CVE-2021-25217 in DHCP
Summary
by MITRE • 05/27/2021
In ISC DHCP 4.1-ESV-R1 -> 4.1-ESV-R16, ISC DHCP 4.4.0 -> 4.4.2 (Other branches of ISC DHCP (i.e., releases in the 4.0.x series or lower and releases in the 4.3.x series) are beyond their End-of-Life (EOL) and no longer supported by ISC. From inspection it is clear that the defect is also present in releases from those series, but they have not been officially tested for the vulnerability), The outcome of encountering the defect while reading a lease that will trigger it varies, according to: the component being affected (i.e., dhclient or dhcpd) whether the package was built as a 32-bit or 64-bit binary whether the compiler flag -fstack-protection-strong was used when compiling In dhclient, ISC has not successfully reproduced the error on a 64-bit system. However, on a 32-bit system it is possible to cause dhclient to crash when reading an improper lease, which could cause network connectivity problems for an affected system due to the absence of a running DHCP client process. In dhcpd, when run in DHCPv4 or DHCPv6 mode: if the dhcpd server binary was built for a 32-bit architecture AND the -fstack-protection-strong flag was specified to the compiler, dhcpd may exit while parsing a lease file containing an objectionable lease, resulting in lack of service to clients. Additionally, the offending lease and the lease immediately following it in the lease database may be improperly deleted. if the dhcpd server binary was built for a 64-bit architecture OR if the -fstack-protection-strong compiler flag was NOT specified, the crash will not occur, but it is possible for the offending lease and the lease which immediately followed it to be improperly deleted.
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Analysis
by VulDB Data Team • 11/23/2025
The vulnerability identified as CVE-2021-25217 affects ISC DHCP server and client implementations across multiple versions including 4.1-ESV-R1 through 4.1-ESV-R16 and 4.4.0 through 4.4.2, with implications extending to older unsupported branches. This issue represents a critical memory corruption vulnerability that manifests differently depending on system architecture and compilation parameters, creating complex operational risks for network infrastructure. The defect originates from improper handling of malformed lease entries during the parsing process, specifically when reading lease files that contain invalid or corrupted data structures. The vulnerability demonstrates characteristics consistent with stack-based buffer overflow conditions, though the exact memory corruption mechanism varies based on the compilation environment and target architecture. The presence of this vulnerability in multiple release series indicates a fundamental flaw in the lease parsing logic that was not adequately addressed through version updates, particularly affecting systems running on 32-bit architectures where stack protection mechanisms may not provide complete coverage.
The operational impact of CVE-2021-25217 varies significantly between the dhclient and dhcpd components, creating different risk profiles for network environments. When affecting dhclient on 32-bit systems, the vulnerability can cause complete process crashes leading to network disconnection as the client fails to maintain DHCP lease negotiation. This scenario directly impacts network connectivity and can result in extended service outages until manual intervention restores the DHCP client process. For dhcpd servers, the consequences are more severe as the vulnerability can trigger service disruption through process termination when encountering malformed lease entries, particularly in 32-bit builds with strong stack protection enabled. The vulnerability also introduces data integrity concerns where the affected lease entry and its subsequent neighbor may be improperly deleted from the lease database, creating potential lease management inconsistencies and possible address conflicts in network environments. This dual impact of service disruption and data corruption makes the vulnerability particularly dangerous for production environments relying on stable DHCP services.
The technical characteristics of this vulnerability align with CWE-121 Stack-based Buffer Overflow and CWE-125 Out-of-bounds Read classifications, representing memory safety issues that can lead to arbitrary code execution or denial of service conditions. The vulnerability's manifestation is influenced by compiler flags such as -fstack-protection-strong which provide additional security layers but may not fully protect against all memory corruption scenarios in 32-bit environments. The architecture-dependent behavior demonstrates the complexity of modern software security issues where the same underlying flaw can produce different outcomes based on compilation targets and system configurations. Network infrastructure administrators face challenges in identifying vulnerable systems as the vulnerability may not manifest consistently across different deployment scenarios, making detection and remediation more difficult. The vulnerability's presence in multiple versions also indicates that the root cause was not properly addressed in security patches, requiring careful version management and system hardening to prevent exploitation.
Mitigation strategies for CVE-2021-25217 require comprehensive system assessment and remediation approaches that address both immediate security concerns and long-term infrastructure stability. Organizations should prioritize upgrading to supported ISC DHCP versions that contain proper fixes for this vulnerability, particularly focusing on systems running 32-bit architectures where the risk is highest. The vulnerability's conditional nature means that systems built with specific compiler flags or running on certain architectures may be more susceptible, requiring targeted patching efforts. Network administrators should implement monitoring solutions to detect potential lease file corruption or service disruptions that may indicate exploitation attempts. The recommended approach includes verifying system architecture, checking compilation parameters, and implementing proper lease file validation procedures to prevent malformed entries from causing service interruptions. Additionally, implementing redundant DHCP services or failover mechanisms can provide resilience against potential service disruptions while patches are deployed, though this requires careful planning to avoid introducing additional complexity into existing network infrastructures. The vulnerability highlights the importance of maintaining current software versions and the risks associated with running unsupported software branches that may contain unaddressed security flaws.