Short Answer 
The notorious backdoor in vsftpd 2.3.4, activated by the input of a smiley face during FTP connections, compromised user security by opening a command shell on port 6200. This incident underscores the risks of open-source software and emphasizes the need for regular updates, security audits, and secure coding practices to mitigate such vulnerabilities.
Step 1: Identify the Vulnerable Version
The version of vsftpd that contained the notorious smiley face backdoor was vsftpd 2.3.4. This specific version came under scrutiny after it was determined that its source code had been compromised. Users must always verify the integrity of any software versions they download to prevent exposure to such vulnerabilities.
Step 2: Understand the Backdoor Activation
The backdoor was designed to activate when a user entered a specific input, namely a smiley face (“:)”, in the user field during an FTP connection attempt. Once triggered, this backdoor opened a command shell remotely on port 6200, allowing unauthorized user access. Such malicious features underline the importance of careful input validation in software development.
Step 3: Recognize the Implications
This incident, which occurred in July 2011, highlights significant security risks associated with open-source software. It emphasizes the need for rigorous code verification and consistently monitoring site security. Users and developers must adopt best practices to safeguard against similar threats, including:
- Regularly updating software to the latest versions
- Conducting thorough security audits
- Utilizing secure coding practices