CVE-2026-109740 in Firefox
Summary
by MITRE • 10/13/2026
Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified security flaw resides within the Widget: Win32 subsystem of Mozilla Firefox, specifically involving an error in how boundary conditions are evaluated during memory access operations. In complex software architectures like a web browser, components responsible for rendering user interface elements must strictly adhere to defined limits when processing data from untrusted sources such as web pages or scripts. The vulnerability arises because the implementation failed to correctly validate these boundaries before performing read or write actions on allocated memory buffers. This lack of rigorous input validation allows an attacker who can craft a malicious webpage containing specially designed content to trigger incorrect boundary checks, thereby bypassing intended security constraints within the browser's sandbox environment.
This type of defect is classically categorized under CWE-125, which describes Out-of-bounds Read vulnerabilities, or potentially CWE-787 if it leads to arbitrary write operations depending on the specific exploitation vector utilized by an attacker. The core issue lies in the failure of the Win32 widget component to ensure that data indices remain within the allocated memory space. When a browser processes content from a website, it relies on sandboxing mechanisms to isolate potentially malicious code from critical system resources and user data. By exploiting incorrect boundary conditions, an adversary can manipulate these isolation boundaries, effectively achieving a sandbox escape. This means that code executing in the context of a web page gains privileges beyond those intended for its environment, allowing access to sensitive information or control over browser processes that should remain restricted.
The operational impact of this vulnerability is severe due to its potential for remote exploitation without user interaction beyond visiting a malicious site. An attacker could construct a webpage containing exploit code designed to trigger the boundary condition error within the Widget: Win32 component. Successful exploitation would allow the execution of arbitrary code with the privileges of the browser process, which often includes access to local files, cookies, session tokens, and other sensitive user data stored by the application. This compromises the confidentiality and integrity of the user's browsing session and potentially their entire system if further privilege escalation techniques are employed following the initial sandbox escape. The ability to break out of the sandbox undermines one of the primary defense-in-depth strategies used in modern web browsers to mitigate risks associated with rendering engine vulnerabilities.
To address this critical security issue, Mozilla released patches in Firefox version 158 and Firefox Extended Support Release (ESR) version 153.5. These updates contain code changes that enforce stricter boundary checks within the Win32 widget component, ensuring that all memory accesses are validated against allocated limits before execution proceeds. Users must upgrade to these patched versions immediately to mitigate the risk of exploitation. Additionally, organizations should ensure their deployment pipelines automatically apply security patches for browser software to maintain a secure endpoint environment. From an ATT&CK perspective, this vulnerability relates to techniques involving sandbox evasion and privilege escalation via client-side code execution, highlighting the importance of keeping all components of the web rendering stack up to date with vendor-provided fixes that address low-level memory safety issues.