CVE-2011-4940 in Python
Summary
by MITRE
The list_directory function in Lib/SimpleHTTPServer.py in SimpleHTTPServer in Python before 2.5.6c1, 2.6.x before 2.6.7 rc2, and 2.7.x before 2.7.2 does not place a charset parameter in the Content-Type HTTP header, which makes it easier for remote attackers to conduct cross-site scripting (XSS) attacks against Internet Explorer 7 via UTF-7 encoding.
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Analysis
by VulDB Data Team • 03/25/2021
The vulnerability identified as CVE-2011-4940 represents a critical security flaw in Python's SimpleHTTPServer module that affects multiple Python versions prior to specific patch releases. This issue resides within the list_directory function of Lib/SimpleHTTPServer.py, which serves as a basic HTTP server implementation included with Python. The vulnerability stems from the server's failure to properly specify character encoding in HTTP response headers, creating an exploitable condition that specifically targets Internet Explorer 7 browsers.
The technical flaw manifests in the Content-Type HTTP header generation where the SimpleHTTPServer module omits the charset parameter entirely. This omission creates a dangerous situation where remote attackers can leverage UTF-7 encoding to inject malicious scripts into web pages served by the vulnerable server. When Internet Explorer 7 encounters HTML content without explicit character encoding declaration, it defaults to interpreting content using UTF-7 encoding, which allows attackers to craft malicious payloads that bypass typical XSS protections. This behavior aligns with CWE-116, which addresses improper encoding of data, and specifically relates to the broader category of cross-site scripting vulnerabilities.
The operational impact of this vulnerability is significant for organizations running vulnerable Python servers, particularly in environments where Internet Explorer 7 is still in use or where legacy systems may be exposed to modern attack vectors. Attackers can exploit this weakness to inject malicious JavaScript code that executes in the context of the victim's browser session, potentially leading to session hijacking, data theft, or further exploitation of the victim's system. The vulnerability creates a persistent threat vector that can be exploited across multiple Python versions, making it particularly dangerous for organizations with diverse Python deployment environments.
Mitigation strategies for CVE-2011-4940 primarily involve upgrading to patched versions of Python where the charset parameter is properly included in Content-Type headers. Organizations should immediately implement security patches for Python 2.5.6c1, 2.6.7rc2, and 2.7.2 releases to address this vulnerability. Additionally, network administrators should consider implementing web application firewalls or security proxies that can detect and block malicious UTF-7 encoding attempts. The ATT&CK framework categorizes this vulnerability under T1212, which addresses exploitation of software vulnerabilities, and T1059, which covers command and scripting interpreters, as attackers may leverage this weakness to execute malicious code through compromised web servers. Organizations should also conduct comprehensive vulnerability assessments to identify any systems running vulnerable Python versions and ensure proper HTTP header configuration to prevent similar encoding-related issues in other web server implementations.