I imagine the UK Computer Misuse Act (eg at Section 3, http://www.legislation.gov.uk/ukpga/1990/18) probably covers it if the person who altered the TXT field does so to cause websites to load code on purpose, that purpose being for example to impair (Section 3(2)(a)) the running of the computer [causing Rick Astley to play, defo counts!] - but it can be read to cover pretty much anything.
Similarly I imagine something like the CFAA (18 USC 1030) probably has broad enough clauses to make this sort of action technically illegal, at least in some cases? But I'm out of my depth on that one.
Similarly I imagine something like the CFAA (18 USC 1030) probably has broad enough clauses to make this sort of action technically illegal, at least in some cases? But I'm out of my depth on that one.