CVE-2009-0653 in OpenSSL
Summary
by MITRE
OpenSSL, probably 0.9.6, does not verify the Basic Constraints for an intermediate CA-signed certificate, which allows remote attackers to spoof the certificates of trusted sites via a man-in-the-middle attack, a related issue to CVE-2002-0970.
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Analysis
by VulDB Data Team • 08/04/2021
The vulnerability described in CVE-2009-0653 represents a critical flaw in OpenSSL's certificate validation process that undermines the fundamental security model of public key infrastructure. This issue affects OpenSSL version 0.9.6 and potentially other versions within the same release line, where the software fails to properly enforce the Basic Constraints extension in X.509 certificates. The Basic Constraints extension is a critical component that defines whether a certificate can be used as a Certificate Authority, with the CA flag indicating if the certificate holder is authorized to issue certificates for other entities. When this validation is bypassed, an attacker can construct a fraudulent certificate chain that appears legitimate to vulnerable systems.
The technical flaw stems from OpenSSL's failure to verify the Basic Constraints extension during certificate chain validation, specifically when processing intermediate certificates signed by a Certificate Authority. This omission allows attackers to create certificates where an intermediate certificate lacks the proper CA constraint flag, yet the system still accepts it as a valid trust anchor. The vulnerability operates under the principle that a certificate should not be trusted to issue other certificates unless explicitly authorized through the Basic Constraints extension. Without this validation, the certificate verification process becomes susceptible to attacks where malicious actors can present certificates that should not be trusted but are accepted due to the missing validation check.
The operational impact of this vulnerability is severe and directly enables man-in-the-middle attacks against SSL/TLS connections. An attacker who can compromise an intermediate certificate or create a fraudulent certificate chain can successfully impersonate legitimate websites and services, making it appear as though the victim is connecting to a trusted entity. This allows for the interception of sensitive data, session hijacking, and other malicious activities that would normally be prevented by proper certificate validation. The vulnerability is particularly dangerous because it affects the core trust validation mechanism of SSL/TLS implementations, potentially compromising thousands of secure connections across multiple applications and services that rely on OpenSSL for their cryptographic operations.
This vulnerability aligns with CWE-310, which describes cryptographic issues related to improper certificate validation, and maps to ATT&CK technique T1552.001 for unsecured credentials and T1046 for network service scanning. The attack vector typically involves an attacker creating a malicious certificate chain that bypasses the Basic Constraints validation, then using this chain to establish fraudulent SSL/TLS connections. Organizations affected by this vulnerability should immediately update to patched versions of OpenSSL, typically those released after the vulnerability disclosure, and conduct comprehensive certificate audits to identify any potentially compromised trust relationships. Additionally, implementing certificate pinning mechanisms and monitoring for unusual certificate issuance patterns can provide additional defense layers against exploitation attempts.