Integer overflow in proto.c in libotr before 4.1.1 on 64-bit platforms allows remote attackers to cause a denial of service (memory corruption and application crash) or execute arbitrary code via a series of large OTR messages, which triggers a heap-based buffer overflow.

The (1) otrl_base64_otr_decode function in src/b64.c; (2) otrl_proto_data_read_flags and (3) otrl_proto_accept_data functions in src/proto.c; and (4) decode function in toolkit/parse.c in libotr before 3.2.1 allocates a zero-length buffer when decoding a base64 string, which allows remote attackers to cause a denial of service (application crash) via a message with the value "?OTR:===.", which triggers a heap-based buffer overflow.