CVE-2011-2523 in vsftpd
Summary
by MITRE
vsftpd 2.3.4 downloaded between 20110630 and 20110703 contains a backdoor which opens a shell on port 6200/tcp.
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Analysis
by VulDB Data Team • 02/14/2025
The vulnerability identified as CVE-2011-2523 represents a critical security flaw in the vsftpd (Very Secure File Transfer Protocol) version 2.3.4 that was distributed during a specific timeframe between June 30 and July 3, 2011. This backdoor was not present in the original source code but was introduced through a compromised release, making it a particularly concerning issue for system administrators who relied on this widely-used FTP server implementation. The backdoor functionality was designed to provide unauthorized remote access to systems running the affected version, creating a persistent security risk that could be exploited by malicious actors without detection. This vulnerability directly violates the principle of least privilege and represents a severe compromise of system integrity and confidentiality.
The technical implementation of this backdoor involves a hardcoded listening socket that binds to TCP port 6200, which is not a standard FTP service port but rather a custom backdoor port. When a remote attacker connects to this port, the server responds by opening a shell session, effectively providing a command-line interface to the compromised system. The backdoor was implemented in such a way that it would activate regardless of authentication status, making it a true backdoor rather than a simple vulnerability that could be exploited through legitimate authentication mechanisms. This design aligns with CWE-94, which describes the execution of code that is not properly validated or sanitized, and represents a classic example of a malicious code injection vulnerability.
The operational impact of this vulnerability extends far beyond the immediate compromise of individual systems. Organizations that had deployed the affected vsftpd version were unknowingly providing attackers with persistent access to their networks, potentially enabling data exfiltration, lateral movement, and establishment of further footholds within the infrastructure. The vulnerability's timing, occurring during a specific window of distribution, suggests that the compromise was likely orchestrated by a sophisticated attacker who understood the widespread adoption of vsftpd and the trust placed in this software by system administrators. This backdoor could remain undetected for extended periods, as it operated on a non-standard port and did not generate typical FTP service logs that might trigger security monitoring systems. The attack pattern aligns with techniques described in the MITRE ATT&CK framework under the T1078 credential access and T1566 initial access categories, where adversaries establish persistent access through compromised software supply chains.
Mitigation strategies for this vulnerability required immediate action including the identification of all systems running the affected vsftpd version, followed by the installation of patched releases that had been properly verified and validated. System administrators needed to conduct thorough network scans to identify any connections to port 6200, as well as review system logs for any unauthorized access attempts. The incident highlighted the importance of software supply chain security and the need for cryptographic verification of software downloads, as the vulnerability was introduced through a compromised release rather than a coding error. Organizations were advised to implement network segmentation to prevent lateral movement and establish monitoring for unusual network traffic patterns on non-standard ports. The vulnerability also underscored the importance of maintaining updated software inventories and implementing processes for rapid response to security incidents, as the backdoor had remained undetected for months within affected networks. This incident became a significant case study in supply chain compromise and demonstrated the critical need for robust software verification processes that could prevent such attacks from succeeding in the first place.