I/O attacks have received increasing attention during the last decade. These attacks are performed by malicious peripherals that make read or write accesses to DRAM memory or to memory embedded in other peripherals, through DMA (Direct Memory Access) requests. Some protection mechanisms have been implemented in modern architectures to face these attacks. A typical example is the IOMMU (Input Output Memory Management Unit). However, such mechanisms may not be properly configured and used by the firmware and the operating system. This paper describes a design weakness that we discovered in the configuration of an IOMMU and a possible exploitation scenario that would allow a malicious peripheral to bypass the underlying protection mechanism. The exploitation scenario is implemented for Intel architectures, with a PCI Express peripheral FPGA, based on Intel specifications and Linux source code analysis. Finally, as a proof of concept, a Linux rootkit based on the attack presented in this paper is implemented.