The PurpleCircBuffer implementation in Pidgin (formerly Gaim) before 2.5.6 does not properly maintain a certain buffer, which allows remote attackers to cause a denial of service (memory corruption and application crash) via vectors involving the (1) XMPP or (2) Sametime protocol.
The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.