CVE-2026-84120 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 115.40, 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 due to use-after-free conditions within the audio and video processing subsystems of Mozilla Firefox. This specific class of flaws occurs when the application attempts to access or modify memory that has already been deallocated by the system's garbage collector or explicit cleanup routines. In complex multimedia frameworks, objects representing media streams, codecs, or playback buffers are frequently created and destroyed in rapid succession as content loads, pauses, seeks, or ends. If a reference to such an object persists after its underlying memory has been freed, any subsequent operation targeting that stale pointer results in undefined behavior. The fix implemented in Firefox version 155, along with the corresponding Extended Support Release versions 115.40, 140.15, and 153.2, addresses this lifecycle mismatch by ensuring proper reference counting or smart pointer management to prevent access to freed memory regions.
From a technical perspective, use-after-free vulnerabilities are particularly dangerous because they allow an attacker who can trigger the specific media processing path to control what data is written into the reclaimed memory space. If the application allocates new objects in that same memory location before executing the stale write operation, the attacker effectively gains arbitrary read and write capabilities within the browser process context. This capability bypasses standard sandboxing mechanisms because the exploit operates entirely within the legitimate address space of the Firefox binary. The vulnerability is categorized under CWE-416, which defines use-after-free as a distinct weakness in software design where pointers are not properly invalidated after memory release. It also aligns with MITRE ATT&CK technique T1203, specifically the exploitation phase for client-side code execution via browser vulnerabilities.
The operational impact of this vulnerability is severe, potentially leading to remote code execution when a user visits a maliciously crafted webpage containing specially constructed audio or video elements. An attacker could design an HTML page that triggers the specific media playback sequence required to reproduce the race condition between object deallocation and subsequent access. Successful exploitation would allow the injection and execution of arbitrary machine code on the victim's system with the privileges of the browser process. This can lead to full system compromise, data exfiltration, or installation of persistent malware payloads. The severity is further amplified by the ubiquity of web-based media consumption, making this a high-value target for drive-by download attacks and targeted phishing campaigns that rely on embedded multimedia content rather than traditional script injection methods.
Mitigation strategies primarily involve immediate patching to the latest stable release as indicated in the vendor advisory. Organizations should enforce automated update policies across all endpoints running Firefox to ensure that version 155 or later is deployed without delay. For environments where immediate upgrading is not feasible, network-level controls such as web proxies can be configured to block access to known malicious domains hosting exploit kits or suspicious media files. Additionally, enabling strict content security policies and disabling unnecessary browser features like autoplay for untrusted sites can reduce the attack surface by limiting the contexts in which vulnerable code paths are triggered. Security teams should also monitor endpoint detection systems for anomalous memory allocation patterns consistent with heap spraying techniques often used to stabilize use-after-free exploits before finalizing remote code execution attempts.