CVE-2016-7426 in ntpd
Summary
by MITRE
NTP before 4.2.8p9 rate limits responses received from the configured sources when rate limiting for all associations is enabled, which allows remote attackers to cause a denial of service (prevent responses from the sources) by sending responses with a spoofed source address.
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Analysis
by VulDB Data Team • 05/13/2026
The vulnerability identified as CVE-2016-7426 affects the Network Time Protocol implementation in versions prior to 4.2.8p9, specifically addressing a flaw in how the protocol handles rate limiting for network associations. This issue arises when rate limiting is enabled for all associations within the NTP configuration, creating a scenario where legitimate responses from configured time sources are inadvertently blocked or filtered out by the rate limiting mechanism.
The technical exploitation of this vulnerability occurs through a spoofing attack vector where remote adversaries craft and transmit NTP response packets with falsified source addresses. The malicious packets are designed to trigger the rate limiting behavior of the NTP daemon, causing it to apply rate limiting rules against what it perceives as legitimate traffic from configured time sources. This misapplication of rate limiting results in the daemon's inability to properly process and respond to genuine time synchronization requests from authorized sources, effectively disrupting the time synchronization process.
From an operational perspective, this vulnerability creates a significant denial of service condition that can severely impact network infrastructure relying on accurate time synchronization. The attack can cause complete disruption of time services, potentially affecting authentication systems, logging mechanisms, network security devices, and any application that depends on synchronized time stamps. The impact extends beyond simple service interruption as it can compromise the integrity of time-sensitive operations and security protocols that rely on precise temporal coordination.
The vulnerability aligns with CWE-400, which categorizes it as a weakness related to resource exhaustion or improper handling of rate limiting mechanisms. This flaw demonstrates how defensive security measures can be subverted by attackers to create their own denial of service conditions. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique involving network denial of service attacks, specifically targeting time synchronization services that are critical infrastructure components.
Mitigation strategies for CVE-2016-7426 primarily involve upgrading to NTP version 4.2.8p9 or later, which includes fixes for the rate limiting implementation. Network administrators should also consider implementing additional security measures such as source address validation, rate limiting controls that distinguish between legitimate and malicious traffic patterns, and monitoring systems to detect anomalous NTP behavior. The fix addresses the core issue by ensuring that rate limiting rules properly differentiate between legitimate time source responses and spoofed packets, preventing the legitimate responses from being incorrectly filtered out by the rate limiting mechanism.