CVE-2016-1547 in ntp
Summary
by MITRE
An off-path attacker can cause a preemptible client association to be demobilized in NTP 4.2.8p4 and earlier and NTPSec a5fb34b9cc89b92a8fef2f459004865c93bb7f92 by sending a crypto NAK packet to a victim client with a spoofed source address of an existing associated peer. This is true even if authentication is enabled.
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Analysis
by VulDB Data Team • 04/20/2025
The vulnerability described in CVE-2016-1547 represents a significant security flaw in the Network Time Protocol implementation that affects versions 4.2.8p4 and earlier of NTP as well as NTPSec up to a specific commit hash. This vulnerability operates under the premise of an off-path attack scenario where an adversary positioned outside the normal communication path between network entities can manipulate the time synchronization process. The attack specifically targets client associations within the NTP ecosystem, exploiting a fundamental weakness in how the protocol handles peer demobilization under certain conditions.
The technical exploitation mechanism involves crafting and transmitting a specially formatted crypto NAK packet to a targeted victim client. This packet contains a spoofed source address that mimics an existing associated peer within the network. The vulnerability stems from the protocol's failure to properly validate the authenticity of demobilization requests, particularly when they originate from addresses that appear to be legitimate peers. The flaw allows an attacker to effectively disrupt legitimate time synchronization processes by causing the client to prematurely terminate its association with a valid time source, even when strong authentication mechanisms are in place.
This vulnerability is particularly concerning because it operates independently of authentication status, meaning that even when proper cryptographic authentication is enabled, the attack remains effective. The impact extends beyond simple disruption to potentially compromising the integrity of time synchronization across affected systems, which can have cascading effects on security protocols that depend on accurate timekeeping. The attack demonstrates a critical weakness in the NTP protocol's peer management and association validation mechanisms, where the system fails to distinguish between legitimate and malicious demobilization requests based on spoofed source addresses.
The operational implications of this vulnerability are substantial for network security infrastructure that relies on NTP for time synchronization. Organizations using affected NTP versions may experience unexpected client disconnections from legitimate time sources, potentially leading to time drift issues that affect security logging, certificate validation, and other time-sensitive operations. The vulnerability also represents a vector for more sophisticated attacks where an attacker could use this capability to isolate specific clients from time sources, potentially creating conditions for further exploitation or disruption of time-dependent security services.
This vulnerability aligns with CWE-284, which addresses improper access control in network protocols, and demonstrates how weaknesses in authentication validation can lead to unauthorized system state modifications. From an ATT&CK framework perspective, this represents a network denial of service technique that can be used to disrupt time synchronization services, potentially enabling other attacks that rely on time manipulation. The attack pattern also aligns with techniques involving spoofing and man-in-the-middle approaches that exploit protocol implementation flaws to achieve unauthorized access or modification of system states.
Mitigation strategies should focus on upgrading to patched versions of NTP that address the demobilization validation issue, implementing proper network segmentation to limit the attack surface, and deploying network monitoring to detect anomalous crypto NAK packet patterns. Organizations should also consider implementing additional authentication layers and regularly auditing their time synchronization infrastructure to identify and remediate similar vulnerabilities. The vulnerability underscores the importance of robust validation mechanisms in network protocols and highlights the need for continuous security assessment of critical infrastructure components.