CVE-2019-17019 in Firefox
Summary
by MITRE
When Python was installed on Windows, a python file being served with the MIME type of text/plain could be executed by Python instead of being opened as a text file when the Open option was selected upon download. *Note: this issue only occurs on Windows. Other operating systems are unaffected.*. This vulnerability affects Firefox < 72.
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Analysis
by VulDB Data Team • 01/09/2020
This vulnerability represents a critical security flaw in how Firefox handles file execution on Windows systems when Python is installed. The issue stems from the browser's handling of MIME type associations and file execution contexts, creating an unexpected execution path for python files. When users download python files from web pages, Firefox incorrectly processes these files with text/plain MIME type, allowing Python interpreters to execute them directly instead of opening them as text documents. This behavior fundamentally violates the principle of least privilege and creates a dangerous execution environment where malicious actors can potentially deliver python-based exploits through web interfaces.
The technical root cause of this vulnerability lies in Firefox's file handling mechanism on Windows platforms, where the browser fails to properly distinguish between file types that should be executed versus those that should be opened as text documents. This flaw operates at the intersection of web browser security, operating system file associations, and application execution contexts. The vulnerability specifically affects Firefox versions prior to 72, where proper MIME type handling and execution context validation was not implemented correctly. According to CWE-74, this represents a Code Injection vulnerability where the browser becomes an unwitting execution agent for malicious code, while the ATT&CK framework would categorize this under T1059.007 for Python-based execution techniques.
The operational impact of this vulnerability is significant for Windows users who have Python installed on their systems, as it creates an attack surface where remote code execution becomes possible through simple web browsing activities. Users downloading files from untrusted sources could unknowingly execute malicious python scripts that exploit this behavior, potentially leading to system compromise, data exfiltration, or further attack escalation. The vulnerability's Windows-specific nature means that organizations with mixed operating system environments may have varying levels of exposure, but Windows users remain particularly vulnerable. This type of vulnerability is particularly dangerous because it leverages legitimate system components to execute malicious code, making detection and prevention more challenging for both users and security systems.
Mitigation strategies for this vulnerability include updating Firefox to version 72 or later where the issue has been addressed through improved MIME type handling and execution context validation. Organizations should also implement browser security policies that restrict file execution permissions and consider deploying additional security measures such as application whitelisting to prevent unauthorized script execution. Users should be educated about the risks of downloading and executing files from untrusted sources, and system administrators should monitor for potential exploitation attempts. The fix implemented by Mozilla addresses the core issue by ensuring proper MIME type interpretation and preventing automatic execution of python files when they are downloaded, thereby aligning with security best practices outlined in the OWASP Top Ten and NIST cybersecurity frameworks.