SPECK is a family of lightweight block ciphers developed by Beaulieu et al. of the US National Security Agency (NSA) for the Internet of Things (IoT). It is an ARX-based design with a Feistel-like structure which supports keys of size ranging from 64 bits to 256 bits. SPECK has been standardised by ISO/IEC for radio frequency identification (RFID) devices. It has drawn the attention of many cryptanalysts and several cryptanalysis results have been published. In this paper, carry flag attacks on the full SPECK ciphers are presented. Depending on the key size and block size, the complexities of our attacks, to nearly ensure success, vary from [Formula: see text] time and [Formula: see text] data to [Formula: see text] time and [Formula: see text] data.