CVE-2026-118021 in Firefox
Summary
by MITRE • 10/27/2026
Sandbox escape due to use-after-free in the XPCOM component. This vulnerability was fixed in Firefox 159.
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Analysis
by VulDB Data Team • 10/27/2026
The identified security flaw resides within the XPCOM (Cross-Platform Component Object Model) subsystem of Mozilla Firefox, specifically manifesting as a use-after-free condition that facilitates sandbox escape. The XPCOM architecture serves as the foundational component infrastructure for many core browser functionalities, managing object lifecycle and inter-process communication mechanisms critical to the browser's operation. When an attacker can trigger this specific vulnerability, they exploit a memory management error where a pointer continues to be used after the memory it references has been deallocated. This scenario typically arises when reference counting logic fails to properly synchronize access or when objects are destroyed while still referenced by active processes or threads within the XPCOM framework.
In the context of modern browser security, the sandbox is a critical isolation boundary designed to restrict the privileges of web content and potentially compromised plugins from accessing sensitive system resources or executing arbitrary code with elevated permissions. A use-after-free vulnerability in such a privileged component allows an attacker to manipulate memory state in ways that bypass these restrictions. By carefully crafting malicious web content or scripts, an adversary can trigger the premature deallocation of an XPCOM object while maintaining a dangling reference. Subsequent operations on this invalid pointer may lead to arbitrary read and write primitives, enabling the attacker to overwrite critical data structures such as function pointers or virtual table entries. This manipulation effectively allows code execution within the context of the browser process with higher privileges than intended by the sandbox policy.
The operational impact of successfully exploiting this vulnerability is severe, potentially leading to full system compromise depending on the user's privilege level and installed software. An attacker could execute arbitrary commands, install malware, steal sensitive data such as credentials or browsing history, or use the compromised browser as a pivot point for further network attacks. The ability to escape the sandbox neutralizes one of the primary defenses against drive-by download attacks and malicious script execution, rendering traditional content isolation measures ineffective against this specific exploit vector. This type of vulnerability is particularly dangerous because it targets core infrastructure components that are frequently invoked during normal browsing activities, increasing the attack surface significantly.
This flaw aligns with Common Weakness Enumeration (CWE) category CWE-416, which describes use-after-free errors resulting from improper memory management practices. From a tactical perspective, exploitation techniques associated with this vulnerability often map to MITRE ATT&CK tactics related to privilege escalation and defense evasion, specifically leveraging memory corruption to bypass sandbox boundaries. The fix implemented in Firefox version 159 addresses the underlying reference counting or lifecycle management logic within the XPCOM component to ensure that objects are not accessed after their resources have been released. Users should immediately update to this patched version to mitigate the risk of exploitation. Additionally, maintaining up-to-date browser versions and employing additional security layers such as content blocking extensions can provide supplementary protection against similar memory corruption vulnerabilities in web browsers.