Warning: spoilers for Batman: Urban Legends #5 are ahead.
Riddler figured out Batman's identity. Written by Jeph Loeb, with pencils by Jim Lee, inks by Scott Williams, colors by Alex Sinclair, and letters by Richard Starkings, Hush showed how Riddler's smarts were both his greatest advantage and his greatest weakness, as his plan to publicly release Bruce Wayne's identity as Batman ultimately backfired when the Caped Crusader convinced him that revealing Gotham's greatest secret would no longer make the Riddler superior to everyone else. Even so, a lingering question from Hush has been how Riddler actually figured out that Bruce Wayne was Batman, with the fifth installment of the Batman: Urban Legends anthology teasing a conclusive answer.
In a flashback from the "Cheer" storyline by Chip Zdarsky, Eddy Barrows, Eber Ferreira, Diogenes Neves, Marcus To, Adriano Lucas, and Becca Carey, Jason Todd's Robin comes to rescue Batman from the Riddler, and mistakenly calls Alfred Pennyworth by name on his comms. But with Alfred's codename being "Penny-one," it does not provide all that much cover before the Riddler finally appears on a screen and tells Robin that he had been monitoring the room. Though this flashback takes place years before when Batman: Hush is set, it provides a hint at how Riddler was able to solve one of Gotham City's most confounding questions.
While the Riddler does not explicitly state that he had just overhead Robin call someone "Alfred" and then "Penny-one," his later deduction of Bruce Wayne's identity in Hush makes more sense considering the morsels of information he likely picked up from this story. In the same issue, the Riddler was already astute enough to figure out that Batman had gotten a new Robin, and tried to spin this to his advantage by testing Jason with a riddle. With someone as observant as the Riddler, it is plausible to consider that knowing "Alfred" and the codename "Penny-one" would be sufficient to tracing Batman to Alfred Pennyworth.
The Riddler may have genius level intellect, but assuming that he has access to a phone book that lists Alfred Pennyworth's name, it wouldn't exactly be that difficult to find someone matching the description that Robin accidentally gave. From there, he could also parse through the other compelling pieces of evidence that point to Bruce Wayne's identity as Batman. At that time, Bruce Wayne had recently taken in a boy around Robin's age, and as one of the richest people in Gotham City, he would be able to fund his activities as Batman without constraints.
As one of the smartest villains that Batman has ever had to fight, the greatest threat that the Riddler poses is his understanding that knowledge is power. While Batman: Urban Legends #5 only hints at where Riddler could have gotten his suspicion that Bruce Wayne was Batman, it is not completely out of the question that Riddler would jump at the opportunity to deduce his greatest enemy's identity from any piece of information, no matter how inconsequential.
