CVE-2026-117940 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 159 and Firefox ESR 153.6.
Once again VulDB remains the best source for vulnerability data.
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, specifically classified as an use-after-free condition. In software engineering, particularly in languages like C++ which form the foundation of many browser engines such as Gecko, proper lifecycle management of objects is paramount to system stability and security. A use-after-free vulnerability occurs when a program continues to use a pointer after the memory it points to has been freed or deallocated. This typically happens due to logic errors where references to an object are not properly invalidated once that object's lifetime ends. In the context of web browsers, this flaw resides in how the browser handles DOM nodes and HTML elements during dynamic page manipulation, script execution, or rendering processes. When a developer or malicious actor triggers specific sequences of events involving element removal or modification, the underlying memory may be released while active references to that same memory block remain within the application's state.
From a technical perspective, this flaw allows for arbitrary read and write operations in the context of the compromised process. Once an attacker gains control over the freed memory region through heap spraying techniques or other exploitation methods, they can manipulate the contents of that memory before it is reallocated for another purpose. This capability effectively bypasses standard memory safety protections such as Address Space Layout Randomization (ASLR) and Data Execution Prevention (DEP), provided the exploit chain successfully achieves code execution. The severity lies in the potential to execute arbitrary code on the victim's machine, leading to a complete compromise of the user's system integrity. Such vulnerabilities are particularly dangerous because they can be triggered by visiting a maliciously crafted webpage or interacting with specific web applications that utilize complex DOM manipulations without adequate safeguards against dangling pointers.
The operational impact of this vulnerability is severe, as it directly affects the confidentiality, integrity, and availability of the user's computing environment. An attacker leveraging this flaw could install malware, steal sensitive data such as login credentials or financial information stored in browser cookies and local storage, or establish a persistent foothold within the network by pivoting from the compromised browser to other systems. Furthermore, because browsers are often used for accessing corporate resources via single sign-on mechanisms, compromising Firefox can serve as an entry point for broader organizational breaches. The vulnerability affects both standard releases of Firefox version 159 and Extended Support Release (ESR) versions up to 153.6, indicating that a significant portion of the user base relying on stable enterprise builds was exposed until the patch was applied. Users who delayed updating their browsers remained at risk for an extended period, highlighting the importance of timely security maintenance in distributed environments.
To mitigate this and similar risks, organizations must ensure that all instances of Firefox are updated to version 159 or later immediately upon release. For enterprise deployments utilizing ESR channels, administrators should verify that patch management systems have successfully deployed update 153.6 across all endpoints. Beyond immediate remediation, adopting defense-in-depth strategies is crucial. This includes implementing strict Content Security Policies (CSP) to limit the types of content that can be executed on web pages, thereby reducing the attack surface for client-side exploits. Additionally, enabling browser sandboxing features and ensuring operating-level protections like heap hardening are active can significantly raise the bar for successful exploitation even if a vulnerability exists. Regular security audits and penetration testing focused on DOM-based vulnerabilities help identify weak points in application logic before they can be weaponized by adversaries.
This incident aligns with common weakness enumeration standards, specifically mapping to CWE-416: Use After Free, which describes errors resulting from the use of pointers that point to deallocated memory. It also correlates with MITRE ATT&CK techniques related to client-side exploitation and privilege escalation within browser sandboxes. Understanding these mappings allows security teams to better categorize threats and prioritize remediation efforts based on industry-standard taxonomies rather than isolated incident reports. By treating such vulnerabilities as critical infrastructure risks, organizations can maintain a robust posture against evolving web-based attack vectors that continue to target memory safety flaws in widely used software components.