CVE-2026-100774 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 application continues to reference a memory block after it has been deallocated or freed by the system. In the context of browser rendering engines, such errors typically arise during complex DOM manipulations where elements are removed from the document tree while references to those elements remain active in JavaScript variables or internal event handlers. When the garbage collector reclaims this memory for other purposes, any subsequent attempt by the vulnerable component to access the now-invalid pointer results in undefined behavior. This category of vulnerability is widely recognized under CWE-416 and poses a severe risk because it allows an attacker who can trick a user into visiting a specially crafted webpage to execute arbitrary code with the privileges of the victim process.
The operational impact of this use-after-free flaw extends beyond simple application crashes, although denial-of-service remains a possible outcome if the memory corruption causes immediate segmentation faults or exceptions. The primary threat lies in remote code execution capabilities. By carefully crafting malicious HTML and JavaScript payloads that trigger specific DOM mutations followed by rapid deallocation cycles, an attacker can manipulate heap layouts to overwrite adjacent objects with controlled data. This technique enables the injection of shellcode into executable regions of memory, facilitating full system compromise without requiring any additional user interaction beyond visiting the targeted site or opening a malicious email attachment containing embedded web content. Such exploits are particularly dangerous in enterprise environments where Firefox is deployed as the standard browser for accessing internal resources and sensitive applications.
Mitigation strategies primarily rely on applying the vendor-provided patches that address these memory safety issues. Users must ensure they have updated to Firefox ESR 153.4, Firefox version 157, or any of the specified Extended Support Release versions including ESR 115.42 and ESR 140.17. These updates contain code changes that enforce stricter lifetime checks on DOM nodes and prevent access to freed memory regions during garbage collection cycles. For organizations managing large fleets of endpoints, automated patch management systems should be configured to prioritize these specific version numbers as critical security fixes. Additionally, deploying browser isolation technologies or web content filtering solutions can provide an additional layer of defense by preventing the execution of untrusted scripts that attempt to exploit DOM manipulation vulnerabilities before they reach the local rendering engine.
From a threat intelligence perspective, this vulnerability aligns with ATT&CK technique T1203 which covers exploitation for client-side attacks. Attackers frequently leverage such browser flaws as part of initial access vectors in targeted campaigns or drive-by download scenarios. The presence of use-after-free bugs in core components like DOM and HTML indicates that the attack surface is broad, affecting any webpage interaction involving dynamic content updates. Security teams should monitor for indicators of compromise associated with known exploit kits that target these specific Firefox versions. Regular security awareness training regarding suspicious links and attachments remains essential to reduce the likelihood of users inadvertently triggering such exploits through social engineering tactics.