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As others suggest, it's most likely whatever password they setup the router with in the first place – which means you never changed it to prevent this from happening

It's possible it wasn't customer-alterable, but your ISP "knowing" your wifi password is nothing at all like "My ISP is storing wifi passwords in plain text".

I'd lay pretty good odds that either:

• all routers they setup use the same password
• they're just telling you whatever was printed on the back of the router
• all routers they setup use a standard format for the password (eg Insight in Lexington KY, prior to being bought-out by TWC, used to use part of your physical address as the password)