CVE-2011-1575 in Pure-FTPd
Summary
by MITRE
The STARTTLS implementation in ftp_parser.c in Pure-FTPd before 1.0.30 does not properly restrict I/O buffering, which allows man-in-the-middle attackers to insert commands into encrypted FTP sessions by sending a cleartext command that is processed after TLS is in place, related to a "plaintext command injection" attack, a similar issue to CVE-2011-0411.
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Analysis
by VulDB Data Team • 11/07/2021
The vulnerability identified as CVE-2011-1575 affects Pure-FTPd versions prior to 1.0.30 and specifically targets the STARTTLS implementation within the ftp_parser.c component. This flaw represents a critical security weakness that undermines the integrity of encrypted file transfer protocols by allowing unauthorized command injection during the secure communication establishment process. The vulnerability stems from improper handling of input/output buffering mechanisms during the transition from plaintext to encrypted communication states.
The technical implementation flaw occurs when the FTP server processes commands in a manner that fails to properly isolate cleartext commands from encrypted ones during the STARTTLS handshake process. Attackers can exploit this by sending malicious cleartext commands before the TLS encryption is fully established, which then get processed after the encryption is in place. This creates a window of opportunity where the system processes commands that should remain isolated, effectively enabling a man-in-the-middle attack vector that bypasses the intended security protections of the encrypted session.
From an operational perspective, this vulnerability significantly compromises the security posture of systems relying on Pure-FTPd for file transfers, particularly in environments where sensitive data is transmitted over untrusted networks. The impact extends beyond simple command injection as it fundamentally undermines the trust model of encrypted communications, potentially allowing attackers to execute arbitrary commands, access sensitive files, or manipulate file transfer operations. The vulnerability is particularly dangerous because it operates silently during the encryption establishment phase, making detection difficult and allowing attackers to gain unauthorized access without raising immediate alerts.
The attack pattern aligns with techniques described in the ATT&CK framework under initial access and command and control phases, where adversaries leverage protocol implementation weaknesses to establish persistent access. This vulnerability also relates to CWE-119, which addresses improper restriction of operations within a limited scope, and CWE-20, which covers improper input validation. Organizations using affected versions should prioritize immediate patching to address the I/O buffering restrictions that allow this attack vector. Additionally, network segmentation, monitoring for unusual command patterns, and implementing proper TLS configuration practices can help mitigate the risk while awaiting patch deployment.
The vulnerability demonstrates how seemingly minor implementation details in cryptographic protocol handling can create significant security weaknesses that affect the entire communication stack. It highlights the importance of proper state management during protocol transitions and the necessity of rigorous input validation even in encrypted communication contexts. Organizations should conduct thorough security assessments of their FTP infrastructure and ensure all components are updated to versions that properly address this class of vulnerability, as the implications extend beyond simple file transfer security to encompass broader network trust and integrity concerns.