CVE-2026-117944 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the Layout 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 • 10/28/2026
The identified vulnerability represents a critical memory management error classified as an out-of-bounds write resulting from a use-after-free condition within the Layout component of Mozilla Firefox. This specific flaw occurs when the browser engine attempts to access or manipulate memory that has already been deallocated by the garbage collector or explicit freeing mechanisms. In complex rendering pipelines, such as those handling CSS Grid and Flexbox layouts, objects representing DOM elements are frequently created, modified, and destroyed in rapid succession. If a reference to an object is retained after its underlying memory has been freed, any subsequent operation on that stale pointer leads to undefined behavior. The specific instance addressed in this update involves the Layout subsystem where internal structures tracking element positions or styles were not properly invalidated upon destruction, allowing maliciously crafted web content to trigger access to freed heap memory.
From a technical perspective, use-after-free vulnerabilities are particularly dangerous because they can be exploited to achieve arbitrary code execution with the privileges of the victim process. When an attacker controls the data written into the reused memory region, they can potentially overwrite critical function pointers or object vtables within the browser's runtime environment. This manipulation allows for control over program flow, enabling the injection and execution of shellcode during the rendering phase. The vulnerability was present in versions prior to Firefox 159, including older Extended Support Release (ESR) branches such as version 115 up to 115.43 and version 153 up to 153.5. These releases contained logic errors where the lifecycle management of layout objects failed to synchronize correctly with memory deallocation events, creating a window of opportunity for exploitation during page rendering or interaction.
The operational impact of this vulnerability is severe, as it compromises the core security model of the browser which relies on process isolation and sandboxing. Successful exploitation could allow an attacker to bypass these protections by executing code within the content process with elevated privileges, potentially leading to full system compromise if combined with other vulnerabilities in operating system components or application plugins. The risk extends beyond simple data theft; it enables persistent access through malware installation, keylogging, or lateral movement within a network environment where users browse untrusted websites. Organizations relying on Firefox ESR for stability must recognize that running outdated versions exposes their infrastructure to remote code execution attacks triggered by visiting specially crafted web pages containing malicious JavaScript and CSS constructs designed to trigger the layout engine's memory handling flaws.
Mitigation strategies primarily involve immediate software updates to ensure all client systems are running patched versions of Mozilla Firefox, specifically version 159 or later for standard releases, and ESR versions 115.44 or 153.6 for enterprise environments. In addition to patching, organizations should implement strict content security policies that restrict the execution of inline scripts and limit access to local resources where possible. Network-level controls such as web filtering proxies can also help by blocking known malicious domains hosting exploit kits that target this specific vulnerability class. Security teams should monitor telemetry data for unusual memory usage patterns in browser processes which may indicate attempted exploitation activities, although detection at runtime is challenging due to the nature of heap corruption attacks.
This incident aligns with Common Weakness Enumeration (CWE) identifier CWE-416, Use After Free, and maps to MITRE ATT&CK technique T1203, Exploitation for Client Execution, which describes adversaries leveraging software vulnerabilities in client applications to execute malicious code on target systems. The fix emphasizes the importance of rigorous memory safety practices within browser engines, particularly when dealing with dynamic layout calculations that involve frequent object creation and destruction cycles. Developers must ensure that all references to objects are nullified or updated immediately upon deallocation to prevent dangling pointers from being dereferenced in subsequent rendering operations.