CVE-2016-9042 in ntp
Summary
by MITRE
An exploitable denial of service vulnerability exists in the origin timestamp check functionality of ntpd 4.2.8p9. A specially crafted unauthenticated network packet can be used to reset the expected origin timestamp for target peers. Legitimate replies from targeted peers will fail the origin timestamp check (TEST2) causing the reply to be dropped and creating a denial of service condition.
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Analysis
by VulDB Data Team • 11/15/2022
The vulnerability identified as CVE-2016-9042 represents a critical denial of service weakness within the Network Time Protocol daemon version 4.2.8p9. This flaw specifically targets the origin timestamp validation mechanism that ntpd employs to verify the authenticity and integrity of time synchronization messages. The vulnerability operates through a sophisticated manipulation of network packet timing that exploits the protocol's trust model and validation procedures. The affected system processes legitimate time synchronization packets but fails to properly validate the timestamp information, creating a condition where valid responses are incorrectly rejected. This fundamental flaw in the timestamp validation process creates a cascading effect that undermines the core functionality of the time synchronization service.
The technical implementation of this vulnerability stems from the ntpd daemon's handling of the TEST2 validation check within the NTP protocol's four-stage handshake mechanism. When an attacker crafts and sends a specially formatted unauthenticated packet to a target ntpd instance, the system resets its expected origin timestamp value for specific peer connections. This manipulation occurs because the daemon does not adequately validate the source of timestamp information or properly handle timestamp updates from unauthenticated sources. The vulnerability manifests when legitimate time synchronization replies arrive from targeted peers, but these replies fail the origin timestamp check because the timestamp values no longer match the reset expectations. This creates a scenario where the legitimate time synchronization process becomes effectively disabled, as all valid responses are filtered out by the corrupted validation logic. The flaw essentially creates a race condition where the attacker can predictably disrupt the time synchronization process by resetting timestamp expectations.
The operational impact of CVE-2016-9042 extends beyond simple service disruption to create broader network stability issues that can affect critical infrastructure systems relying on accurate time synchronization. Organizations depending on ntpd for time coordination across distributed systems may experience cascading failures as time synchronization becomes unreliable or completely fails. The vulnerability affects systems where ntpd serves as a time server or time client, potentially impacting everything from network security systems that rely on synchronized timestamps for log correlation to financial systems requiring precise timekeeping for transaction processing. The attack requires minimal resources and can be executed from any network location, making it particularly dangerous as it can be exploited by remote attackers without authentication credentials. This vulnerability aligns with CWE-284 Access Control Issues and represents a specific implementation weakness in the NTP protocol's security model, as documented in the NIST National Vulnerability Database. The attack vector operates through the standard NTP network protocols, making it difficult to detect and mitigate without proper network monitoring and protocol-level security controls.
Mitigation strategies for CVE-2016-9042 require both immediate patching and architectural considerations to prevent exploitation. The primary solution involves updating ntpd to version 4.2.8p10 or later, which includes fixes for the timestamp validation logic and proper handling of unauthenticated timestamp information. Network administrators should implement proper access controls and authentication mechanisms to limit exposure to unauthenticated network traffic that could trigger this vulnerability. The implementation of NTP access control lists and restricted peer configurations can help reduce the attack surface by limiting which systems can interact with the time synchronization service. Additionally, network monitoring should be enhanced to detect unusual timestamp patterns and potential exploitation attempts, as the vulnerability creates detectable anomalies in network traffic. Organizations should also consider implementing redundant time synchronization sources to maintain service availability even if one time server becomes compromised. This vulnerability demonstrates the importance of proper timestamp validation in time synchronization protocols and aligns with ATT&CK technique T1499.001 for Network Denial of Service attacks, where the attack specifically targets service availability through protocol manipulation rather than direct network disruption. The fix implemented in subsequent ntpd versions addresses the core validation logic and implements proper timestamp handling that prevents the reset condition from occurring in legitimate network operations.