CVE-2016-4955 in ntpd
Summary
by MITRE
ntpd in NTP 4.x before 4.2.8p8, when autokey is enabled, allows remote attackers to cause a denial of service (peer-variable clearing and association outage) by sending (1) a spoofed crypto-NAK packet or (2) a packet with an incorrect MAC value at a certain time.
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Analysis
by VulDB Data Team • 01/27/2025
The vulnerability described in CVE-2016-4955 affects the Network Time Protocol daemon (ntpd) in NTP versions 4.x prior to 4.2.8p8, specifically when the autokey feature is enabled. This flaw represents a significant security concern as it allows remote attackers to disrupt time synchronization services through carefully crafted network packets that exploit weaknesses in the cryptographic authentication mechanisms. The vulnerability operates within the broader context of network time synchronization protocols where maintaining accurate time across distributed systems is critical for security operations, logging, and various time-sensitive applications. When autokey is enabled, ntpd implements cryptographic authentication to verify the legitimacy of time synchronization messages, but this implementation contains a critical flaw that can be exploited by malicious actors.
The technical exploitation of this vulnerability occurs through two distinct attack vectors that leverage weaknesses in the peer authentication process. Attackers can send spoofed crypto-NAK packets or packets containing incorrect Message Authentication Code (MAC) values at specific timing intervals to trigger the vulnerability. When ntpd receives these malformed packets, it triggers an internal state corruption that leads to the clearing of peer variables and subsequent association outages. This behavior stems from improper handling of authentication failures within the autokey subsystem, where the daemon fails to properly validate packet authenticity and maintain stable peer connections. The vulnerability specifically targets the cryptographic authentication state machine, causing it to enter an inconsistent state that ultimately results in service disruption rather than complete system compromise.
The operational impact of CVE-2016-4955 extends beyond simple denial of service as it can severely disrupt network time synchronization across affected systems and potentially impact downstream services that depend on accurate timekeeping. When peer variables are cleared and associations go out of service, the affected ntpd instances lose their ability to maintain synchronized time with other peers, leading to cascading effects throughout the network infrastructure. This disruption can affect critical systems such as authentication services, logging mechanisms, certificate validation, and any application that relies on synchronized timestamps. The vulnerability's exploitation requires minimal network access and can be executed remotely, making it particularly dangerous in environments where network segmentation is not properly implemented. Organizations relying on NTP for time synchronization may experience intermittent time drift, service degradation, or complete time synchronization failures that can compromise security operations and audit trails.
Mitigation strategies for this vulnerability primarily focus on applying the official patch released by the NTP project, which addresses the cryptographic authentication handling flaws in ntpd versions prior to 4.2.8p8. System administrators should immediately update their NTP implementations to the patched versions and verify that autokey functionality is properly configured. Additionally, network administrators can implement packet filtering rules to limit exposure by restricting access to NTP ports and implementing rate limiting for NTP traffic. The vulnerability aligns with CWE-200, which addresses improper handling of authentication failures, and can be categorized under ATT&CK technique T1499.3 for network denial of service attacks. Organizations should also consider implementing network monitoring to detect anomalous NTP traffic patterns and establish baseline behavior for their NTP infrastructure to quickly identify potential exploitation attempts. Regular security assessments and vulnerability scanning should include verification of NTP configuration and autokey implementation to prevent similar vulnerabilities from affecting operational time synchronization services.