[The Saucepan Man]

I do not believe that Rivendell was under attack during the War of the Ring. Certainly there is, as far as I am aware, no reference to it being so in any of Tolkien's works.

Red Feather's post makes a lot of sense to me. There was no way that Sauron could have got forces to Rivendell in any meaningful numbers. I am sure that the Orcs in Moria, while a threat to the Fellowship, would not have presented any danger to a stronghold which held out for a good while when Sauron's forces had overrun most of Eregion in the Second Age.

Unfinished Tales tells us that Gandalf's main reason for encouraging Thorin into the Quest of Erebor was to vanquish Smaug and see Dale and Erebor fortified so that Sauron could not use the northern passes to attack Rivendell. This seems to suggest that, while Men and Dwarves barred the northern passes and Lothlorien and Rohan held the other routes to Eregion (as they all did during the War of the Ring), Rivendell would be safe. Indeed, in relating the background to the Quest of Erebor to Frodo, Gimli et al, Gandalf paints an attack on Rivendell as the nightmare scenario, suggesting that it was something that never happened.