Is CBC block cipher mode susceptible to reordering attacks? Regarding reordering attacks, is CBC block cipher as vulnerable as ECB? why? If CBC can prevent reordering attacks (that is, after reordering, the decrypted plaintext will make nonsense so the receiver can detect that it must have been tampered with during transmission), why do we still need mechanisms like sequence number to prevent reordering attack?
CBC is almost as vulnerable to reordering attacks as ECB. Sometimes you can get garbage, but you can certainly move e.g. two blocks and know that the second block will decrypt to the same plaintext as the original.

With ECB you cannot easily change the last block if padding is used. If the last block doesn't contain correct padding then an error may be generated during decryption. With CBC this is extended to (the last part) of the first-to-last block of the ciphertext.

All this is quite useless though. To protect modes such as ECB and CBC against change you should simply add an authentication tag over the ciphertext (and IV, if that is send). If active attacks are possible then padding oracle attacks may be possible as well; an authentication tag would protect against both.

So adding an authentication tag (MAC) or using authenticated encryption (such as GCM mode) is the best way to protect the message integrity and authenticity. That way you don't have to rely on the plaintext message / padding to detect errors. 



You can of course have two ways of reordering ciphertext when it comes to CBC. You can reorder the blocks in a message or reorder the messages themselves. Sequence numbers protect against the latter.

This is quite a separate issue; reordering messages can be performed regardless of which mode is used. CBC certainly won't protect against the reordering of messages.