CVE-2026-117929 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 159, Firefox ESR 115.44, and Firefox ESR 153.6.
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Analysis
by VulDB Data Team • 10/27/2026
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 pages involving dynamic content updates, script execution, or rapid structural changes, the internal pointers referencing these nodes may not be properly invalidated after deletion. This creates a window of opportunity where stale references are retained and subsequently dereferenced by subsequent operations within the browser's layout engine or JavaScript interpreter. Such scenarios often arise during event handling sequences where asynchronous callbacks trigger modifications to the DOM structure while previous handlers still hold active references to elements that have since been removed from the document tree.
From a technical perspective, this flaw falls under CWE-416 which defines use-after-free errors as accessing memory after it has been freed without proper reinitialization or nullification of pointers. The vulnerability resides in the core parsing and rendering logic rather than external plugins, indicating an intrinsic weakness in how Firefox manages object lifecycles for HTML elements. When a script removes a node from the DOM but retains a reference to it, subsequent operations such as style recalculation, layout adjustment, or attribute modification may attempt to read or write data at the memory address previously occupied by that element. Since this memory region is now available for reallocation, any new object allocated in its place could be controlled by an attacker through crafted web content. This misalignment between logical DOM state and physical memory allocation undermines the integrity of the browser's execution environment.
The operational impact of exploiting a use-after-free vulnerability in a core browser component is severe and multifaceted. An attacker who successfully exploits this flaw can achieve arbitrary code execution with the privileges of the victim user, effectively taking full control over the compromised system. Beyond direct exploitation for malware installation or data theft, such vulnerabilities facilitate sophisticated drive-by download attacks where malicious scripts execute silently upon visiting a specially crafted webpage. The attack surface is broad because it affects standard web browsing activities and does not require specific plugin installations or elevated permissions beyond normal user access. Furthermore, since this issue impacts the core HTML parsing engine, it remains relevant across various operating systems supported by Firefox including Windows, macOS, Linux, Android, and iOS versions of the browser.
In terms of threat intelligence frameworks, this vulnerability aligns with MITRE ATT&CK technique T1203 which covers exploitation for client-side execution. The attack vector typically involves social engineering or malicious advertising networks delivering compromised web content that triggers the specific sequence of DOM manipulations required to activate the bug. Defense-in-depth strategies must account for the fact that modern browsers employ various mitigations such as heap hardening and sandboxing, but these are not foolproof against well-crafted exploits targeting core engine logic. Security researchers often utilize fuzzing tools like AFL or Honggfuzz combined with memory sanitizers such ASan to detect such issues during development phases before they reach production builds.
Mitigation strategies primarily rely on the timely application of vendor-provided patches as indicated by the fixed versions Firefox 159, Firefox ESR 115.44, and Firefox ESR 153.6. Organizations should enforce automated update policies to ensure all endpoints are running these patched versions or newer releases where similar issues may have been addressed in subsequent updates. For environments unable to immediately patch client software, network-level controls such as web filtering proxies can be configured to block access to known malicious domains hosting exploit kits targeting this specific vulnerability class. Additionally, enabling strict Content Security Policy headers on internal websites reduces the risk of executing untrusted scripts that might trigger the vulnerable code paths within the DOM engine.
Long-term remediation involves rigorous adherence to secure coding practices in browser development lifecycles. This includes implementing robust ownership models for DOM nodes where references are automatically invalidated upon removal from the tree structure. Code review processes should specifically target memory management logic and pointer lifecycle tracking to prevent stale reference retention. Regular integration of static analysis tools that detect potential use-after-free patterns can help identify these flaws early in the development cycle. Furthermore, participating in bug bounty programs allows external security researchers to contribute findings before they are weaponized by threat actors, thereby enhancing the overall resilience of the browser ecosystem against memory corruption attacks.