indiscreet
Not careful about keeping secrets or private information.
Indiscreet means failing to keep private information to yourself, or acting without thinking about how your words or actions might cause problems. Someone who is indiscreet might blurt out a friend's secret, share embarrassing details about someone else, or say something thoughtless that hurts feelings or creates awkward situations.
An indiscreet person speaks or acts without considering the consequences. If your teacher tells you something in confidence and you immediately tell three classmates, you're being indiscreet. If someone asks you not to mention their surprise party but you accidentally bring it up in front of them, that's an indiscreet mistake.
Being indiscreet isn't always malicious. Sometimes people are indiscreet simply because they're excited, not thinking clearly, or don't realize certain information should stay private. But indiscreet behavior can damage trust. When you share something you shouldn't, people may become reluctant to confide in you. They remember that you couldn't keep a secret or that you spoke without thinking about the impact of your words.
The opposite quality is discretion: being thoughtful about what you share and what you keep private.