Isn't "game-over flaw" a bit overstated? It's a serious blunder, but if the data you're authenticating is highly structured then in many cases it won't be exploitable because the final padding from the original message becomes part of the body of the extended message, as unparseable gibberish.
I'm talking about exploitation of this vulnerability in general, not the specific case of Flickr. Not everything is a sequence of key-value pairs formatted in UTF-8.
Most of the crypto you're going to run across as a pentester will be in apps written in Java and C#, and in almost every one of those cases, garbage characters won't break a parse. ".split()" works just fine even if you have 16 characters of random high ASCII. It's one of those things C programmers definitely have to unlearn.
I trust a generated parser more than a hand-written one. Any parsing algorithm that involves the use of split() is almost certain to be weakly thought-out and have ill-defined behavior for unexpected input. A well-written parser will never read past the first unexpected character, or at least token.
