CVE-2021-39602 in Miniftpd
Summary
by MITRE • 08/24/2021
A Buffer Overflow vulnerabilty exists in Miniftpd 1.0 in the do_mkd function in the ftpproto.c file, which could let a remote malicious user cause a Denial of Service.
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Analysis
by VulDB Data Team • 08/26/2021
The buffer overflow vulnerability identified as CVE-2021-39602 resides within the Miniftpd 1.0 file transfer protocol implementation, specifically within the do_mkd function located in the ftpproto.c source file. This vulnerability represents a classic stack-based buffer overflow condition that occurs when the application fails to properly validate input length before copying data into a fixed-size buffer. The flaw manifests when processing the MKD (Make Directory) command from remote FTP clients, where maliciously crafted directory names can exceed the allocated buffer space, leading to memory corruption and potential system instability.
The technical execution of this vulnerability follows a well-established pattern that aligns with CWE-121 Stack-based Buffer Overflow, where insufficient bounds checking allows attackers to overwrite adjacent memory locations including return addresses and function pointers. When a remote attacker sends a specially crafted MKD command containing an excessively long directory name, the ftpd server process attempts to store this input in a local buffer without proper size validation. This condition creates a predictable memory corruption scenario that can be exploited to either crash the service or potentially execute arbitrary code depending on memory layout and protection mechanisms in place. The vulnerability operates at the application layer protocol level, making it particularly dangerous as it requires no special privileges to exploit and can be triggered through standard FTP client interactions.
From an operational impact perspective, this vulnerability creates a significant denial of service risk for systems running Miniftpd 1.0, as a remote attacker can reliably crash the FTP service by sending malicious MKD commands. The service interruption affects legitimate users who depend on FTP functionality for file transfers and directory management operations. Additionally, the vulnerability may provide a foothold for more sophisticated attacks if the buffer overflow can be carefully crafted to overwrite critical program execution flow, potentially leading to privilege escalation or remote code execution scenarios. The attack surface is particularly concerning given that FTP services are commonly exposed to external networks without proper network segmentation, and the vulnerability affects a widely used open source FTP server implementation.
Mitigation strategies for CVE-2021-39602 should prioritize immediate patching of affected Miniftpd installations to the latest stable releases that contain proper input validation fixes. Organizations should implement network-level controls such as firewall rules that restrict FTP service access to trusted networks and consider implementing intrusion detection systems to monitor for suspicious MKD command patterns. The vulnerability demonstrates the importance of proper input validation and bounds checking in network services, aligning with ATT&CK technique T1203 Exploitation for Client Execution and T1499 Endpoint Denial of Service. Additionally, system administrators should conduct comprehensive vulnerability assessments to identify other instances of similar buffer overflow vulnerabilities in legacy network services and implement proper application security testing including fuzzing and static code analysis to prevent similar issues in future deployments.