CVE-2026-117906 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the CSS Parsing and Computation 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 Cascading Style Sheets parsing and computation subsystem of Mozilla Firefox browsers. Specifically classified as an use-after-free condition, this flaw occurs when the application continues to reference a block of memory that has already been deallocated by the system. In the context of CSS processing, such errors typically arise during complex style recalculations or layout adjustments where pointers to internal data structures are not properly invalidated after their associated objects are destroyed. This creates a window of opportunity for an attacker who can trigger specific rendering sequences to access freed memory regions that may have been reallocated for other purposes.
From a technical perspective, the CSS engine in Firefox is responsible for parsing style rules and computing final computed values for elements on a webpage. When this component fails to manage object lifecycles correctly, it retains references to objects that are no longer valid. If an attacker can craft a malicious web page or manipulate existing stylesheets to trigger these specific code paths, they may induce the browser to read from or write to arbitrary memory locations. This capability is particularly dangerous because it allows for potential information disclosure, where sensitive data residing in adjacent memory blocks could be leaked, or more critically, arbitrary code execution if the attacker can control the content of the freed memory and redirect program flow through function pointers or virtual table entries embedded within those structures.
The operational impact of this vulnerability extends beyond simple application instability. While a crash resulting from accessing invalid memory is a common outcome, which leads to denial of service for the user session, the primary concern remains the potential for remote code execution. An attacker hosting a specially crafted website could exploit this flaw when a victim visits it or interacts with specific elements on the page. Successful exploitation would grant the attacker the same privileges as the current user, potentially allowing them to install malware, steal credentials stored in the browser, bypass security controls like Same-Origin Policy, and take full control of the affected system. The severity is heightened by the fact that CSS parsing occurs frequently during normal browsing activities, increasing the attack surface significantly compared to less common features.
This vulnerability aligns with Common Weakness Enumeration identifier CWE-416, which defines use-after-free errors as a category of memory corruption vulnerabilities where pointers are used after they have been freed. In terms of offensive security frameworks, this flaw facilitates techniques associated with MITRE ATT&CK tactic Tactic 12: Impact and potentially Technique T1059 Command and Scripting Interpreter if the exploitation leads to code execution that allows further command-line operations within the user context. The vulnerability underscores the complexity of managing memory in high-performance rendering engines where performance optimizations sometimes complicate strict adherence to safe memory management practices.
Mitigation strategies primarily rely on applying the vendor-provided patches released for Firefox 159, Firefox Extended Support Release version 115.44, and Firefox ESR version 153.6. Organizations should prioritize updating all endpoints running these browser versions to ensure that the underlying memory management logic in the CSS parser is corrected. For environments where immediate patching is not feasible, network-level controls such as web filtering proxies can be configured to block access to known malicious domains or restrict JavaScript execution on untrusted sites, thereby reducing the likelihood of triggering the vulnerable code paths. Additionally, enabling browser sandboxing features and keeping operating system security patches up to date provides additional layers of defense that limit the impact if an exploitation attempt is successful by restricting the privileges available to the compromised process.