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.