CVE-2013-4073 in Mac OS X
Summary
by MITRE
The OpenSSL::SSL.verify_certificate_identity function in lib/openssl/ssl.rb in Ruby 1.8 before 1.8.7-p374, 1.9 before 1.9.3-p448, and 2.0 before 2.0.0-p247 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.
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Analysis
by VulDB Data Team • 06/01/2021
The vulnerability described in CVE-2013-4073 represents a critical flaw in Ruby's OpenSSL implementation that undermines the fundamental security guarantees of SSL/TLS certificate validation. This issue specifically affects the OpenSSL::SSL.verify_certificate_identity function within Ruby's ssl.rb library, where improper handling of null characters in domain names within X.509 certificates creates a pathway for sophisticated man-in-the-middle attacks. The flaw stems from the inability of the verification process to correctly parse and validate domain names containing null characters, which are typically rejected by standard certificate validation mechanisms but can be exploited when present in the Subject Alternative Name field of certificates.
The technical implementation of this vulnerability lies in how Ruby's OpenSSL library processes certificate subject alternative names during SSL certificate validation. When a certificate contains a null character within a domain name field, the verification function fails to properly sanitize or reject the malformed input, allowing attackers to craft certificates that appear valid to Ruby applications while actually containing malicious domain specifications. This behavior directly violates the certificate validation standards established by x509 and PKI protocols, where domain name validation should be strict and consistent across all implementations. The vulnerability operates at the application layer rather than the protocol level, making it particularly dangerous as it can bypass standard security controls that rely on proper certificate validation.
The operational impact of this vulnerability extends far beyond simple certificate validation failures, creating significant risks for any Ruby application that relies on SSL/TLS for secure communications. Attackers can exploit this weakness to create fraudulent certificates that will be accepted by vulnerable Ruby applications, enabling them to intercept and manipulate encrypted traffic between clients and servers. This capability undermines the entire trust model of SSL/TLS, allowing adversaries to impersonate legitimate services and potentially access sensitive data, credentials, or private communications. The vulnerability affects multiple Ruby versions including 1.8.7 before 1.8.7-p374, 1.9 before 1.9.3-p448, and 2.0 before 2.0.0-p247, indicating a widespread exposure across the Ruby ecosystem. The related nature to CVE-2009-2408 suggests this represents a recurring pattern of inadequate null character handling in certificate validation processes.
Security mitigations for this vulnerability require immediate patching of affected Ruby versions to ensure proper null character handling in certificate validation routines. Organizations should prioritize updating their Ruby installations to versions that include the corrected OpenSSL::SSL.verify_certificate_identity function, which should properly reject certificates containing null characters in domain name fields. Additionally, system administrators should implement certificate pinning strategies where possible to provide defense-in-depth against certificate-based attacks. The vulnerability aligns with ATT&CK technique T1552.001 for credential access through certificate manipulation and CWE-295 for improper certificate validation, highlighting the need for comprehensive security controls that address both the immediate patching requirements and broader certificate management practices. Network monitoring should be enhanced to detect unusual certificate validation behaviors, and security teams should review their certificate trust stores to ensure no malicious certificates have been inadvertently accepted due to this vulnerability.