CVE-2026-0672 in CPython
Summary
by MITRE • 01/21/2026
When using http.cookies.Morsel, user-controlled cookie values and parameters can allow injecting HTTP headers into messages. Patch rejects all control characters within cookie names, values, and parameters.
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Analysis
by VulDB Data Team • 04/22/2026
The vulnerability identified as CVE-2026-0672 represents a critical security flaw in the Python http.cookies.Morsel implementation that enables HTTP header injection attacks through improperly sanitized cookie data. This issue arises when applications process user-controlled cookie values and parameters without adequate validation, creating an avenue for malicious actors to inject arbitrary HTTP headers into server responses. The flaw specifically targets the Morsel class which is responsible for parsing and generating HTTP cookie headers, making it a fundamental component in web application security. The vulnerability exists because the implementation fails to properly validate control characters within cookie names, values, and parameters, allowing attackers to inject malicious headers that could be interpreted by HTTP clients or servers. This type of vulnerability falls under the CWE-113 category for Improper Neutralization of CRLF Sequences in HTTP Headers, which is a well-documented weakness in web application security. The attack vector leverages the fact that HTTP headers are terminated by CRLF sequences, and when these sequences appear within cookie data, they can be interpreted as header terminators, enabling header injection attacks. The implications extend beyond simple data manipulation as this vulnerability can be exploited to perform various malicious activities including cache poisoning, cross-site scripting attacks, and session hijacking. The patch implemented to address this vulnerability specifically rejects all control characters within cookie names, values, and parameters, effectively preventing the injection of HTTP headers through cookie manipulation. This mitigation aligns with the principle of input validation and follows the security principle of least privilege by ensuring that only valid cookie data is accepted. The vulnerability is particularly concerning in environments where user input directly influences cookie creation, such as web applications that generate cookies based on user preferences or authentication data. Attackers can leverage this weakness to manipulate HTTP responses by injecting headers that could redirect traffic, modify content, or bypass security controls. The flaw demonstrates the importance of proper input sanitization in web applications and highlights how seemingly benign components like cookie handling can become attack vectors. The ATT&CK framework categorizes this vulnerability under T1190 for Proxy Execution and T1071.1000 for Application Layer Protocol: Web Protocols, as it involves manipulation of HTTP protocols through cookie injection. Organizations using Python web frameworks or applications that rely on the standard library's http.cookies module must address this vulnerability to prevent potential exploitation. The patch implementation provides a robust solution by implementing strict validation rules that reject any control characters, ensuring that cookie data cannot be used to inject HTTP headers. This approach follows the defensive programming principle of rejecting invalid input rather than attempting to sanitize it, which is more secure and reliable. The vulnerability underscores the critical need for comprehensive security testing of standard library components and highlights how even core language features can contain security flaws that require careful attention. Proper implementation of this patch ensures that web applications remain resilient against header injection attacks while maintaining compatibility with legitimate cookie usage patterns. The fix also reinforces the importance of validating all user-supplied data at multiple points in the application lifecycle, particularly in HTTP header generation and processing functions.