CVE-2026-84118 in Firefox
Summary
by MITRE • 09/01/2026
Use-after-free in the JavaScript: GC component. This vulnerability was fixed in Firefox 155 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 within the garbage collection subsystem of the Mozilla Gecko engine, specifically affecting its JavaScript execution environment. As a use-after-free flaw, this defect occurs when the application attempts to access or manipulate memory that has already been deallocated by the system's automatic memory manager. In complex web browsers like Firefox, the JavaScript engine frequently allocates and frees objects during runtime as part of normal garbage collection cycles designed to optimize performance and prevent memory leaks. When a pointer remains valid in code logic but points to freed memory due to a race condition or incorrect reference counting, any subsequent read or write operation targets an unpredictable location in the heap. This scenario creates a high-risk environment where the integrity of the program's execution flow is compromised, potentially leading to crashes, data corruption, or more severe security breaches depending on how the attacker manipulates the freed memory region.
From a technical perspective, this flaw falls under the Common Weakness Enumeration category CWE-416, which defines use-after-free errors as a subset of improper resource management vulnerabilities. The vulnerability likely stems from a failure to properly nullify pointers after deallocation or a lapse in reference counting that allows an object to be collected while still being referenced by active JavaScript code paths. During the garbage collection process, if the engine incorrectly determines that an object is no longer needed and reclaims its memory space without updating all existing references within the execution context, subsequent operations on those stale pointers will access invalid memory addresses. This can result in arbitrary code execution if an attacker can carefully craft a malicious webpage or script to control the contents of the freed memory block before it is reused by another allocation request. By overwriting the freed memory with controlled data structures such as function pointers or object headers, an adversary may hijack the program counter and execute arbitrary commands within the context of the browser process.
The operational impact of this vulnerability extends beyond simple application instability to significant security risks for end-users browsing untrusted web content. If exploited successfully, a remote attacker could leverage this flaw to achieve code execution on the victim's machine with the privileges of the current user. This capability allows for the installation of malware, exfiltration of sensitive data such as cookies and session tokens stored in browser memory, or further pivoting into internal network resources if the browser is configured to access local services. The vulnerability affects both standard releases like Firefox 155 and extended support release versions like Firefox ESR 153.2, indicating that the flaw existed across multiple maintenance branches before being addressed. Users relying on these browsers for secure web interactions are exposed to potential compromise until they apply the provided patches which correct the memory management logic within the garbage collection component.
Mitigation strategies primarily involve immediate software updates as the vendor has released fixes in Firefox 155 and Firefox ESR 153.2 that address the underlying reference counting or pointer invalidation issues. Organizations should prioritize patching their browser fleets to ensure all endpoints are protected against this specific memory corruption technique. In addition to updating, implementing strict content security policies can help mitigate some attack vectors by restricting the types of scripts and resources that browsers execute from untrusted origins. Furthermore, enabling advanced browser sandboxing features ensures that even if an exploit is attempted, its impact is contained within a restricted environment rather than affecting the host operating system directly. Security teams should also monitor for indicators of compromise related to memory corruption events in browser logs and consider deploying endpoint detection solutions capable of identifying anomalous behavior patterns associated with heap spraying or use-after-free exploitation attempts as defined by MITRE ATT&CK techniques involving memory manipulation and code injection.