CVE-2015-0205 in OpenSSL
Summary
by MITRE
The ssl3_get_cert_verify function in s3_srvr.c in OpenSSL 1.0.0 before 1.0.0p and 1.0.1 before 1.0.1k accepts client authentication with a Diffie-Hellman (DH) certificate without requiring a CertificateVerify message, which allows remote attackers to obtain access without knowledge of a private key via crafted TLS Handshake Protocol traffic to a server that recognizes a Certification Authority with DH support.
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Analysis
by VulDB Data Team • 03/02/2022
The vulnerability described in CVE-2015-0205 represents a critical flaw in OpenSSL's TLS handshake implementation that undermines the fundamental security assumptions of certificate-based authentication. This issue affects OpenSSL versions prior to 1.0.0p and 1.0.1k, specifically within the ssl3_get_cert_verify function located in s3_srvr.c. The vulnerability stems from an improper validation mechanism during the SSL/TLS handshake process where the server fails to require the standard CertificateVerify message when processing client authentication with Diffie-Hellman certificates. This weakness creates a scenario where an attacker can bypass the normal authentication flow and gain unauthorized access to systems without possessing the corresponding private key that should be required for legitimate certificate validation.
The technical flaw manifests in the SSL/TLS protocol implementation where the server accepts client certificates that utilize Diffie-Hellman key exchange mechanisms without performing the necessary cryptographic verification through the CertificateVerify message. This message serves as a critical cryptographic proof that the client actually possesses the private key corresponding to the public key presented in the certificate. When this verification step is omitted, the server effectively accepts any client certificate that matches the expected Diffie-Hellman parameters, regardless of whether the client truly controls the associated private key. The vulnerability specifically targets servers that have been configured to recognize Certification Authorities with DH support, creating a dangerous attack vector where malicious actors can exploit this gap in validation logic to impersonate legitimate clients.
From an operational impact perspective, this vulnerability enables remote attackers to perform unauthorized access attempts against systems that rely on client certificate authentication for security. The attack requires crafting specific TLS Handshake Protocol traffic that exploits the missing validation step, making it particularly insidious as it can be executed without direct access to the private key material. This weakness essentially allows attackers to perform what is known as a "man-in-the-middle" attack variant where they can establish authenticated connections to servers without proper cryptographic proof. The implications extend beyond simple unauthorized access, as this vulnerability can potentially enable further exploitation including privilege escalation, data exfiltration, and system compromise depending on the access controls and permissions granted to authenticated users. The vulnerability aligns with CWE-310, which addresses cryptographic weaknesses in authentication mechanisms, and represents a failure in the proper implementation of certificate validation protocols.
Organizations affected by this vulnerability should implement immediate mitigations including upgrading to OpenSSL versions 1.0.0p or 1.0.1k, which contain the necessary patches to enforce proper CertificateVerify message validation. System administrators should also consider implementing additional monitoring and detection mechanisms to identify anomalous TLS handshake patterns that might indicate exploitation attempts. The vulnerability demonstrates the importance of comprehensive protocol validation and highlights the critical need for proper cryptographic implementation even in well-established security libraries. From an ATT&CK framework perspective, this vulnerability maps to techniques involving credential access and privilege escalation, as it allows attackers to bypass authentication mechanisms and potentially gain elevated privileges within targeted systems. Regular security assessments and vulnerability scanning should be conducted to ensure that all systems utilizing OpenSSL are properly patched and that the certificate validation mechanisms are functioning as intended, preventing exploitation of this and similar cryptographic weaknesses.