The Collaborative International Dictionary of English v.0.48:

blabbermouthed \blabbermouthed\ adj.
   1. tending to talk excessively.

   Syn: bigmouthed, blabby, talkative.
        [WordNet 1.5]

   2. prone to revealing secrets; -- of people.

   Syn: leaky, talebearing(prenominal), tattling(prenominal).
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

blabbermouthed
    adj 1: unwisely talking too much [syn: bigmouthed,
           blabbermouthed, blabby, talkative]
    2: prone to communicate confidential information [syn:
       blabbermouthed, leaky, talebearing(a), tattling(a)]