No, it's not the same as length extension. SHA1 is vulnerable to Chosen Prefix collisions. SHA2 doesn't have any known collision attacks faster than the birthda

No, it's not the same as length extension. SHA1 is vulnerable to Chosen Prefix collisions. SHA2 doesn't have any known collision attacks faster than the birthday bounded brute force attack, let alone any chosen prefix collisions, but both do have length extension attacks. Also length extension isn't specific to Merkle–Damgård, though all Merkle–Damgård hashes are vulnerable to it without mitigations (like truncation of the output).