AES-CBC, or Advanced Encryption Standard in Cipher Block Chaining mode, is a common way to encrypt data in fixed-size blocks. Each plaintext block is mixed with the previous ciphertext block before encryption, so identical data does not always produce identical ciphertext. It is widely used in older web applications and protocols, especially where backward compatibility matters.
In security work, AES-CBC is important because the mode itself does not provide integrity. If an application reveals different errors for different decryption failures, attackers may gain a padding oracle: a side channel that helps them test ciphertexts, recover plaintext, or sometimes forge valid encrypted values. That is why defenses focus on using authenticated encryption such as AES-GCM, or combining encryption with strong message authentication and uniform error handling. For legacy systems that still rely on AES-CBC, eliminating distinguishable failure signals is critical to prevent decryption oracles from becoming larger server-side attacks.