CVE-2015-3405 in ntp
Summary
by MITRE
ntp-keygen in ntp 4.2.8px before 4.2.8p2-RC2 and 4.3.x before 4.3.12 does not generate MD5 keys with sufficient entropy on big endian machines when the lowest order byte of the temp variable is between 0x20 and 0x7f and not #, which might allow remote attackers to obtain the value of generated MD5 keys via a brute force attack with the 93 possible keys.
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Analysis
by VulDB Data Team • 12/15/2022
The vulnerability described in CVE-2015-3405 affects the ntp-keygen utility within the Network Time Protocol implementation, specifically impacting versions prior to 4.2.8p2-RC2 and 4.3.12. This flaw resides in the cryptographic key generation process and represents a significant weakness in the security infrastructure of time synchronization systems. The issue manifests exclusively on big endian architectures where the random number generation process becomes predictable due to insufficient entropy in the key generation algorithm. The vulnerability is particularly concerning because it directly impacts the security of NTP authentication mechanisms that rely on MD5 keys for protecting time synchronization communications.
The technical root cause of this vulnerability stems from a flaw in the random number generation algorithm used by ntp-keygen on big endian systems. When generating MD5 keys, the utility utilizes a temporary variable where the lowest order byte falls within the range of 0x20 to 0x7f but excludes the character '#'. This specific condition creates a predictable pattern in the key generation process that reduces the entropy of the resulting MD5 keys. The vulnerability maps to CWE-330 which describes insufficient entropy in random number generation, and more specifically to CWE-331 which addresses insufficient entropy in a cryptographic key. The flaw essentially creates a reduced search space for attackers attempting to brute force the generated keys, as they only need to consider 93 possible values rather than the full cryptographic space.
The operational impact of this vulnerability extends beyond simple cryptographic weakness to potentially compromise the integrity and authenticity of time synchronization across networked systems. When attackers can predict or significantly reduce the search space for MD5 keys used in NTP authentication, they gain the ability to forge time synchronization messages and potentially manipulate system time across multiple devices. This represents a serious threat to network security infrastructure since NTP is fundamental to maintaining accurate time across distributed systems, and compromised time synchronization can affect log integrity, authentication mechanisms, and security protocol timing. The vulnerability aligns with ATT&CK technique T1070.004 which involves the use of network time protocol for time synchronization and can be exploited to undermine system integrity and security monitoring capabilities.
The security implications of this vulnerability are particularly severe in environments where NTP authentication is critical for maintaining system integrity and where attackers have network access to systems using vulnerable NTP versions. The reduced entropy makes the keys susceptible to brute force attacks that could be executed within reasonable timeframes, especially when considering that the 93 possible key values represent a very small subset of the full MD5 key space. Organizations using affected NTP versions should immediately implement mitigation strategies including upgrading to patched versions, disabling vulnerable NTP key generation where possible, and implementing additional network security controls to monitor for potential NTP-related attacks. The vulnerability demonstrates the critical importance of proper entropy management in cryptographic implementations and highlights the need for thorough testing across different architectural platforms during security development lifecycle processes.