CVE-2018-5733 in DHCP
Summary
by MITRE
A malicious client which is allowed to send very large amounts of traffic (billions of packets) to a DHCP server can eventually overflow a 32-bit reference counter, potentially causing dhcpd to crash. Affects ISC DHCP 4.1.0 -> 4.1-ESV-R15, 4.2.0 -> 4.2.8, 4.3.0 -> 4.3.6, 4.4.0.
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Analysis
by VulDB Data Team • 04/26/2025
The vulnerability identified as CVE-2018-5733 represents a critical denial-of-service weakness in the Internet Systems Consortium DHCP server implementation that affects multiple version ranges including 4.1.0 through 4.1-ESV-R15, 4.2.0 through 4.2.8, 4.3.0 through 4.3.6, and 4.4.0. This flaw manifests when a malicious client establishes a connection to a DHCP server and begins transmitting an extraordinarily high volume of traffic measured in billions of packets. The attack exploits a fundamental design limitation in the server's packet processing mechanism where a 32-bit reference counter becomes saturated due to the excessive traffic load. The vulnerability falls under the category of CWE-128 integer overflow or wraparound, which is classified as a weakness in software design that allows attackers to manipulate counter values beyond their intended range. The attack pattern aligns with ATT&CK technique T1499.004 which involves resource exhaustion attacks that target network services.
The technical implementation of this vulnerability involves the exploitation of a 32-bit unsigned integer counter that tracks DHCP packet processing activities within the dhcpd server daemon. When a malicious client floods the server with billions of packets, the counter eventually reaches its maximum value of 4294967295 and wraps around to zero, causing the server to lose track of its packet processing state. This counter overflow condition directly impacts the server's ability to maintain proper session tracking and packet handling mechanisms. The vulnerability is particularly concerning because it does not require authentication or elevated privileges to exploit, making it accessible to any attacker who can establish network connectivity to the target DHCP server. The server's response to this overflow condition results in a complete crash of the dhcpd service, rendering the network unable to perform dynamic host configuration for devices attempting to connect to the network.
The operational impact of CVE-2018-5733 extends beyond simple service disruption to encompass broader network reliability and availability concerns. When the dhcpd service crashes, network infrastructure dependent on dynamic IP assignment becomes compromised, potentially affecting hundreds or thousands of devices simultaneously depending on the network size and scope. Network administrators may experience extended downtime while troubleshooting and implementing fixes, particularly in environments where DHCP serves as the primary method for IP address allocation. The vulnerability also creates opportunities for attackers to use this as a stepping stone for more sophisticated attacks, as the service disruption can mask other malicious activities or provide cover for additional exploitation attempts. The crash condition affects all versions within the specified ranges, indicating a persistent design flaw that required multiple patch releases to address properly.
Mitigation strategies for CVE-2018-5733 involve both immediate remediation and long-term architectural improvements to network infrastructure. The primary solution requires updating all affected ISC DHCP server installations to versions that contain the patched counter handling logic, which typically includes applying the official security patches released by ISC. Network administrators should implement rate limiting and traffic monitoring on DHCP server interfaces to detect and prevent abnormal packet volume patterns that could indicate exploitation attempts. Additional defensive measures include configuring firewalls to restrict DHCP traffic to authorized clients only and implementing network segmentation to limit the potential impact of a successful attack. The vulnerability also highlights the importance of proper input validation and counter management in network services, particularly those handling high-volume packet processing. Organizations should conduct regular vulnerability assessments to identify similar integer overflow conditions in other network services and ensure proper testing of boundary conditions in software implementations. The fix typically involves replacing the vulnerable 32-bit counter with a larger integer type or implementing additional overflow detection mechanisms that can prevent the wraparound condition from causing service disruption.