Cloud computing environments increasingly provision field-programmable gate arrays (FPGAs) for their programmability and hardware-level parallelism. While FPGAs are typically used by one tenant at a time, multitenant schemes supporting spatial sharing of cloud FPGA resources have been proposed in the literature. However, the spatial multitenancy of FPGAs opens up new attack surfaces. Investigating potential security threats to multitenant FPGAs is thus essential for better understanding and eventually mitigating the security risks. This work makes a notable step forward by systematically analyzing the combined threat of FPGA power wasters and satisfiability don’t-care hardware Trojans in shared cloud FPGAs. We demonstrate a successful remote undervolting attack that activates a hardware Trojan concealed within a victim FPGA design and exploits the payload. The attack is carried out entirely remotely, assuming two spatially colocated FPGA users isolated from one another. The victim user’s circuit is infected with a Trojan, triggered by a pair of don’t-care signals that never reach the combined trigger condition during regular operation. The adversary, targeting the exploitation of the Trojan, deploys power waster circuits to lower the supply voltage of the FPGA. The assumption is that, under the effect of the lowered voltage, don’t-care signals may reach the particular state that triggers the Trojan. We name this exploit X-Attack and demonstrate its feasibility on embedded FPGA and real-world cloud FPGA instances. Additionally, we study the effects of various attack tuning parameters on the exploit’s success. Finally, we discuss potential countermeasures against this security threat and present a lightweight self-calibrating countermeasure. To the best of our knowledge, this is the first work on undervolting-based fault-injection attacks in multitenant FPGAs to demonstrate the attack on commercially available cloud FPGA instances.