CVE-2026-117921 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 after it has been deallocated or freed by the system. In the context of CSS processing, such errors typically arise during complex style recalculations where objects representing style rules or computed values are destroyed while still being accessed by other parts of the rendering engine. This scenario is particularly dangerous because the memory previously occupied by the object may be reallocated for a different purpose, leading to unpredictable behavior when the stale pointer attempts to read from or write to that now-invalid address space.
From a technical perspective, this vulnerability falls under the Common Weakness Enumeration category CWE-416, which defines use-after-free as a condition where software uses memory after it has been freed. The root cause often involves incorrect reference counting mechanisms or race conditions in multi-threaded environments where one thread frees an object while another is still processing references to it. In browser engines, the CSS parser and compositor operate on high-frequency data structures that are subject to rapid allocation and deallocation cycles. If a pointer to a style rule or computed value is not properly nullified after deletion, subsequent operations such as layout adjustments or paint commands may attempt to access this dangling pointer. This can lead to memory corruption, application crashes, or in more sophisticated exploitation scenarios, arbitrary code execution if the attacker can control the data that overwrites the freed memory region.
The operational impact of exploiting a use-after-free vulnerability in a web browser is severe and extends beyond simple denial of service. While an unexploited crash would merely disrupt the user's browsing session by closing the tab or crashing the entire application, successful exploitation allows for remote code execution with the privileges of the current user. Attackers can craft malicious web pages containing specially designed CSS rules that trigger this flaw during rendering. By carefully controlling the contents written to the freed memory region, an attacker could inject shellcode or manipulate function pointers within the browser process. This effectively bypasses many modern security mitigations such as address space layout randomization and data execution prevention if the vulnerability allows for precise control over memory writes. Consequently, this represents a high-severity threat that compromises both confidentiality and integrity of user systems.
Industry frameworks categorize this type of flaw under specific attack vectors within the MITRE ATT&CK framework. The exploitation technique aligns with T1203, which covers Exploitation for Client Execution, as it involves tricking a victim into visiting a malicious webpage that triggers code execution in their browser client. Furthermore, the underlying weakness is directly mapped to CWE-416 and often intersects with CWE-825 regarding expired pointer dereference. Understanding these mappings helps security teams prioritize remediation efforts based on established risk models rather than treating each vulnerability as an isolated incident. The presence of such a flaw indicates gaps in memory safety guarantees within the legacy C++ codebase that powers much of Firefox's rendering engine, highlighting the ongoing challenge browsers face in maintaining robustness against complex memory errors.
Mitigation strategies for this vulnerability primarily rely on timely software updates and proactive security hardening measures. Users must immediately upgrade to Mozilla Firefox version 159 or later, as well as the extended support release versions 115.44 and 153.6, which contain patches addressing these specific memory management issues. For organizations managing large fleets of browsers, automated patch deployment systems should be configured to prioritize updates for browser components due to their high exposure surface area. Additionally, deploying content security policies that restrict the loading of external resources can reduce the attack surface by limiting the ability of malicious scripts or stylesheets to interact with sensitive parts of the DOM and rendering engine. Security awareness training is also recommended to help users recognize suspicious links that might lead to pages designed to exploit such browser vulnerabilities.
Long-term resilience against use-after-free errors requires architectural shifts toward memory-safe languages for critical components like CSS parsing. While Firefox continues to rely heavily on C++, ongoing efforts by the Mozilla community and broader industry initiatives aim to migrate high-risk modules to Rust, a language that enforces memory safety at compile time without garbage collection overhead. Until such migration is complete, rigorous code review processes focusing on pointer lifecycle management and automated fuzzing tools targeting CSS parsers are essential for identifying similar flaws before they reach production releases. Continuous monitoring of browser update channels and adherence to vendor security advisories remain the most effective defense against exploitation of these complex memory corruption vulnerabilities in web browsers.