openssl_encrypt versions prior to 1.4.9 contain a cryptographic design flaw where an unkeyed SHA-256 hash of the plaintext is stored in cleartext within the file header metadata. This allows attackers who can access encrypted files to read the hash without knowing the encryption password. The vulnerability enables offline plaintext confirmation attacks, where an attacker can verify guessed plaintext values against the exposed hash. Additionally, it enables fingerprinting of identically encrypted files across separate encryption operations, breaking confidentiality expectations. The issue is classified as a plaintext confirmation oracle. Users are advised to upgrade to version 1.4.9 or later to remediate the vulnerability.
