Which of the following is a common vulnerability in operating systems?

Weak password policies are indeed a common vulnerability in operating systems. This vulnerability arises when systems allow users to create easily guessable passwords or when they do not enforce rules regarding password complexity, length, or expiration. Such policies can lead to unauthorized access, as attackers often exploit weak passwords through techniques like brute-force attacks or social engineering.

In contrast, high processing speed is a characteristic of the hardware and does not inherently pose a security risk. Frequent software updates often bolster security by patching known vulnerabilities and improving system stability, making them a critical part of a well-maintained operating system. Meanwhile, robust firewalls serve to protect systems from unauthorized access by filtering incoming and outgoing traffic, thereby enhancing security rather than presenting any vulnerabilities.