CVE-2009-5013 in pyftpdlib
Summary
by MITRE
Memory leak in the on_dtp_close function in ftpserver.py in pyftpdlib before 0.5.2 allows remote authenticated users to cause a denial of service (memory consumption) by sending a QUIT command during a data transfer.
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Analysis
by VulDB Data Team • 09/27/2021
The vulnerability identified as CVE-2009-5013 represents a critical memory management flaw within the pyftpdlib FTP server implementation that affects versions prior to 0.5.2. This issue manifests specifically within the on_dtp_close function of the ftpserver.py module, where improper handling of memory resources during data transfer termination creates a persistent memory leak condition. The vulnerability operates under the context of authenticated remote attackers who can exploit this weakness by sending a QUIT command while a data transfer is actively in progress, leading to gradual memory consumption that can ultimately result in system resource exhaustion.
The technical root cause of this vulnerability stems from inadequate memory deallocation practices within the data transfer closure mechanism. When a QUIT command is received during an active data transfer, the on_dtp_close function fails to properly release allocated memory buffers and associated resources, causing them to remain in memory indefinitely. This memory leak occurs because the function does not properly execute cleanup routines that would normally occur during normal termination sequences, leaving behind references to data structures that should have been freed. The flaw operates at the application layer and specifically targets the FTP protocol implementation within the pyftpdlib framework, making it particularly dangerous in environments where FTP services are heavily utilized.
The operational impact of this vulnerability extends beyond simple resource consumption to create significant service availability risks for affected systems. Remote authenticated users can systematically consume available memory resources through repeated exploitation attempts, gradually degrading system performance until complete service unavailability occurs. This type of denial of service attack can be particularly effective in resource-constrained environments or when multiple concurrent connections are active, as each successful exploitation event compounds the memory consumption problem. The vulnerability affects systems where pyftpdlib is used as an FTP server implementation, making it relevant to a wide range of network services that rely on this Python-based FTP library for file transfer operations.
Mitigation strategies for CVE-2009-5013 primarily focus on immediate software updates and system hardening measures. The most effective solution involves upgrading to pyftpdlib version 0.5.2 or later, where the memory leak has been addressed through proper resource management implementation. Organizations should also implement connection monitoring and resource usage alerts to detect abnormal memory consumption patterns that may indicate exploitation attempts. Network segmentation and access control measures can limit the attack surface by restricting which authenticated users can initiate FTP connections and perform data transfers. Additionally, implementing connection timeouts and automatic resource cleanup mechanisms can help mitigate the impact of successful exploitation attempts. This vulnerability aligns with CWE-401, which describes improper release of memory, and represents a classic example of how inadequate resource management in network services can create persistent security weaknesses that enable denial of service attacks. The ATT&CK framework categorizes this vulnerability under privilege escalation and denial of service tactics, as it allows authenticated users to consume system resources and potentially disrupt service availability for legitimate users.