads to arbitrary code execution under the context of the user running the browser, granting full control over the compromised system. This aligns directly with MITRE ATT&CK technique T1203 which covers exploitation for client-side attacks, specifically targeting software applications through memory corruption flaws. Furthermore, it corresponds to Common Weakness Enumeration CWE-416, which defines use-after-free errors as a significant category of input validation and resource management failures in modern software development.
Mitigation strategies primarily involve immediate patching by upgrading the browser infrastructure to include the fixes present in Firefox 155 or the specified ESR branches. Organizations relying on Extended Support Releases should prioritize deploying version 140.15 or 153.2 depending on their specific release track requirements to ensure continuity of security updates without introducing unstable features. In environments where immediate patching is not feasible, network-level controls such as web content filtering proxies can be configured to block access to domains known for hosting exploit kits targeting browser vulnerabilities. Additionally, enabling strict Content Security Policy headers and disabling unnecessary media plugins or JavaScript execution on untrusted sites reduces the attack surface available to potential adversaries attempting to trigger this memory corruption flaw. Regular security awareness training regarding suspicious multimedia links also serves as a secondary defense layer against social engineering tactics often paired with such technical exploits.