CVE-2016-0701 in OpenSSL
Summary
by MITRE
The DH_check_pub_key function in crypto/dh/dh_check.c in OpenSSL 1.0.2 before 1.0.2f does not ensure that prime numbers are appropriate for Diffie-Hellman (DH) key exchange, which makes it easier for remote attackers to discover a private DH exponent by making multiple handshakes with a peer that chose an inappropriate number, as demonstrated by a number in an X9.42 file.
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Analysis
by VulDB Data Team • 07/06/2022
The vulnerability identified as CVE-2016-0701 resides within the OpenSSL cryptographic library's implementation of the Diffie-Hellman key exchange mechanism. This flaw specifically affects OpenSSL versions 1.0.2 through 1.0.2e, where the DH_check_pub_key function fails to properly validate the primality and appropriateness of prime numbers used in the Diffie-Hellman key exchange process. The issue stems from insufficient validation of public key components, allowing attackers to potentially exploit weak prime numbers that do not meet the mathematical requirements for secure key exchange. This vulnerability is particularly concerning because it enables a class of attacks known as small subgroup confinement attacks, where an attacker can manipulate the key exchange process to force the use of weak parameters.
The technical flaw manifests in the cryptographic validation process where OpenSSL does not adequately verify that the prime numbers and public key values used in Diffie-Hellman operations meet the necessary security criteria. When a peer presents a public key that uses an inappropriate prime number, the DH_check_pub_key function fails to reject it properly, allowing the insecure key exchange to proceed. This weakness is particularly evident when dealing with X9.42 formatted files, which contain standardized parameters for cryptographic operations. The vulnerability creates a scenario where an attacker can perform multiple handshake attempts, each time selecting different prime numbers, and through statistical analysis or mathematical techniques, gradually deduce the private exponent. This represents a significant weakening of the cryptographic security model that Diffie-Hellman key exchange is designed to provide.
The operational impact of CVE-2016-0701 extends beyond simple cryptographic weakness to encompass potential compromise of secure communications across numerous systems utilizing OpenSSL. Attackers can exploit this vulnerability to perform passive eavesdropping on encrypted communications, potentially decrypting sensitive data transmitted over protocols such as HTTPS, SSH, and SSL/TLS connections. The vulnerability is particularly dangerous in environments where long-term key exchange security is critical, as it allows for extended periods of undetected compromise. Organizations using affected OpenSSL versions may experience data breaches, loss of confidential information, and potential regulatory violations. The attack vector requires only that an attacker can influence or observe the key exchange process, making it particularly insidious in networked environments where man-in-the-middle attacks are possible.
Mitigation strategies for CVE-2016-0701 primarily involve upgrading to OpenSSL version 1.0.2f or later, which includes proper validation of Diffie-Hellman prime numbers and public key components. System administrators should conduct comprehensive inventory assessments to identify all systems running affected OpenSSL versions and implement immediate patching procedures. Organizations should also consider implementing additional security measures such as monitoring for unusual handshake patterns and implementing stricter cryptographic parameter validation at the network level. The vulnerability aligns with CWE-327, which addresses the use of weak cryptographic algorithms and improper implementation of cryptographic functions. From an ATT&CK framework perspective, this vulnerability maps to techniques involving credential access through cryptographic attacks and can be leveraged as part of broader exploitation campaigns targeting secure communication channels. Regular security audits and cryptographic assessments should be conducted to ensure that all systems maintain appropriate security postures against evolving threats.