That particular case isn't vulnerable to length extension, since the key is placed at the end of the message. This MAC breaks down, however, when the function u

That particular case isn't vulnerable to length extension, since the key is placed at the end of the message. This MAC breaks down, however, when the function used is not collision-resistant. HMAC offers some level of protection against collision-weak hash functions, and that's why you can still use HMAC-MD5 today and be pretty OK (although I don't recommend it!).