KASUMI is an 8-round Feistel block cipher used in the confidentiality and the integrity algorithms of the 3GPP mobile communications. As more and more 3GPP networks are being deployed, more and more users use KASUMI to protect their privacy. Previously known attacks on KASUMI can break up to 6 out of the 8 rounds faster than exhaustive key search, and no attacks on the full KASUMI have been published. In this paper we apply the recently introduced related-key boomerang and rectangle attacks to KASUMI, resulting in an attack that is faster than exhaustive search against the full cipher. We also present a related-key boomerang distinguisher for 6-round KASUMI using only 768 adaptively chosen plaintexts and ciphertexts encrypted or decrypted under four related keys. Recently, it was shown that the security of the entire encryption system of the 3GPP networks cannot be proven using only the “ordinary” assumption that the underlying cipher (KASUMI) is a Pseudo-Random Permutation. It was also shown that if we assume that KASUMI is also secure with respect to differential-based related-key attacks then the security of the entire system can be proven. Our results show that theoretically, KASUMI is not secure with respect to differential-based related-key attacks, and thus, the security of the entire encryption system of the 3GPP cannot be proven at this time.