CVE-2026-100757 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the Widget component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical memory management error classified as an out-of-bounds write within the Widget component of Mozilla Firefox browsers. This specific flaw is categorized under CWE-416, which describes situations where a program uses a pointer to memory that has already been freed, leading to undefined behavior and potential security compromises. In this instance, the issue stems from improper handling of object lifecycles during widget rendering or interaction events. When certain user interactions trigger state changes in the Widget component, the underlying system fails to correctly manage the reference counts for associated data structures. Consequently, memory that should have been retained is prematurely deallocated while still being referenced by active processes within the browser engine.
The technical mechanism behind this vulnerability involves a race condition or logic error where the application attempts to access or modify memory segments after they have been returned to the system allocator. This out-of-bounds write allows an attacker who can craft malicious web content containing specially designed JavaScript and HTML elements to overwrite adjacent heap metadata or other critical data structures in memory. Such overwrites are particularly dangerous because they do not necessarily cause immediate application crashes, which might alert users or security monitoring systems. Instead, the corruption occurs silently until a subsequent operation triggers the use of the corrupted pointer, potentially leading to arbitrary code execution with the privileges of the user running the browser.
From an operational impact perspective, this vulnerability poses a severe risk to end-users browsing untrusted websites. An attacker could host a malicious webpage that exploits this flaw when visited by the victim. Upon successful exploitation, the attacker gains the ability to execute arbitrary commands on the affected system. This effectively bypasses standard security boundaries such as same-origin policies and sandboxing mechanisms designed to isolate web content from the local operating environment. The severity is compounded by the fact that Widget components are frequently involved in rendering complex UI elements, increasing the attack surface for potential exploitation through various vector types including drag-and-drop operations, context menu interactions, or dynamic DOM manipulation events.
Mitigation strategies primarily rely on applying the vendor-provided patches released in Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17. These updates contain code changes that enforce stricter memory management protocols within the Widget component, ensuring that pointers are nullified after deallocation or that reference counts are accurately maintained to prevent premature freeing. Organizations should prioritize updating all instances of affected browser versions across their infrastructure immediately. For environments where immediate patching is not feasible due to compatibility concerns, implementing network-level controls such as web application firewalls can help filter out known exploit patterns associated with this vulnerability type. Additionally, enabling strict content security policies and disabling unnecessary JavaScript features in high-risk browsing contexts can reduce the likelihood of successful exploitation by limiting the capabilities available to malicious scripts attempting to trigger the memory corruption flaw.
This incident aligns with ATT&CK technique T1203, which covers Exploitation for Client Execution, highlighting how vulnerabilities in client-side software are leveraged to establish initial access or persistence on target systems. The out-of-bounds write nature of this bug also relates to CWE-787, emphasizing the importance of rigorous bounds checking and memory safety practices in modern web browser development. Continuous monitoring for anomalous behavior resulting from heap corruption and maintaining up-to-date vulnerability databases are essential components of a robust defense-in-depth strategy against such high-severity client-side vulnerabilities.