From a tactical and realistic sense, this is a very stupid and foolhardy thing to do. Almost every time Sups has been in trouble or defeated by a bad guy they were just following his advise and used some form of kryptonite against him.

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    When did Superman share that information? Every retelling I've seen has had Luthor or some other villain discovering it, or Superman himself discovering it the vulnerability in a public moment when there are people around to witness it. – Nerrolken Sep 8 '15 at 15:32
  • I would say that telling a reporter qualifies as sharing that information. – Morgan Sep 8 '15 at 18:15
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    Again, when did that happen? Are you talking about the cover in Jack's answer? Superman is just talking about Kryptonite generally in that image, not revealing that he's vulnerable to it, and the other parts of Jack's answer touch on retellings in which Luthor discovered kryptonite's effect in other ways. The Superman story has been retold thousands of times, and I can't find a single one in which Superman was the one who outed his own weakness. – Nerrolken Sep 8 '15 at 18:27

At least in the first Superman movie (1978) Lex Luther has this to say:

You mean, to us, they're just meteorites. Fair enough. But the level of specific radioactivity is so high, to anyone from the planet Krypton, this substance is lethal!

Now, Lex hides his kryptonite in a lead box. Superman revealed to Lois Lane that he couldn't see through lead, which she subsequently published in her paper. Superman probably wasn't smart to do that.

In the DCAU "A Little Piece of Home"

Superman's attempts to stop the criminals are progressing nicely until he comes near a display of minerals. He sinks to the ground in pain, allowing the criminals to escape.

Luthor discovers the phenomenon and has the display analyzed to discover what happened to Superman. He learns that one of the rocks is from an foreign solar system and realizes that he now has the perfect weapon against Superman.

On the other hand, if you can believe a comic cover, then yes, Superman is an idiot who reveals his weaknesses to his enemies.

enter image description here

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    As the saying goes. "Loose lips sink ships." – Morgan Sep 8 '15 at 18:17
  • He isn't revealing anything in this picture. It seems to be an educational program he's narrating, and the Kryptonite effect is already known (at least by Jimmy Olson, who seems to have come prepared). – Nerrolken Sep 8 '15 at 18:24
  • As a note, I don't know about this specific cover, but based on every other cover I recall, ypu can't trust the cover. I have one where on the cover, superman, half in his Clark suit tries to convince Lois (Lana?) he's Superman, and she's not buying it. – Longspeak Sep 10 '17 at 19:07

I don't know the exact issue but sometime after Superboy discovered his own vulnerability to Kryptonite, a scientist found a meteor, figured out that it was a piece of Krypton and arranged to "present" it to Superboy at a public forum as a gift. Whoops.

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