CVE-2016-1549 in ntp
Summary
by MITRE
A malicious authenticated peer can create arbitrarily-many ephemeral associations in order to win the clock selection algorithm in ntpd in NTP 4.2.8p4 and earlier and NTPsec 3e160db8dc248a0bcb053b56a80167dc742d2b74 and a5fb34b9cc89b92a8fef2f459004865c93bb7f92 and modify a victim's clock.
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Analysis
by VulDB Data Team • 04/20/2025
The vulnerability described in CVE-2016-1549 represents a critical flaw in the Network Time Protocol implementation that affects ntpd versions 4.2.8p4 and earlier, as well as specific NTPsec versions. This vulnerability stems from a weakness in the clock selection algorithm that governs how network time servers determine the most reliable time source among multiple candidates. The flaw allows an authenticated malicious peer to exploit the system's association management mechanism by creating numerous ephemeral associations, effectively manipulating the clock selection process through a form of algorithmic manipulation that can lead to time synchronization compromise.
The technical implementation of this vulnerability exploits the inherent design of the NTP protocol's clock selection algorithm, which traditionally relies on a peer selection process that evaluates various time sources to determine the most accurate reference. When an authenticated attacker can create multiple ephemeral associations, they can manipulate the selection criteria that determines which time source will be chosen as the authoritative reference for the victim system. This manipulation occurs through the clock selection algorithm's vulnerability to association flooding, where the attacker's multiple connections can skew the selection process in their favor, ultimately allowing them to modify the victim's system clock to a time controlled by the attacker.
The operational impact of this vulnerability extends beyond simple time manipulation and represents a significant security risk that can undermine the integrity of time-sensitive systems and cryptographic operations. When an attacker successfully modifies a victim's clock, they can potentially disrupt time-based security mechanisms such as Kerberos authentication, SSL/TLS certificate validation, and other time-dependent cryptographic protocols that rely on accurate time synchronization. The vulnerability also creates opportunities for more sophisticated attacks, including the potential to bypass time-based access controls, disrupt network services that depend on synchronized time, and create conditions for further exploitation through time-based attacks. This vulnerability directly aligns with attack patterns documented in the MITRE ATT&CK framework under the T1070.004 technique for "Indicator Removal on Host: File Deletion" and can be considered a form of time-based deception that affects system integrity and trust relationships.
Mitigation strategies for CVE-2016-1549 focus on addressing the core weakness in the clock selection algorithm and strengthening the authentication and association management mechanisms within NTP implementations. Organizations should immediately upgrade to patched versions of ntpd or NTPsec that address this vulnerability by implementing proper rate limiting and association management controls that prevent an attacker from creating arbitrarily many ephemeral associations. The fix typically involves enhancing the peer selection algorithm to better detect and mitigate association flooding attacks, implementing stricter authentication requirements, and adding controls to limit the number of associations that can be established by a single authenticated peer. Additionally, system administrators should implement network segmentation to limit the exposure of NTP servers to untrusted networks, configure proper access controls, and monitor for unusual association patterns that could indicate exploitation attempts. This vulnerability also highlights the importance of implementing the principle of least privilege and proper network access controls as recommended by NIST SP 800-53 security controls, particularly in relation to time synchronization services that form the foundation of many network security operations.