Re:Postel’s Principle is a Bad Idea
by tedunangst on Sunday February 03, 2013 @08:30PM
(#5161975)
The problem is if you decode a particular byte sequence that causes a bad action (if that's possible with step files) in a different way than some other program that is supposed to keep you safe.
In the case of ie, ie decoded one way and forum software might decode a different way. So the forum software says the string is safe for the browser (according to its decoding rules) but then the browser applies different rules and gets a bad string.
You may not be seeing the danger because you implicitly think a step file from unsafe sources is always unsafe. But imagine if you had a safe file detector program, except it applied different rules than the program you're actually going to open the file with.
As jbert pointed out, if your program's main job is to say whether or not something is safe, and it liberally says "Oh yeah, I think that's safe", that's pretty much the exact opposite of "be conservative in what you do".
Trying to do something with 8-bit characters -- whether skipping them, indicating an illegal character in the string, or trying to guess what was really meant -- cannot make that situation any worse.