CVE-2019-9740 in Python
Summary
by MITRE
An issue was discovered in urllib2 in Python 2.x through 2.7.16 and urllib in Python 3.x through 3.7.2. CRLF injection is possible if the attacker controls a url parameter, as demonstrated by the first argument to urllib.request.urlopen with \r\n followed by an HTTP header or a Redis command.
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Analysis
by VulDB Data Team • 05/19/2025
The vulnerability identified as CVE-2019-9740 represents a critical CRLF injection flaw affecting Python's standard library HTTP handling modules. This issue exists within both Python 2.x versions up to 2.7.16 and Python 3.x versions up to 3.7.2, specifically impacting the urllib2 module in Python 2 and urllib in Python 3. The vulnerability stems from insufficient input validation when processing URL parameters that contain carriage return and line feed characters, allowing malicious actors to inject arbitrary HTTP headers or Redis commands into requests.
The technical exploitation of this vulnerability occurs when an attacker can control a URL parameter that gets directly processed by the urllib modules without proper sanitization. When the vulnerable code encounters a URL containing CRLF sequences, these characters are not properly escaped or filtered, enabling attackers to inject additional HTTP headers or commands that can be executed by the target system. This injection can occur in the first argument passed to urllib.request.urlopen, where the attacker-controlled URL parameter contains the malicious CRLF sequences that get interpreted as part of the HTTP request structure.
The operational impact of CVE-2019-9740 is significant across multiple attack vectors and system components. Attackers can leverage this vulnerability to perform HTTP header injection attacks, potentially leading to session hijacking, cross-site scripting, or cache poisoning attacks. The vulnerability is particularly dangerous in web applications that use Python's urllib modules to process user-supplied URLs or in systems that rely on HTTP request parsing for authentication or authorization decisions. Additionally, when the vulnerable code interacts with Redis servers, attackers can inject Redis commands, potentially leading to remote code execution or data manipulation within the Redis environment. This vulnerability aligns with CWE-113, which specifically addresses improper neutralization of CRLF characters in HTTP headers, and maps to ATT&CK technique T1071.004 for application layer protocol manipulation.
Mitigation strategies for CVE-2019-9740 require immediate action to address the root cause through proper input validation and sanitization. Organizations should upgrade to patched versions of Python, specifically Python 2.7.17 or later and Python 3.7.3 or later, which contain the necessary fixes for this vulnerability. In environments where immediate upgrades are not feasible, developers should implement strict input validation that filters or escapes CRLF characters from URL parameters before processing them through urllib modules. Additionally, network-level protections such as web application firewalls can help detect and block suspicious CRLF injection patterns in HTTP requests. Security teams should also conduct thorough code reviews to identify all instances where user-controlled input is passed directly to urllib functions without proper sanitization, ensuring that the vulnerable patterns are eliminated from production code. The vulnerability demonstrates the critical importance of proper input validation in network communication libraries, particularly when handling user-supplied data that may be interpreted as protocol control characters.