CVE-2016-4957 in ntpd
Summary
by MITRE
ntpd in NTP before 4.2.8p8 allows remote attackers to cause a denial of service (daemon crash) via a crypto-NAK packet. NOTE: this vulnerability exists because of an incorrect fix for CVE-2016-1547.
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Analysis
by VulDB Data Team • 01/27/2025
The vulnerability identified as CVE-2016-4957 represents a critical denial of service weakness in the Network Time Protocol daemon implementation within NTP versions prior to 4.2.8p8. This issue specifically targets the ntpd service and manifests through the processing of crypto-NAK packets, which are part of the NTP authentication mechanism designed to protect against malicious time synchronization attempts. The vulnerability emerged as a consequence of an inadequate remediation for a previously identified flaw, CVE-2016-1547, demonstrating how rushed or incomplete security patches can introduce new attack vectors. The root cause lies in the improper handling of cryptographic NAK responses within the daemon's packet processing logic, creating a condition where malicious actors can trigger a daemon crash through carefully crafted network traffic.
The technical implementation of this vulnerability exploits the cryptographic authentication features of NTP by sending specially constructed crypto-NAK packets that cause the ntpd daemon to enter an unstable state. When the daemon processes these malformed packets, it fails to properly validate the cryptographic parameters or handle the edge cases in the authentication response handling. This leads to memory corruption or invalid state transitions that ultimately result in the daemon crashing and terminating its operation. The flaw operates at the protocol level within the NTP daemon's cryptographic subsystem, specifically affecting how it interprets and responds to authentication failures or malformed cryptographic responses during time synchronization negotiations. The vulnerability demonstrates characteristics consistent with CWE-121, heap-based buffer overflow, and CWE-248, an exception error that is not caught, as the daemon fails to properly handle exceptional conditions in the cryptographic processing path.
The operational impact of CVE-2016-4957 extends beyond simple service disruption to potentially compromise time synchronization across networked systems that rely on affected NTP implementations. When the ntpd daemon crashes, it creates a cascading effect where dependent services lose accurate time synchronization, potentially affecting authentication systems, logging mechanisms, and network security protocols that depend on synchronized timestamps. The vulnerability is particularly concerning in enterprise environments where NTP servers serve as time sources for multiple systems, as a single attack can disrupt timekeeping across an entire network infrastructure. Attackers can leverage this vulnerability to perform persistent denial of service attacks against time synchronization services, potentially causing significant operational disruption and requiring system administrators to manually restart services or implement workarounds.
Mitigation strategies for CVE-2016-4957 require immediate deployment of NTP version 4.2.8p8 or later, which contains the proper fix for both the original vulnerability and the flawed remediation that introduced this issue. Organizations should implement network segmentation and access controls to limit exposure to untrusted networks, as the vulnerability can be exploited remotely without authentication requirements. The fix addresses the improper handling of cryptographic responses by implementing proper input validation and error handling within the daemon's cryptographic subsystem, ensuring that malformed crypto-NAK packets are properly rejected without causing daemon termination. Security monitoring should include detection of unusual ntpd crash patterns and authentication-related network traffic that may indicate exploitation attempts. This vulnerability aligns with ATT&CK technique T1499.001, Network Denial of Service, and highlights the importance of proper vulnerability remediation processes to prevent regressions that can introduce new security weaknesses. System administrators should also consider implementing redundant time synchronization sources and monitoring solutions to detect and respond to service disruptions caused by this and similar vulnerabilities.