CVE-2026-100794 in Firefox
Summary
by MITRE • 09/29/2026
Sandbox escape due to incorrect boundary conditions in the Internationalization component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, and Firefox ESR 140.17.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 09/29/2026
The identified security flaw resides within Mozilla Firefox's internationalization subsystem, specifically involving a critical failure in boundary condition handling that facilitates sandbox escape. In modern web browsers, the rendering engine operates under strict isolation principles where untrusted content from websites is executed within a restricted environment known as the browser sandbox. This architecture prevents malicious code from accessing sensitive system resources or user data directly. However, when processing complex international text strings and locale-specific formatting rules, the underlying C++ components responsible for string manipulation and layout calculation failed to properly validate input boundaries against allocated memory buffers.
This technical deficiency allows an attacker who controls a crafted webpage containing specially constructed Unicode sequences or specific localization payloads to trigger an out-of-bounds read or write operation within the browser's process space. By exploiting these incorrect boundary checks, it is possible to corrupt adjacent memory structures that govern sandbox permissions and execution contexts. The vulnerability effectively bridges the gap between the untrusted web content zone and the more privileged browser utility processes, allowing arbitrary code execution with higher privileges than intended by the security model.
From a classification perspective, this issue aligns closely with CWE-125 Out-of-bounds Read or CWE-787 Out-of-bounds Write depending on the specific memory access pattern exploited during the boundary check failure. In terms of adversary tactics and techniques, such an exploit maps directly to MITRE ATT&CK technique T1059 Command and Scripting Interpreter through browser-based execution vectors, specifically leveraging sandbox escape mechanisms categorized under privilege escalation paths within web application environments. The ability to break out of the renderer process isolation represents a severe compromise of the core security architecture designed to protect users from drive-by download attacks or malicious script injection.
The operational impact of this vulnerability is significant as it enables remote code execution without user interaction beyond visiting a compromised website. Attackers can leverage this flaw to install malware, steal credentials stored in browser profiles, conduct session hijacking by accessing cookies and local storage data protected by the same-origin policy boundaries that are now bypassed due to sandbox escape, or pivot further into internal network resources if the victim is on an enterprise LAN with less stringent endpoint protection. The lack of proper boundary validation means that even well-intentioned code paths within the internationalization library can be manipulated to execute attacker-controlled logic in a privileged context.
Mitigation strategies primarily involve applying the vendor-provided patches released for Firefox ESR 153.4, Firefox 157, and Firefox ESR 140.17 immediately across all affected installations. Organizations should enforce automated update policies to ensure that browser versions remain current with security fixes. Additionally, implementing strict content security policies on internal web applications can reduce the attack surface by limiting script execution sources. For high-security environments, deploying application whitelisting solutions and network segmentation controls provides defense-in-depth measures that limit the potential damage even if a sandbox escape is successfully executed at the browser level. Continuous monitoring for anomalous process behavior originating from browser processes can also aid in detecting exploitation attempts of this nature before significant data exfiltration occurs.