You can file a bogus tax return for someone in the US just as easily. I'd be surprised if that weren't the case in most countries. I believe there have been occasional incidents of people doing so to collect other peoples' tax refunds, but I don't hear about it very often.
VAT statements in Germany are a bit riskier though: If you give the Revenue Service permission to withdraw money and I file a VAT statement for you which shows a large income they'll just go and collect 20%. It's a bit an edge case but that's the reason I manually send the VAT collected each month. I'd feel much safer if the interface was authenticated in some way.
