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Intel's Software Guard Extensions (SGX) enables isolated execution environments, called enclaves, on untrusted operating systems (OS), and thus it can improve the security for various applications and online services. However, SGX has also well-known limitations. First, its remote attestation mechanism is vulnerable to relay attacks that allow the attacker to redirect attestation and the following provisioning of secrets to an unintended platform. Second, attestation keys have been shown to leak thus enabling attackers to fake the secure attested environment by emulating it. Third, there exists no secure way to let enclaves communicate with the I/O devices and as a consequence the user. To address these shortcomings, we propose a hardened variant of SGX attestation using proximity verification. We design and implement a system called ProximiTEE, where a simple embedded device with a low TCB is attached to the target platform. The embedded device verifies the proximity of the attested enclave by using distance bounding and secure boot-time initialization, thus allowing secure attestation regardless of a compromised OS or leaked attestation keys. Our boot-time initialization can be seen as a novel variant of ``trust on first use'' (TOFU) that makes deployment of secure attestation easier, reduces the system's attack surface and enables secure revocation. We further leverage the embedded device to build a trusted I/O path between peripherals (e.g., keyboards, displays) and enclaves, by letting it securely mediate every I/O communication between them. Our prototype implementation shows that such proximity verification is reliable in practice …