CVE-2018-20679 in BusyBox
Summary
by MITRE
An issue was discovered in BusyBox before 1.30.0. An out of bounds read in udhcp components (consumed by the DHCP server, client, and relay) allows a remote attacker to leak sensitive information from the stack by sending a crafted DHCP message. This is related to verification in udhcp_get_option() in networking/udhcp/common.c that 4-byte options are indeed 4 bytes.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 06/09/2025
The vulnerability identified as CVE-2018-20679 represents a critical out-of-bounds read flaw within the BusyBox network stack components, specifically affecting the udhcp functionality that serves as the foundation for DHCP client, server, and relay operations. This issue exists in BusyBox versions prior to 1.30.0 and demonstrates how seemingly minor validation logic can create significant security implications in network services. The flaw manifests when the udhcp_get_option() function in networking/udhcp/common.c fails to properly verify that 4-byte DHCP options are indeed exactly 4 bytes in length, creating a scenario where malformed packets can trigger memory access violations that expose sensitive stack data to remote attackers.
The technical implementation of this vulnerability stems from inadequate input validation within the DHCP option parsing mechanism. When the udhcp_get_option() function processes DHCP messages, it assumes that 4-byte options will always contain exactly 4 bytes of data, without performing proper bounds checking against the actual message length. This assumption becomes problematic when attackers craft malicious DHCP packets containing options that appear to be 4-byte but are actually shorter or longer than expected, causing the function to read beyond allocated memory boundaries. The specific nature of this vulnerability aligns with CWE-129, which addresses improper validation of array indices, and CWE-787, which covers out-of-bounds write operations that can result from inadequate bounds checking.
The operational impact of CVE-2018-20679 extends beyond simple information disclosure, as it creates opportunities for attackers to extract sensitive data from the stack memory of affected systems. This includes potentially valuable information such as cryptographic keys, session tokens, memory addresses, and other confidential data that may be present in the stack at the time of the out-of-bounds read. The remote nature of the attack means that adversaries can exploit this vulnerability from outside the network perimeter, making it particularly dangerous for network infrastructure devices that rely on BusyBox for DHCP services. The vulnerability affects all components that utilize the udhcp library, including DHCP servers, clients, and relay agents, creating widespread exposure across network environments where BusyBox is deployed.
Organizations should prioritize immediate patching of all affected BusyBox installations to address this vulnerability, as the exposure window remains significant until remediation is complete. The recommended mitigation strategy involves upgrading to BusyBox version 1.30.0 or later, which includes proper bounds checking in the udhcp_get_option() function to prevent out-of-bounds memory access. Network administrators should also consider implementing network segmentation and monitoring to detect anomalous DHCP traffic patterns that might indicate exploitation attempts. From a security posture perspective, this vulnerability highlights the importance of proper input validation and memory safety practices in embedded systems and network infrastructure components, aligning with ATT&CK technique T1059.007 for execution through DHCP services and T1082 for discovery of system information through memory access patterns. Additionally, organizations should review their network monitoring capabilities to detect and respond to malformed DHCP packets that could be used to probe for this specific vulnerability.