CVE-2015-0204 in Internet Directory
Summary
by MITRE
The ssl3_get_key_exchange function in s3_clnt.c in OpenSSL before 0.9.8zd, 1.0.0 before 1.0.0p, and 1.0.1 before 1.0.1k allows remote SSL servers to conduct RSA-to-EXPORT_RSA downgrade attacks and facilitate brute-force decryption by offering a weak ephemeral RSA key in a noncompliant role, related to the "FREAK" issue. NOTE: the scope of this CVE is only client code based on OpenSSL, not EXPORT_RSA issues associated with servers or other TLS implementations.
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Analysis
by VulDB Data Team • 04/17/2023
The vulnerability described in CVE-2015-0204 represents a critical security flaw in OpenSSL's client-side implementation that enables man-in-the-middle attackers to perform RSA-to-EXPORT_RSA downgrade attacks. This issue specifically affects the ssl3_get_key_exchange function within the s3_clnt.c file and impacts OpenSSL versions prior to 0.9.8zd, 1.0.0p, and 1.0.1k. The vulnerability stems from the client's improper handling of ephemeral RSA key exchanges during SSL/TLS handshakes, creating a pathway for attackers to force the use of weak cryptographic parameters.
The technical flaw manifests when an attacker-controlled SSL server presents a weak ephemeral RSA key during the key exchange phase of the SSL handshake process. This weakness allows the client to accept a key that is significantly weaker than what would normally be expected, effectively downgrading the security level from standard RSA to EXPORT_RSA encryption. The vulnerability operates through a specific implementation flaw where the client fails to properly validate the strength of the ephemeral RSA key offered by the server, particularly when the server presents a key size that falls below the minimum acceptable threshold for secure communication.
From an operational standpoint, this vulnerability creates a significant risk for any system using affected OpenSSL versions as an SSL client. Attackers can exploit this weakness by intercepting communications between vulnerable clients and servers, forcing the use of weak cryptographic parameters that can be brute-forced or cracked using modern computational resources. The impact extends beyond simple data confidentiality breaches, as successful exploitation can lead to full decryption of communications, session hijacking, and potential credential theft. This vulnerability directly relates to the FREAK (Factoring RSA Export Keys) attack vector that affected numerous TLS implementations across the internet.
The security implications of this vulnerability align with CWE-327, which addresses the use of weak cryptographic algorithms, and can be mapped to ATT&CK technique T1552.001 for unsecured credentials and T1046 for network service scanning. Organizations using vulnerable OpenSSL clients face substantial risk of cryptographic downgrade attacks that can compromise the integrity and confidentiality of their communications. The vulnerability is particularly concerning because it operates at the protocol level during handshake negotiation, making it difficult to detect through standard network monitoring tools.
Mitigation strategies for this vulnerability require immediate patching of affected OpenSSL versions to the recommended secure releases. Organizations should also implement network monitoring to detect unusual handshake patterns and consider configuring clients to reject weak ephemeral RSA keys. Additional defensive measures include enforcing strong cryptographic policies, implementing certificate pinning where appropriate, and conducting regular security audits of SSL/TLS implementations. The vulnerability demonstrates the critical importance of proper cryptographic parameter validation in SSL/TLS implementations and highlights the need for robust security practices in protocol-level code to prevent downgrade attacks that can compromise entire communication channels.