CVE-2018-5105 in Firefox
Summary
by MITRE
WebExtensions can bypass user prompts to first save and then open an arbitrarily downloaded file. This can result in an executable file running with local user privileges without explicit user consent. This vulnerability affects Firefox < 58.
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Analysis
by VulDB Data Team • 12/30/2019
The vulnerability identified as CVE-2018-5105 represents a critical security flaw in the Firefox browser's WebExtensions architecture that undermines user consent mechanisms for file operations. This issue specifically targets the browser's handling of file downloads and execution permissions, creating a pathway for malicious extensions to circumvent intended security controls. The vulnerability exists within Firefox versions prior to 58, where the WebExtensions API implementation failed to properly enforce user interaction requirements for file handling operations. This weakness allows malicious actors to craft extensions that can automatically save and execute downloaded files without requiring explicit user confirmation, effectively bypassing the browser's intended security model.
The technical implementation of this vulnerability stems from insufficient validation within Firefox's WebExtensions subsystem where the browser fails to properly verify user intent before executing file operations. When a WebExtension attempts to download and execute a file, the system should require explicit user consent through a prompt interface. However, the flaw permits extensions to bypass this user interaction step entirely, enabling automatic execution of potentially malicious files. This bypass occurs at the API level where the extension's request to save and open files is processed without proper user verification. The vulnerability is particularly concerning because it operates at the browser extension level, which typically operates with elevated privileges and can access system resources without the same restrictions applied to regular web content.
The operational impact of CVE-2018-5105 extends beyond simple privilege escalation as it enables attackers to execute arbitrary code with local user privileges without explicit user consent. This creates a significant attack surface where malicious extensions can silently download and execute payloads such as malware, trojans, or other harmful software. The vulnerability can be exploited through the installation of malicious WebExtensions, which can then leverage this flaw to automatically execute downloaded files. This represents a direct violation of user security expectations and browser security policies, as users believe they have control over file operations through explicit prompts. The consequences include potential data theft, system compromise, and unauthorized access to sensitive information, as the malicious code runs with the same privileges as the user who installed the extension.
From a cybersecurity perspective, this vulnerability aligns with CWE-693, which addresses protection mechanism failures, and demonstrates how insufficient access control can lead to privilege escalation. The flaw also connects to ATT&CK technique T1176, which covers "Browser Extensions" as a method of persistence and privilege escalation. Organizations and users affected by this vulnerability should immediately update to Firefox version 58 or later, where the issue has been resolved through proper implementation of user consent mechanisms. Additionally, administrators should review installed extensions and ensure they come from trusted sources, while implementing monitoring for unusual file download and execution patterns. The fix implemented in Firefox 58 addresses the root cause by enforcing proper user interaction requirements before allowing automatic file execution, thereby restoring the intended security model for WebExtensions and preventing unauthorized code execution.
This vulnerability highlights the importance of proper sandboxing and privilege separation in browser extension architectures, as well as the critical need for comprehensive testing of user consent mechanisms. The security implications extend beyond the immediate execution of malicious files, as this flaw demonstrates how seemingly minor API implementation issues can create significant security risks. The vulnerability serves as a reminder of the complex security landscape in modern browsers where extension systems must balance functionality with user protection, and where a single flaw can undermine the entire security model. Regular security updates and proper extension vetting processes become essential components of browser security management to prevent exploitation of similar vulnerabilities in the future.