CVE-2026-100791 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the DOM: Core & HTML 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 within the Document Object Model core and HTML components of Mozilla Firefox browsers. Specifically classified as an use-after-free condition, this flaw occurs when the browser's rendering engine attempts to access or manipulate DOM elements that have already been deallocated from system memory. In complex web environments where dynamic content is frequently added, removed, or modified via JavaScript, the internal state tracking mechanisms may fail to correctly invalidate references to these objects before they are freed. This discrepancy allows a maliciously crafted webpage to trigger operations on stale pointers, leading to undefined behavior within the browser's execution context. Such vulnerabilities are particularly dangerous because they can be exploited during normal browsing sessions if a user is tricked into visiting a specially constructed website that leverages this timing window between object deallocation and subsequent access attempts.
From a technical perspective, use-after-free errors fall under CWE-416 in the Common Weakness Enumeration standard, which highlights issues related to referencing memory after it has been freed. In the context of web browsers, these flaws often stem from complex interactions between JavaScript engines like SpiderMonkey and the C++ based DOM implementation layers. When a script removes an element from the document tree, the associated memory should be immediately marked as inaccessible or nullified in all active references. However, if asynchronous operations or event handlers retain lingering pointers to that now-freed memory block, accessing it can result in reading corrupted data or executing arbitrary code paths controlled by the attacker. The severity of this issue is further amplified by its potential alignment with MITRE ATT&CK techniques related to client-side exploitation and privilege escalation within the browser sandbox environment.
The operational impact of such a vulnerability extends beyond simple application crashes, although denial-of-service remains a likely outcome if memory corruption leads to segmentation faults or stack overflows. More critically, successful exploitation can lead to remote code execution with the privileges of the current user. Attackers could potentially bypass security mitigations like ASLR and DEP by carefully crafting heap layouts to achieve predictable memory states, thereby gaining control over instruction pointers. This capability allows for data exfiltration, installation of persistent malware, or further lateral movement within a compromised network if the browser is used as an initial entry point. Given that Firefox ESR versions are widely deployed in enterprise environments where stability and security are paramount, this vulnerability poses a significant risk to organizational infrastructure integrity.
Mitigation strategies primarily involve applying the vendor-provided patches released for Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17 immediately across all affected endpoints. Security administrators should prioritize updating these specific versions to ensure that the underlying memory management logic in the DOM core is corrected. In addition to patching, organizations can implement browser hardening policies such as disabling JavaScript execution on untrusted domains or utilizing content security policies to restrict dynamic code evaluation where possible. Regular vulnerability scanning and monitoring for new CVE disclosures related to web rendering engines are essential practices to maintain a robust defense posture against memory corruption exploits that continue to evolve in modern web applications.