CVE-2020-5204 in uftpd
Summary
by MITRE
In uftpd before 2.11, there is a buffer overflow vulnerability in handle_PORT in ftpcmd.c that is caused by a buffer that is 16 bytes large being filled via sprintf() with user input based on the format specifier string %d.%d.%d.%d. The 16 byte size is correct for valid IPv4 addresses (len('255.255.255.255') == 16), but the format specifier %d allows more than 3 digits. This has been fixed in version 2.11
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Analysis
by VulDB Data Team • 03/19/2024
The vulnerability identified as CVE-2020-5204 represents a classic buffer overflow condition within the uftpd FTP server implementation. This flaw exists in the handle_PORT function located within the ftpcmd.c source file and affects versions prior to 2.11. The vulnerability stems from a fundamental mismatch between the allocated buffer size and the potential data volume that can be written to it through improper input handling mechanisms.
The technical execution of this vulnerability occurs when the ftpd server processes PORT command requests from clients. The buffer allocated for storing IP address information is precisely 16 bytes in size, which appears sufficient for valid IPv4 addresses formatted as "255.255.255.255" with a length of exactly 15 characters plus null terminator. However, the sprintf() function call utilizes the %d format specifier which permits integers of any length, allowing malicious users to input values exceeding three digits in each octet. This discrepancy enables attackers to write more data into the 16-byte buffer than it can accommodate, leading to memory corruption and potential code execution.
This buffer overflow vulnerability directly maps to CWE-121, which describes heap-based buffer overflow conditions where insufficient bounds checking allows data to overwrite adjacent memory locations. The operational impact of this vulnerability extends beyond simple memory corruption, as it creates opportunities for arbitrary code execution within the context of the ftpd process. Attackers could potentially exploit this condition to gain unauthorized access to systems running vulnerable versions of uftpd, particularly when the service operates with elevated privileges. The vulnerability is particularly concerning because it occurs during normal FTP operations when handling network connection requests, making it accessible to remote attackers without requiring authentication.
The attack surface for this vulnerability is significant within environments that rely on uftpd for file transfer services, especially in network configurations where FTP servers are exposed to untrusted networks. The exploitability of this condition follows ATT&CK technique T1203, which involves the use of valid accounts to perform command and control operations, though in this case the vulnerability allows for privilege escalation through memory corruption. Organizations using affected versions should prioritize immediate patching to version 2.11 or later, as this update resolves the buffer size calculation and input validation issues. Additionally, network segmentation and firewall rules should be implemented to limit exposure of FTP services to trusted networks, while monitoring systems should be configured to detect unusual PORT command patterns that might indicate exploitation attempts. The remediation process should include thorough testing of the patched version to ensure that legitimate FTP operations remain functional while the vulnerability is eliminated.