Walid Muhammad Salih bin Mubarak bin Attash is a Yemeni prisoner held at the United States' Guantanamo Bay detention camp under terrorism-related charges and is suspected of playing a key role in the early stages of the 9/11 attacks. The Office of the Director of National Intelligence has described him as a "scion of a terrorist family". American prosecutors at the Guantanamo military commissions allege that he helped in the preparation of the 1998 East Africa Embassy bombings and the USS Cole bombing and acted as a bodyguard to Osama bin Laden, gaining himself the reputation of an "errand boy". He is formally charged with selecting and helping to train several of the hijackers of the September 11 attacks.

The Guantanamo Bay detention camp is a United States military prison within the Guantanamo Bay Naval Base, also referred to as Gitmo, on the coast of Guantánamo Bay in Cuba. As of April 2023, of the 779 people detained there since January 2002 when the military prison first opened after the September 11 attacks, 740 had been transferred elsewhere, 30 remained there, and nine had died while in custody.