Twitter has advised its users to change their passwords.
The social media network said a bug had caused "unmasked" passwords to be stored internally.
It added that there was "no indication" that the details had been misused but warned users to update their login information as a precaution.
The company said in a statement on Thursday: "We recently found a bug that stored passwords unmasked in an internal log.
"We fixed the bug and have no indication of a breach or misuse by anyone.
"As a precaution, consider changing your password on all services where you've used this password."