AES-GCM is authenticated encryption; AES-CBC is not: using CBC without a MAC is dangerous

AES-GCM (Galois/Counter Mode) provides authenticated encryption with associated data (AEAD). It encrypts the plaintext AND produces a 128-bit authentication tag. Any modification to the ciphertext makes the tag verification fail and decryption is rejected. AES-CBC encrypts correctly but produces no integrity check. An attacker who can flip bits in a CBC ciphertext will produce different decrypted plaintext and the receiver accepts it silently. This is the basis of padding oracle attacks. Always use an AEAD mode (GCM, ChaCha20-Poly1305) for any new system.