Direct Memory Access (DMA) attacks can allow attackers to access memory directly, bypassing OS supervision or software protections. In this work, we put forth and benchmark a cryptographically secure attestation scheme, which detects DMA attacks. In fact, our scheme detects any attack in a more general class of attacks which we call "direct injection". We prove security of our scheme under a realistic machine model which extends in a non-trivial manner a cryptographic model proposed by Lipton, Ostrovsky, and Zikas (ICALP 2016.) Despite the fact that our scheme, in its current form, protects against write-only attacks, both our security model and our scheme can be extended to allow the attacker to have additional read access to memory---thereby capturing leakage---as well as detecting more types of memory corruptions such as bit flips.