Looking at the source code makes it much faster to understand exactly how the program works. This includes bugs in both the client **and** the server that can allow for arbitrary code execution and thus malware.
Now that it’s many times easier to understand the code for them, people can find a bug that sends bad data to a server and take it over. Then they can find a bug in the client that lets a taken-over server send bad data to the client and run malware on the player’s computer.
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