CVE-2013-5211 in ntpd
Summary
by MITRE
The monlist feature in ntp_request.c in ntpd in NTP before 4.2.7p26 allows remote attackers to cause a denial of service (traffic amplification) via forged (1) REQ_MON_GETLIST or (2) REQ_MON_GETLIST_1 requests, as exploited in the wild in December 2013.
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Analysis
by VulDB Data Team • 01/26/2025
The vulnerability identified as CVE-2013-5211 represents a critical security flaw in the Network Time Protocol daemon implementation that enables remote attackers to exploit traffic amplification capabilities for denial of service attacks. This vulnerability specifically affects ntpd versions prior to 4.2.7p26 and stems from the implementation of the monlist feature in the ntp_request.c file. The monlist functionality was originally designed to allow administrators to query the list of clients that have recently synchronized with the NTP server, but it became a vector for malicious exploitation due to its inherent design flaws.
The technical exploitation occurs through forged network requests that target the REQ_MON_GETLIST and REQ_MON_GETLIST_1 message types, which are part of the NTP protocol's monitoring interface. These requests, when sent with spoofed source IP addresses, can trigger the NTP server to respond with voluminous amounts of data to the victim's IP address rather than the attacker's actual location. The amplification factor can reach several hundred times the original request size, making these attacks particularly effective for overwhelming network resources and causing service disruption. This mechanism operates by leveraging the NTP server's response to monitoring queries, which are designed to return detailed information about connected clients and their synchronization status.
The operational impact of this vulnerability extends beyond simple service disruption, as it enables attackers to conduct large-scale distributed denial of service attacks using the amplification technique. When exploited in the wild during December 2013, these attacks demonstrated the potential for generating massive traffic volumes that could overwhelm network infrastructure and render services unavailable to legitimate users. The vulnerability aligns with CWE-400, which categorizes issues related to excessive resource consumption, and represents a classic example of a reflection/amplification attack pattern that has since become a common vector in DDoS campaigns. The attack requires minimal resources from the attacker while potentially causing significant damage to targeted systems.
Security mitigations for this vulnerability primarily involve upgrading to NTP version 4.2.7p26 or later, which includes patches that disable or restrict the monlist functionality by default. Network administrators should also implement access control measures such as firewall rules that limit access to NTP monitoring ports and disable the monlist feature entirely when it is not required for legitimate administrative purposes. The mitigation strategy aligns with ATT&CK technique T1498.001, which covers direct network denial of service attacks, and emphasizes the importance of proper protocol implementation and access control measures. Organizations should also consider implementing rate limiting and monitoring for unusual traffic patterns that might indicate exploitation attempts, as the vulnerability demonstrates how legitimate protocol features can be abused for malicious purposes when not properly secured.