CVE-2019-9794 in Firefox
Summary
by MITRE
A vulnerability was discovered where specific command line arguments are not properly discarded during Firefox invocation as a shell handler for URLs. This could be used to retrieve and execute files whose location is supplied through these command line arguments if Firefox is configured as the default URI handler for a given URI scheme in third party applications and these applications insufficiently sanitize URL data. *Note: This issue only affects Windows operating systems. Other operating systems are unaffected.*. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 11/26/2025
This vulnerability represents a critical security flaw in Mozilla Firefox and Thunderbird applications that specifically impacts Windows operating systems. The issue stems from improper handling of command line arguments when Firefox is invoked as a shell handler for URL protocols, creating a potential attack vector that could be exploited by malicious actors. The vulnerability is particularly concerning because it leverages the trust relationship between applications and the operating system's default URI handler configuration, allowing for arbitrary code execution through specially crafted URL schemes.
The technical flaw manifests when Firefox is configured as the default handler for specific URI schemes such as http, https, or custom protocols. During normal operation, Firefox receives command line arguments that should be properly sanitized and discarded, but in this case, certain arguments containing file paths or execution directives are not adequately filtered. This allows attackers to craft malicious URLs that, when processed by Firefox, could trigger unintended file operations or command execution. The vulnerability specifically affects Windows systems because of how the Windows shell handles command line arguments when launching applications, creating a path traversal and execution flaw that bypasses normal security boundaries.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it can be exploited through third-party applications that configure Firefox as their default handler for specific protocols. When these applications fail to properly sanitize URL data before passing it to Firefox, attackers can inject malicious command line arguments that get executed by the browser. This creates a multi-layered attack surface where the vulnerability can be triggered through various vectors including email clients, web browsers, or desktop applications that rely on Firefox for protocol handling. The exploitation requires a specific combination of factors including Firefox being configured as a default handler, insufficient input sanitization in third-party applications, and the presence of maliciously crafted URLs in the attack chain.
Organizations should implement immediate mitigations including updating to affected versions of Firefox, Thunderbird, and Firefox ESR where available, as well as reviewing and modifying default URI handler configurations to limit Firefox's exposure. System administrators should also consider disabling or restricting Firefox's ability to act as a default handler for potentially dangerous URI schemes, particularly those that could lead to file system access or command execution. Security teams should monitor for suspicious URL patterns and implement network-level controls to detect and block potentially malicious protocol handling attempts. This vulnerability aligns with CWE-78 and CWE-88 categories related to command injection and improper argument handling, and maps to ATT&CK techniques involving execution through command shell and privilege escalation through application misconfiguration.
The vulnerability highlights the importance of proper input validation and argument sanitization in cross-platform applications, particularly when dealing with system-level integration points. Organizations should conduct comprehensive security assessments of their application ecosystems to identify other potential misconfigurations that could create similar attack vectors. Regular security testing and code reviews focusing on command line argument handling, especially in applications that interface with system-level components, should be implemented as part of ongoing security hygiene practices. This vulnerability serves as a reminder that seemingly benign configuration options can create significant security risks when combined with insufficient input validation and proper security architecture design principles.