a computer file that was developed by the European Institute for Computer Antivirus Research (EICAR) and Computer Antivirus Research Organization to test the response of computer antivirus programs. Instead of using real malware, which could cause real damage, this test file allows people to test anti-virus software without having to use real malware.

Hook (29% TomatoMeter).

But it was released in 1991, so it wouldn’t count for the XKCD version. Also the audience score is 76%, so not really an unpopular opinion I guess.