CVE-2016-4953 in ntpd
Summary
by MITRE
ntpd in NTP 4.x before 4.2.8p8 allows remote attackers to cause a denial of service (ephemeral-association demobilization) by sending a spoofed crypto-NAK packet with incorrect authentication data at a certain time.
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Analysis
by VulDB Data Team • 12/22/2024
The vulnerability identified as CVE-2016-4953 affects the Network Time Protocol daemon ntpd in versions 4.x prior to 4.2.8p8, representing a critical denial of service weakness that can be exploited remotely. This flaw resides within the cryptographic authentication mechanism of the NTP protocol implementation, specifically targeting how the system handles authentication failures and association management. The vulnerability manifests when an attacker crafts and sends a spoofed crypto-NAK packet containing incorrect authentication data to a vulnerable NTP server, triggering a specific sequence of events that leads to service disruption.
The technical root cause of this vulnerability lies in the improper handling of cryptographic NAK (Not Acknowledged) packets within the NTP protocol stack. When ntpd receives a spoofed crypto-NAK packet with invalid authentication data, the daemon enters a state where it demobilizes ephemeral associations, which are temporary network connections used for time synchronization. This behavior constitutes a weakness categorized under CWE-284, specifically related to improper access control during cryptographic operations, and aligns with ATT&CK technique T1499.001 for network denial of service attacks. The flaw exploits the timing-sensitive nature of NTP authentication mechanisms, where the system's response to invalid authentication data triggers an unintended state transition that results in service degradation.
The operational impact of this vulnerability extends beyond simple service interruption, as it can lead to widespread time synchronization failures across networks that depend on the affected NTP servers. When exploited successfully, the vulnerability causes ephemeral associations to be prematurely terminated, forcing clients to re-establish connections and potentially leading to cascading failures in time-sensitive applications. Network administrators may observe increased network traffic as clients repeatedly attempt to synchronize time, while monitoring systems might report unusual association demobilization patterns. The attack requires minimal resources from the attacker, who only needs to send a single spoofed packet, making this a particularly dangerous vulnerability for critical infrastructure environments where time synchronization reliability is paramount.
Mitigation strategies for CVE-2016-4953 primarily focus on updating to NTP version 4.2.8p8 or later, which includes patches addressing the cryptographic authentication handling flaw. Organizations should implement network segmentation and access control measures to limit exposure of NTP servers to untrusted networks, as well as deploy intrusion detection systems that can identify suspicious NAK packet patterns. Additional protective measures include configuring NTP servers to restrict access based on source IP addresses, implementing rate limiting for authentication-related packets, and monitoring for unusual association demobilization events. The vulnerability also highlights the importance of maintaining up-to-date security patches across all network infrastructure components, as the issue demonstrates how cryptographic protocol implementations can introduce unexpected denial of service vectors. Security teams should consider implementing automated patch management processes to ensure rapid deployment of security updates and establish monitoring procedures that can detect exploitation attempts against time synchronization services.