CVE-2026-109763 in Firefox
Summary
by MITRE • 10/13/2026
Mitigation bypass in the File Handling 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 issue represents a critical failure within the file handling subsystem of Mozilla Firefox, specifically categorized as a mitigation bypass. In modern web browsers, mitigations are designed to restrict how user data is accessed or written by web content, thereby preventing malicious scripts from exfiltrating sensitive information or performing unauthorized operations on the local filesystem. The vulnerability in question indicates that these protective mechanisms were circumvented through specific interactions with file handling APIs, allowing an attacker to potentially access files outside of intended sandbox boundaries or perform write operations that should have been blocked by security policies. This type of flaw undermines the fundamental trust model between the browser and its user, as it allows web-based attacks to escalate privileges beyond what is permitted for standard webpage execution contexts.
From a technical perspective, file handling vulnerabilities often stem from improper validation of input parameters or incorrect implementation of sandboxing rules within the operating system interface layer. When a website attempts to interact with local files through mechanisms such as drag-and-drop APIs, download managers, or explicit user-initiated save dialogs, the browser must verify that the operation complies with security constraints like same-origin policies and content security directives. In this instance, the logic governing these checks contained a flaw that permitted actions which should have been denied. This could involve bypassing restrictions on file overwrites, accessing restricted directories, or manipulating file metadata in ways that expose system configuration details to malicious scripts. The complexity of modern browser architectures means that such errors can remain undetected until they are exploited by sophisticated threat actors who understand the internal state management and permission models of the application.
The operational impact of this vulnerability is significant for users relying on Firefox for secure web browsing, particularly those handling sensitive documents or working in environments with strict data loss prevention requirements. An attacker could craft a malicious webpage that exploits this bypass to read local files without explicit user consent beyond initial page load interactions, potentially stealing credentials stored locally, accessing private documents, or modifying system configuration files if the browser is running with elevated privileges. Furthermore, the ability to write to unintended locations can lead to persistence mechanisms where malware embeds itself in legitimate directories, making detection and removal more difficult for endpoint security solutions. The risk is amplified when users are tricked into visiting malicious sites through phishing campaigns or drive-by downloads, as the exploit requires no additional software installation beyond the browser itself.
To address this issue effectively, Mozilla released patches in Firefox version 158 and Firefox Extended Support Release (ESR) version 153.5. These updates contain code changes that correct the logic errors within the file handling component, ensuring that all mitigation rules are strictly enforced during file operations. Users must update their browsers immediately to close this security gap. Beyond updating software, organizations should enforce strict content security policies and disable unnecessary browser features such as automatic downloads or drag-and-drop functionality for untrusted domains where possible. Regular monitoring of browser console logs can also help detect anomalous behavior indicative of exploitation attempts.
This vulnerability aligns with common weakness enumerations related to improper access control and sandbox escape, specifically reflecting patterns found in CWE-284 Improper Access Control and CWE-755 Improper Handling of Exceptional Conditions. In terms of adversary tactics, it relates to techniques used for data staging or collection via browser-based attacks, consistent with MITRE ATT&CK framework entries concerning client-side exploitation. The resolution underscores the importance of continuous code review and fuzzing in core browser components to prevent privilege escalation through seemingly minor logic errors in file I/O operations. Maintaining up-to-date software versions remains the primary defense against such vulnerabilities, as vendors actively patch these flaws once they are identified by internal security teams or external researchers.