Recent works have demonstrated the possibility of extracting secrets from a cryptographic core running on an FPGA by means of remote power analysis attacks. To mount these attacks, an adversary implements a voltage fluctuation sensor in the FPGA logic, records the power consumption of the target cryptographic core, and recovers the secret key by running a power analysis attack on the recorded traces. Despite showing that the power analysis could also be performed without physical access to the cryptographic core, these works were mostly carried out on dedicated FPGA boards in a controlled environment, leaving open the question about the possibility to successfully mount these attacks on a real system deployed in the cloud. In this paper, we demonstrate, for the first time, a successful key recovery attack on an AES cryptographic accelerator running on an Amazon EC2 F1 instance. We collect the power traces using a delay-line based voltage drop sensor, adapted to the Xilinx Virtex Ultrascale+ architecture used on Amazon EC2 F1, where CARRY8 blocks do not have a monotonic delay increase at their outputs. Our results demonstrate that security concerns raised by multitenant FPGAs are indeed valid and that countermeasures should be put in place to mitigate them.