Which of the following is a well-known type of malware?

Conficker is a well-known type of malware that emerged in 2008. It is a worm that primarily spreads through vulnerabilities in the Windows operating system and removable drives, leveraging network shares and the Autorun feature to propagate itself. Conficker is notable for its ability to infect millions of computers worldwide and for its use of advanced techniques to avoid detection and removal. The worm also has a complex update mechanism that allows it to download additional malicious payloads, making it particularly dangerous.

In the context of the options provided, the other terms refer to different concepts or types of software. Heartbleed is a security vulnerability related to OpenSSL that allows attackers to steal sensitive data from servers. Shellshock is a vulnerability in the Bash shell that can allow unauthorized command execution. Ubuntu, on the other hand, is a widely-used Linux operating system. These terms relate to security issues but do not classify as malware themselves, which is why Conficker stands out in this context as the correct answer.