CVE-2016-2107 in Communications Application Session Controller
Summary
by MITRE
The AES-NI implementation in OpenSSL before 1.0.1t and 1.0.2 before 1.0.2h does not consider memory allocation during a certain padding check, which allows remote attackers to obtain sensitive cleartext information via a padding-oracle attack against an AES CBC session. NOTE: this vulnerability exists because of an incorrect fix for CVE-2013-0169.
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Analysis
by VulDB Data Team • 01/05/2025
The vulnerability identified as CVE-2016-2107 represents a critical flaw in OpenSSL's cryptographic implementation that specifically affects versions prior to 1.0.1t and 1.0.2h. This issue resides within the AES-NI hardware acceleration implementation, which is designed to optimize AES encryption operations on processors that support the AES-NI instruction set. The vulnerability stems from an improper handling of memory allocation during a specific padding validation process, creating a condition that enables attackers to exploit padding oracle vulnerabilities. The flaw was introduced as a misguided attempt to address CVE-2013-0169, which itself was a padding oracle vulnerability in the SSL/TLS implementation, but the subsequent fix created this new vulnerability that undermines the security of encrypted communications.
The technical execution of this vulnerability occurs through a padding oracle attack that leverages timing differences in memory allocation patterns during the AES cipher block chaining decryption process. When OpenSSL processes encrypted data with AES-CBC mode, it performs padding validation checks that should be constant-time operations to prevent information leakage. However, the flawed implementation causes memory allocation patterns to vary based on whether padding validation succeeds or fails, creating observable timing differences that attackers can exploit. This timing variation allows adversaries to perform statistical analysis and gradually reconstruct the plaintext content of encrypted sessions. The vulnerability specifically targets the hardware-accelerated AES-NI implementation, making it particularly dangerous as it affects high-performance cryptographic operations that are widely deployed in production environments.
The operational impact of CVE-2016-2107 extends beyond simple information disclosure, as it can compromise the confidentiality of entire encrypted sessions that rely on AES-CBC encryption. Attackers can use this vulnerability to decrypt sensitive data transmitted over HTTPS, SSH, and other protocols that utilize affected OpenSSL versions, potentially accessing user credentials, personal information, financial data, and proprietary business information. The vulnerability affects systems where OpenSSL is used for TLS/SSL encryption, including web servers, email servers, database servers, and any application that implements secure communication using the affected cryptographic libraries. Given that OpenSSL is deployed across millions of systems worldwide, the potential attack surface is enormous, with numerous high-value targets including financial institutions, government agencies, healthcare providers, and enterprise networks that rely on secure communications.
The remediation strategy for CVE-2016-2107 requires immediate deployment of updated OpenSSL versions that contain proper fixes for both the original padding oracle vulnerability and this subsequent flaw. Organizations must upgrade to OpenSSL 1.0.1t or 1.0.2h and later versions, which implement correct constant-time padding validation and proper memory allocation handling. System administrators should conduct comprehensive inventory checks to identify all systems running vulnerable OpenSSL versions and prioritize updates accordingly. Additionally, organizations should consider implementing network monitoring to detect potential exploitation attempts and review their cryptographic configurations to ensure that AES-CBC mode is not being used in environments where padding oracle attacks could be mounted. This vulnerability also highlights the importance of careful testing when implementing cryptographic fixes, as demonstrated by the fact that the remediation for one vulnerability inadvertently introduced another. The issue aligns with CWE-20, "Improper Input Validation," and CWE-310, "Cryptographic Issues," while mapping to ATT&CK technique T1552.001, "Unsecured Credentials," and T1071.004, "Application Layer Protocol: DNS," as attackers may use this vulnerability to extract credentials from encrypted communications. Organizations should also consider implementing alternative encryption modes such as AES-GCM that are less susceptible to padding oracle attacks and maintain robust patch management processes to prevent similar issues in the future.