No, it is not legal to release a computer virus that puts a funny message on people's screens although it does not damage files because it still constitutes a computer invasion because it requires changing the system without permission in order to do the intended action.
What is a virus?
A computer virus is a specific kind of computer program that, when run, replicates itself by altering other software and adding new code. In other words, a computer virus is a kind of malware—malicious software—that travels between computers and corrupts software and data.
Computer viruses try to interfere with systems, create significant operational problems, and lead to data loss and leakage. Computer viruses are made to propagate across programs and systems, which is an important fact to understand.
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