libpurple in Finch in Pidgin before 2.6.6, when an XMPP multi-user chat (MUC) room is used, does not properly parse nicknames containing <br> sequences, which allows remote attackers to cause a denial of service (application crash) via a crafted nickname.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.