This paper presents a security analysis of the optical image encryption scheme based on interference and fixed phase masks. Unlike the traditional interference-based scheme, which uses only one phase-only mask (POMs) generated during the encryption process as the private key, the improved system incorporates fixed phase masks as the additional private keys. While it appears that the security strength of the interference-based scheme is enhanced by expanding the key space, our analysis reveals that these fixed phase masks are independent of the plaintext and can be recovered through a known-plaintext attack (KPA). Once these additional private keys are obtained, the number of unknown private keys is reduced to one, allowing the plaintext information encrypted by the improved interference-based scheme to be recovered from any arbitrarily given ciphertexts without any knowledge of the private key. To show the vulnerability of this cryptosystem, we propose a hybrid attack combining the KPA and ciphertext-only attack (COA) which can break the improved interference-based cryptosystem. Numerical simulations have been performed to confirm the feasibility and effectiveness of the proposed hybrid attack.