Abstract: The lightweight cryptosystem MIBS was proposed at the CANS conference in 2009. It has high efficiency in both hardware implementation and software implementation. MIBS can resist against classical cryptanalysis, such as differential analysis and linear analysis, etc. It is suitable for the resource-limited devices in the Internet of things. This paper proposes new ciphertext-only fault analysis of the MIBS cryptosystem. The attackers can apply a new fault model of Double AND and two novel distinguishers of Parzen-HW and Parzen-HW-MLE to break MIBS. The experiments only require at least 72 fault injections to recover the secret key with a success probability of no less than 99%. The method can further reduce fault injections and time, and effectively improve the attacking efficiency. It shows that the ciphertext-only fault analysis poses a serious threaten to the security of MIBS. The research also provides an important reference for the security analysis of other lightweight cryptosystems.