Comment by Leo_Verto

To add onto this, the reason ECB mode is so horribly bad, is that it produces the same cyphertext for blocks with the same plaintext.

This allows an attacker to detect repeating plain text segments, e.g. reused passwords.

The Wikipedia article on block cypher modes illustrates this problem rather well [1].

[1]: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation...