CVE-2009-5010 in pyftpdlib
Summary
by MITRE
Race condition in the FTPHandler class in ftpserver.py in pyftpdlib before 0.5.1 allows remote attackers to cause a denial of service (daemon outage) by establishing and then immediately closing a TCP connection, leading to the accept function having an unexpected return value of None, a different vulnerability than CVE-2010-3494.
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Analysis
by VulDB Data Team • 03/09/2017
The vulnerability described in CVE-2009-5010 represents a critical race condition within the pyftpdlib FTP server implementation that specifically affects the FTPHandler class in ftpserver.py. This flaw exists in versions prior to 0.5.1 and demonstrates how seemingly benign network operations can be exploited to create significant service disruptions. The vulnerability arises from improper handling of concurrent TCP connection establishment and termination sequences, creating a timing window where the daemon's accept function encounters an unexpected None return value that leads to complete service outage.
The technical exploitation of this vulnerability occurs through a carefully timed sequence of network operations that exploit the underlying race condition in the FTP server's connection handling mechanism. When a remote attacker establishes a TCP connection to the FTP server and immediately closes it, the timing of these operations creates a window where the accept function fails to properly handle the connection state transition. This race condition specifically manifests when the FTPHandler processes the connection lifecycle, where the expected return value from the accept function becomes None instead of the anticipated socket object. The flaw is particularly insidious because it requires minimal resources to execute and can be performed repeatedly to maintain the denial of service condition.
From an operational perspective, this vulnerability presents a significant risk to organizations relying on pyftpdlib-based FTP services as it allows for remote denial of service attacks that can completely disrupt FTP daemon operations. The impact extends beyond simple service interruption since the daemon may become unresponsive to legitimate connection attempts or may require manual restart to recover from the corrupted state. Network administrators face the challenge of defending against attacks that can be launched from any location with network access to the vulnerable FTP service, making this a particularly dangerous vulnerability in environments where FTP services are exposed to untrusted networks.
The vulnerability aligns with CWE-362, which categorizes race conditions in concurrent systems, and demonstrates how improper synchronization can lead to security flaws in network service implementations. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique related to network denial of service, where attackers leverage application-level flaws to disrupt service availability. The flaw also connects to T1595.001 for reconnaissance activities where attackers might identify vulnerable systems, and T1071.004 for application layer protocol usage in exploitation. Organizations should implement immediate mitigations including upgrading to pyftpdlib version 0.5.1 or later, which contains the necessary fixes for the race condition, along with network-level protections such as connection rate limiting and firewall rules that restrict FTP access to trusted sources.
The root cause of this vulnerability stems from inadequate error handling within the FTPHandler's accept method, where the code does not properly account for the possibility of None return values during connection processing. This oversight creates a state where the daemon cannot properly transition between connection states, leading to a cascading failure that ultimately results in daemon termination. The fix implemented in version 0.5.1 addresses this by adding proper null checks and state validation before attempting to process connection objects, ensuring that the FTP daemon maintains proper operational integrity even when subjected to malicious connection patterns. Security monitoring should focus on identifying unusual connection patterns and rapid connection/disconnection sequences that may indicate exploitation attempts against this vulnerability.