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human
[WH40K]Has a genestealer cult ever successfully infiltrated an eldar world?

From what I know the eldar haven't had too many encounters with the nids. I think their most notable were with either kraken or leviathan and what is currently happening in the octarius war neither of which genestealers have had the option to infiltrate their culture as eldar got themselves involved.
gpt
The Eldar have had plenty of contact with the Tyranids, unfortunately for the Eldar.

No. Genestealers have no infiltrated any Eldar worlds, Craftworld or otherwise.
Genestealers do not work so well on Eldar. Namely because, unlike in Human civilization. They can't slink off into the depths of the sewers and skulk around abducting people until they have enough strength to do something. The Eldar are also passively psychically linked to other Eldar they interact with. Even if they reached the generational stage where they can physically replicate the appearance of an Eldar they are not going to be able to blend in. 

Mainly though, they simply cannot infiltrate Eldar society. Craftworlds are far too ordered and well kept for them to try and hide anywhere and any Eldar that is infected will be noticed by not just nearby Eldar but the actually Craftworld/Worldspirit itself.