CVE-2011-1521 in Python
Summary
by MITRE
The urllib and urllib2 modules in Python 2.x before 2.7.2 and 3.x before 3.2.1 process Location headers that specify redirection to file: URLs, which makes it easier for remote attackers to obtain sensitive information or cause a denial of service (resource consumption) via a crafted URL, as demonstrated by the file:///etc/passwd and file:///dev/zero URLs.
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Analysis
by VulDB Data Team • 08/04/2024
The vulnerability described in CVE-2011-1521 represents a significant security flaw in Python's urllib and urllib2 modules that affected versions prior to 2.7.2 and 3.2.1. This issue stems from improper handling of HTTP redirect responses that contain Location headers pointing to file:// URLs, creating a dangerous pathway for attackers to exploit. The flaw allows remote adversaries to craft malicious URLs that could trigger unintended file system access patterns through the HTTP redirection mechanism, effectively bypassing normal network security controls that would typically prevent direct file system access.
The technical implementation of this vulnerability occurs when Python's HTTP client modules encounter a redirect response with a Location header containing a file:// URL scheme. Instead of properly rejecting or sanitizing such redirections, the urllib modules process these file:// URLs as if they were regular HTTP URLs, leading to potential information disclosure and resource consumption attacks. Attackers can construct URLs that, when accessed through the vulnerable Python modules, cause the application to attempt to read system files or consume system resources through the file:// protocol handler. This behavior creates a direct pathway for attackers to access sensitive system information or to perform resource exhaustion attacks by pointing redirects to devices like /dev/zero that continuously generate data.
The operational impact of this vulnerability extends beyond simple information disclosure to include potential denial of service conditions. When an application using vulnerable Python versions processes a redirect to a file:// URL, it can result in excessive resource consumption as the system attempts to read from special device files or access protected system files. The file:///etc/passwd example demonstrates how attackers can obtain sensitive system information through simple URL manipulation, while file:///dev/zero illustrates how the vulnerability can be exploited for resource exhaustion attacks that could crash applications or consume system resources. This vulnerability particularly affects web applications and scripts that rely on Python's built-in HTTP client functionality for processing user-provided URLs or API responses containing redirects.
From a cybersecurity perspective, this vulnerability aligns with CWE-20: Improper Input Validation and CWE-78: Improper Neutralization of Special Elements used in OS Command Injection, as it represents a failure to properly validate and sanitize input from external sources. The attack pattern follows ATT&CK techniques related to command and control through web services and privilege escalation via information disclosure. Organizations running Python applications that process external URLs or HTTP redirects are particularly at risk, as this vulnerability can be exploited through simple web requests without requiring any special privileges or complex attack vectors. The remediation strategy requires updating Python installations to versions 2.7.2 or later and 3.2.1 or later, where the urllib and urllib2 modules have been patched to properly reject or sanitize Location headers containing file:// URLs, preventing the unintended file system access patterns that the vulnerability enabled.
The broader implications of this vulnerability highlight the importance of proper input validation and the potential risks associated with HTTP client libraries that do not adequately sanitize redirect targets. This issue demonstrates how seemingly benign functionality in standard library components can create significant security risks when they fail to properly handle edge cases in URL processing. Security practitioners should ensure that all Python installations are updated to patched versions and that applications are reviewed for proper URL validation before implementing any custom redirect handling logic that might inadvertently expose similar vulnerabilities. Organizations should also implement network monitoring to detect unusual patterns of file system access that might indicate exploitation attempts against this vulnerability.