In complex systems like'Internet of Things' and'Cyber Physical Systems', reliable communication is made possible by the use of error control codes like turbo codes. Such systems are vulnerable to attack by Trojans during the design or fabrication phases of IC manufacturing. This leads to a severe performance degradation of the entire system. This paper aims at analyzing the effect of a sign-bit-flipping Trojan on a turbo coded communication system. Simulations were carried out on a conventional iterative turbo decoder and the results point out that the Trojan affects the performance of the decoder at SNRs greater than -2 dB. This performance degradation is seen even with minimal activation of the Trojan and a finite error persists even at very high SNRs. © 2019 Association for Computing Machinery.