In an email to staff, Musk said an unnamed employee made unspecified coding changes to its manufacturing operating system and sent sensitive data to unnamed third parties.
The matter will be investigated, Mr Musk said.
"The full extent of his actions are not yet clear, but what he has admitted so far is pretty bad," he said in the email.
"His stated motivation is that he wanted a promotion that he did not receive.
"As you know there are a long list of organisations that want Tesla to die," he added, citing Wall Street short-sellers, oil and gas companies and rival car makers as being among them.
In a separate email to staff, Musk said there had been a small fire at a Tesla factory on Sunday. It was quickly extinguished and did not cause any injuries or significant damage, he added.
Last week, Mr Musk said 9% of Tesla's workforce would be cut as part of a restructuring of the carmaker, which has not made an annual profit since it was created nearly 15 years ago.