CVE-2026-117954 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the Layout: Positioned 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 Mozilla Gecko rendering engine, specifically affecting the Layout: Positioned component responsible for handling CSS positioning logic. This issue is classified as an out-of-bounds write resulting from a use-after-free condition, which falls under CWE-416 in the Common Weakness Enumeration taxonomy. The flaw occurs when the browser retains access to memory that has already been deallocated during the layout calculation process. In complex web pages utilizing advanced CSS positioning properties such as absolute or fixed positioning, the engine may attempt to reference a DOM element or style object after its underlying memory structure has been freed by the garbage collector or explicit cleanup routines. This discrepancy between logical state and physical memory availability creates an opportunity for malicious actors to exploit the unstable pointer references.
From an operational perspective, this vulnerability allows for arbitrary code execution with the privileges of the target process if successfully exploited. An attacker can craft a specially designed web page that triggers the use-after-free condition during layout recalculation. By carefully controlling the allocation and deallocation patterns in JavaScript or through specific CSS interactions, it is possible to overwrite adjacent memory structures. This could lead to heap corruption, allowing for control flow hijacking techniques such as return-oriented programming (ROP) or direct shellcode injection. The attack vector aligns with ATT&CK technique T1203 Exploitation for Client Execution, where the browser serves as the initial compromise point leading further into a networked environment.
The impact extends beyond simple data leakage to include full system compromise depending on the user's privilege level and security configurations in place. While modern browsers implement various mitigations such as heap hardening, sandboxing via process isolation, and memory protection mechanisms like DEP and ASLR, these defenses are not impenetrable against sophisticated exploits targeting low-level rendering engines. The vulnerability affects all supported versions of Firefox prior to the patch release, including standard releases and Extended Support Release (ESR) branches used by enterprise environments. This broad impact necessitates immediate attention from system administrators managing fleets of workstations or servers running affected browser versions.
Mitigation strategies primarily involve applying the software updates released in Firefox version 159, as well as Firefox ESR 115.44 and Firefox ESR 153.6. These patches correct the memory management logic within the positioned layout component to ensure that references are invalidated immediately upon deallocation or checked for validity before access. Organizations should prioritize patching their endpoints through automated deployment tools to minimize exposure windows. Additionally, implementing application control policies can restrict the execution of unsigned scripts and limit browser capabilities in untrusted environments. Users should also be educated on avoiding interaction with suspicious websites until patches are verified as applied across all client systems within the infrastructure.