Synonyms for Blabbermouthed (same or very similar meaning)
WordNet sense 1 (unwisely talking too much):
talkative, bigmouthed, blabbyWordNet sense 2 (lacking discretion; injudicious):
indiscreetWordNet sense 3 (prone to communicate confidential information):
leaky, talebearing, tattlingWordNet sense 4 (able or tending to communicate):
communicative, communicatoryFrom the ODE community, based on WordNetadd/edit