ECB: the mode that draws the penguin
Electronic Codebook encrypts block by block without chaining. Consequence: identical inputs produce identical outputs, and structure leaks.
Why it is weak
An image encrypted in ECB still shows its outlines (the famous Tux penguin). Beyond aesthetics: an attacker spots repeated blocks, reorders or reuses ciphertext blocks (cut-and-paste), and with an encryption oracle recovers secrets one byte at a time (byte-at-a-time ECB decryption).
C = ECB(known_prefix || SECRET)
# align the secret to block boundaries and guess it byte by byteDefence
Do not use ECB. Use authenticated modes (GCM) with a unique IV/nonce per message. If you only need confidentiality, CBC with a random IV still beats ECB, but GCM is the correct choice today.