The definite_length_decoder function in lib/util/quickder.c in Mozilla Network Security Services (NSS) before 3.16.2.4 and 3.17.x before 3.17.3 does not ensure that the DER encoding of an ASN.1 length is properly formed, which allows remote attackers to conduct data-smuggling attacks by using a long byte sequence for an encoding, as demonstrated by the SEC_QuickDERDecodeItem function's improper handling of an arbitrary-length encoding of 0x00.