I came across a few articles about length extension attacks on cryptographic hash functions. I was very surprised to read that SHA-256 and SHA-512 are prone to length extension attacks. This seems contradictory to the fact that—to the best of my knowledge—there are no known collisions on these algorithms.
According to the comparison of SHA functions table from Wikipedia the capacity against length extension attacks is 0 for both SHA-256 and SHA-512 which should mean that this attack does not require much computing resources.
Aren't length extension attacks considered as collisions as well?