The fact that log4j is such a ubiquitous piece of software is what makes this such a big deal. Imagine if a common type of lock used by millions of people to keep their doors shut was suddenly discovered to be ineffective. Switching a single lock for a new one is easy, but finding all the millions of buildings that have that defective lock would take time and an immense amount of work.

Log4j is part of the Java programming language, which is one of the foundational ways software has been written since the mid-90s. Huge swaths of the computer code that modern life runs on use Java and contain log4j. Cloud storage companies such as Google, Amazon and Microsoft, which provide the digital backbone for millions of other apps, are affected. So are giant software sellers whose programs are used by millions, such as IBM, Oracle and Salesforce. Devices that connect to the Internet such as TVs and security cameras are at risk as well. Hackers who try to break into digital spaces to steal information or plant malicious software suddenly have a massive new opportunity to try to get into nearly anywhere they want. That doesn’t mean everything will be hacked, but it just got a lot easier to do so — just as if the locks on half of the homes and businesses in a city suddenly stopped working all at once.

On top of all that, the vulnerability is straightforward to take advantage of. In the Minecraft video game, it’s as easy as typing a line of malicious code into the public chat box during a game. On Twitter, some people changed their display names to strings of bad code, Wired reported.