CVE-2016-5699 in CPython
Summary
by MITRE
CRLF injection vulnerability in the HTTPConnection.putheader function in urllib2 and urllib in CPython (aka Python) before 2.7.10 and 3.x before 3.4.4 allows remote attackers to inject arbitrary HTTP headers via CRLF sequences in a URL.
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Analysis
by VulDB Data Team • 09/14/2022
The CVE-2016-5699 vulnerability represents a critical cross-site scripting and header injection flaw affecting the Python programming language's standard library components. This vulnerability specifically targets the HTTPConnection.putheader function within urllib2 and urllib modules, which are fundamental components for handling HTTP requests in Python applications. The flaw arises from insufficient input validation and sanitization of URL parameters, allowing malicious actors to inject carriage return line feed sequences that can manipulate HTTP headers. This vulnerability impacts all versions of Python prior to 2.7.10 and 3.x versions before 3.4.4, making it particularly widespread across numerous Python deployments and applications that rely on these standard library functions for web communications.
The technical exploitation of this vulnerability occurs through carefully crafted URLs containing CRLF sequences that are processed by the putheader function without proper sanitization. When a Python application processes such malicious URLs, the CRLF characters are interpreted as header terminators rather than part of the URL path or query parameters. This allows attackers to inject additional HTTP headers into the request, potentially enabling various malicious activities including cache poisoning, session hijacking, and cross-site request forgery attacks. The vulnerability operates at the HTTP protocol level, directly manipulating the header structure of outgoing requests and can be leveraged to bypass security controls that depend on proper header validation.
The operational impact of CVE-2016-5699 extends beyond simple header injection, as it can enable more sophisticated attacks that compromise application security and data integrity. Attackers can use this vulnerability to manipulate HTTP headers in ways that may bypass authentication mechanisms, alter request routing, or inject malicious content into web applications. The vulnerability is particularly dangerous in web applications that process user-provided URLs or redirect users to external resources, as these applications become potential vectors for header injection attacks. This flaw directly relates to CWE-113, which describes improper neutralization of CRLF sequences in HTTP headers, and aligns with ATT&CK technique T1071.004 for application layer protocol manipulation.
Mitigation strategies for CVE-2016-5699 primarily focus on upgrading affected Python installations to versions that contain the necessary security patches. Organizations should prioritize updating their Python environments to Python 2.7.10 or later, or Python 3.4.4 and later versions where the vulnerability has been addressed. Additionally, applications should implement proper input validation and sanitization for URLs before processing them through urllib functions, particularly when dealing with user-supplied data. Security measures including web application firewalls and header validation mechanisms can provide additional protection layers, though the most effective approach remains maintaining up-to-date software versions. Organizations should also conduct thorough security assessments of their Python applications to identify and remediate any code that may be vulnerable to similar injection attacks, as this vulnerability demonstrates the importance of proper input handling in network communication libraries.