Coach Joachim Low announced his squad for the match with Greece to his players at 13:30CET on Friday afternoon, and a player in attendance at that meeting reportedly leaked the team-sheet to a German news agency, who published the news less than an hour later.
The squad in question was a controversial one, with Mario Gomez, Thomas Muller and Lukas Podolski left out, and Miroslav Klose, Andre Schurrle and Marco Reus coming in.
Lahm is furious that a player within the camp has been leaking information, but he admitted they are unlikely to find the person responsible.
"It is an issue, I must admit it and I think now everyone should be aware who he passes the line-up to," Lahm told The Telegraph.
"It's a pity if leaks occur. The whole nation is behind us so it's a disgrace for the line-up to be in the public eye so early.
"The one with the problem is the guy who has done this because he is damaging the hopes of the entire nation. It's a shame, but we won't be hiring a detective."
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"We must be extra careful now. Whether someone plays or they do not play, it is out of the question to pass the line-up to advisors," he added.
"Exchanges with closest family are understandable, but that is it."
Lahm praised the ability within the squad, and is only focusing on winning the final on Sunday.
"We have 23 top-category players and the coach is spoilt for choice," he continued. "The strength of our squad was visible in the last two matches.
"We are content with four wins in a row and being in another semi-final but the objective is to win in Ukraine next Sunday."