When Encrypted Cookies Aren't Secure: The CBC Bit Flipping Vulnerability

Many web developers assume that encrypting session cookies provides complete security, but CBC mode without integrity checks leaves applications vulnerable to bit-flipping attacks. Rather than requiring knowledge of the encryption key, attackers can systematically modify ciphertext to alter the decrypted session data—potentially escalating privileges or hijacking sessions. This practical methodology is critical for security professionals to understand both defensive measures and attack vectors in real-world applications.