CVE-2013-7440 in CPython
Summary
by MITRE
The ssl.match_hostname function in CPython (aka Python) before 2.7.9 and 3.x before 3.3.3 does not properly handle wildcards in hostnames, which might allow man-in-the-middle attackers to spoof servers via a crafted certificate.
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Analysis
by VulDB Data Team • 08/23/2022
The ssl.match_hostname function in CPython versions prior to 2.7.9 and 3.x versions before 3.3.3 contained a critical vulnerability in the certificate validation process that undermined the security of SSL/TLS connections. This flaw specifically affected the handling of wildcard certificates, which are commonly used to secure multiple subdomains under a single certificate authority. The vulnerability stems from improper validation of wildcard patterns in hostname matching, creating a potential attack vector for man-in-the-middle adversaries who could exploit this weakness to impersonate legitimate servers. The issue is classified under CWE-295 which addresses improper certificate validation and certificate path validation, making it a fundamental security flaw in the certificate verification mechanism. This vulnerability directly impacts the integrity of SSL/TLS security protocols by allowing attackers to craft certificates that would pass validation despite not actually belonging to the claimed server.
The technical implementation flaw occurs when the ssl.match_hostname function processes wildcard certificates containing asterisk characters in the subject alternative name (SAN) field of X.509 certificates. In affected versions, the function fails to correctly validate wildcard patterns according to the RFC 6125 standard for hostname matching, particularly when dealing with multiple levels of subdomain wildcards. The function incorrectly matches hostnames against wildcard patterns, allowing a certificate issued for *.example.com to be accepted for a connection to subdomain.example.com when it should only be valid for specific subdomains like www.example.com or mail.example.com. This improper matching enables attackers to create certificates that appear legitimate to the validation function while actually being malicious. The vulnerability exists because the implementation does not properly enforce the wildcard matching rules that specify wildcards can only match a single label in the domain name and cannot span multiple subdomain levels.
The operational impact of this vulnerability is significant as it allows attackers to perform successful man-in-the-middle attacks against SSL/TLS connections that rely on hostname verification. An attacker with access to a certificate authority or ability to obtain a certificate for a wildcard domain can create a malicious certificate that will be accepted by vulnerable applications using affected Python versions. This compromises the trust model of SSL/TLS connections, potentially allowing interception of sensitive data, credential theft, and unauthorized access to protected services. The vulnerability affects any application that uses Python's ssl module for certificate validation, including web applications, email clients, and network services. The attack surface extends beyond simple web browsing to any protocol that uses SSL/TLS with hostname verification, making it particularly dangerous for enterprise environments where Python-based applications handle critical data transfers and authentication processes. Organizations using affected Python versions are exposed to potential data breaches and security incidents that could result in regulatory compliance violations and financial losses.
Organizations should immediately upgrade to Python versions 2.7.9 or later and 3.3.3 or later to address this vulnerability, as these releases contain the corrected ssl.match_hostname implementation that properly validates wildcard patterns according to RFC 6125 standards. Additionally, system administrators should conduct comprehensive inventory assessments to identify all applications and services running vulnerable Python versions and ensure proper patching across all environments. The mitigation strategy should include implementing additional security controls such as certificate pinning for critical applications, monitoring for suspicious certificate usage, and employing network-level security measures to detect and prevent man-in-the-middle attacks. From an ATT&CK framework perspective, this vulnerability maps to techniques involving credential access through network sniffing and man-in-the-middle attacks, specifically T1046 for network service scanning and T1566 for credential harvesting via phishing or network attacks. Security teams should also consider implementing certificate transparency monitoring to detect unauthorized certificate issuance and establish incident response procedures for potential exploitation of this vulnerability. The vulnerability demonstrates the critical importance of proper certificate validation implementation and highlights the need for continuous security assessment of cryptographic libraries used in production environments.