CVE-2016-0703 in OpenSSL
Summary
by MITRE
The get_client_master_key function in s2_srvr.c in the SSLv2 implementation in OpenSSL before 0.9.8zf, 1.0.0 before 1.0.0r, 1.0.1 before 1.0.1m, and 1.0.2 before 1.0.2a accepts a nonzero CLIENT-MASTER-KEY CLEAR-KEY-LENGTH value for an arbitrary cipher, which allows man-in-the-middle attackers to determine the MASTER-KEY value and decrypt TLS ciphertext data by leveraging a Bleichenbacher RSA padding oracle, a related issue to CVE-2016-0800.
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Analysis
by VulDB Data Team • 07/08/2022
The vulnerability described in CVE-2016-0703 represents a critical weakness in OpenSSL's SSLv2 implementation that fundamentally compromises the security of encrypted communications. This flaw exists within the get_client_master_key function in the s2_srvr.c file and specifically affects OpenSSL versions prior to 0.9.8zf, 1.0.0r, 1.0.1m, and 1.0.2a. The vulnerability stems from improper validation of client master key parameters during the SSLv2 handshake process, creating a pathway for attackers to exploit weaknesses in the cryptographic protocol. The issue is particularly concerning because it affects the foundational SSLv2 protocol which, despite being deprecated, remains in use in legacy systems and can be exploited in various attack scenarios.
The technical implementation flaw occurs when the get_client_master_key function accepts a nonzero CLIENT-MASTER-KEY CLEAR-KEY-LENGTH value that is inconsistent with the actual cipher being negotiated. This parameter mismatch allows attackers to manipulate the key derivation process and exploit a Bleichenbacher RSA padding oracle vulnerability. The vulnerability is classified under CWE-20 as "Improper Input Validation" and specifically relates to improper handling of cryptographic parameters. When an attacker can manipulate the key length parameter, they can effectively bypass normal cryptographic protections and potentially reconstruct the master key used for encryption. This creates a scenario where the cryptographic integrity of the TLS connection is fundamentally compromised.
The operational impact of this vulnerability is severe and directly enables man-in-the-middle attacks that can decrypt sensitive TLS ciphertext data. Attackers leveraging this vulnerability can determine the master key value through a series of carefully crafted cryptographic operations that exploit the RSA padding oracle. This allows them to decrypt communications between clients and servers, potentially accessing confidential information such as passwords, personal data, financial transactions, and other sensitive communications. The vulnerability is particularly dangerous because it can be exploited without requiring any special privileges or access to the cryptographic keys themselves. The attack vector aligns with ATT&CK technique T1041 which describes data compression and encryption techniques used by adversaries to avoid detection and maintain access to compromised systems.
Mitigation strategies for this vulnerability require immediate patching of affected OpenSSL versions to the recommended secure releases. Organizations must ensure that all systems running vulnerable OpenSSL versions are updated to prevent exploitation. Additionally, administrators should disable SSLv2 support entirely, as this protocol is fundamentally insecure and has been deprecated for years. The vulnerability highlights the importance of proper input validation in cryptographic implementations and the necessity of following secure coding practices. Security teams should implement monitoring for any attempts to use SSLv2 connections and establish policies to phase out legacy cryptographic protocols. The remediation process should include comprehensive testing to ensure that updates do not break existing applications while maintaining strong cryptographic security standards. This vulnerability serves as a reminder of the critical importance of maintaining up-to-date cryptographic libraries and the dangers of continuing to support deprecated security protocols.