TLBleed is a cryptographic side-channel attack that uses machine learning to exploit a timing side-channel via the translation look-aside buffer (TLB) on modern microprocessors that use simultaneous multithreading.As of June 2018, the attack has only been demonstrated experimentally on Intel processors; it is speculated that AMD and other processors may also potentially be vulnerable to a variant of the attack, but no proof of concept has been demonstrated.

The attack led to the OpenBSD project disabling simultaneous multithreading on Intel microprocessors.Theo de Raadt has stated that, while the attack could theoretically be prevented by preventing tasks with different security contexts from sharing physical cores, such a fix is currently impractical because of the complexity of the problem.