CVE-2013-4238 in Python
Summary
by MITRE
The ssl.match_hostname function in the SSL module in Python 2.6 through 3.4 does not properly handle a \0 character in a domain name in the Subject Alternative Name field of an X.509 certificate, which allows man-in-the-middle attackers to spoof arbitrary SSL servers via a crafted certificate issued by a legitimate Certification Authority, a related issue to CVE-2009-2408.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 05/21/2021
The ssl.match_hostname function in Python versions 2.6 through 3.4 contains a critical security flaw that undermines SSL certificate validation mechanisms. This vulnerability specifically affects the handling of null characters within domain names found in the Subject Alternative Name field of X.509 certificates. The flaw allows malicious actors to perform man-in-the-middle attacks by crafting certificates that appear legitimate but contain null characters in their domain specifications, thereby bypassing standard certificate validation procedures that should prevent such spoofing attempts.
This vulnerability represents a direct violation of the certificate validation standards established in x509 certificate specifications and demonstrates a failure in proper input sanitization within the Python SSL module implementation. The technical flaw stems from insufficient validation of domain name strings during SSL certificate hostname matching, where null characters are not properly handled or rejected during the certificate verification process. This weakness enables attackers to insert null bytes into domain name fields, which the function fails to properly sanitize or detect, allowing forged certificates to pass validation checks that should have rejected them.
The operational impact of this vulnerability is severe as it fundamentally compromises the security guarantees provided by SSL/TLS certificate validation. Attackers can exploit this weakness to create malicious certificates that appear valid to Python applications, enabling them to intercept and decrypt communications between clients and servers. This vulnerability affects a wide range of applications that rely on Python's SSL module for secure communications, including web applications, email servers, and any system using Python 2.6 through 3.4 for SSL certificate validation. The attack vector is particularly dangerous because it leverages legitimate certificate authorities, making the forged certificates appear trustworthy to applications using vulnerable Python versions.
The vulnerability aligns with CWE-264, which addresses permissions, privileges, and access controls in certificate validation, and relates to ATT&CK technique T1552.001 for credentials in files and T1552.006 for unsecured credentials in SSL/TLS certificates. Organizations using affected Python versions should immediately upgrade to patched versions or implement workarounds such as manual null character validation in certificate processing, though the most effective mitigation involves updating to Python versions that properly handle null characters in certificate domain names. The issue also highlights the importance of proper input validation and the need for robust certificate validation routines that adhere to established security standards and prevent character injection attacks in cryptographic validation processes.