An integer underflow vulnerability exists in the "nextstate()" function in "gpsd/packet.c" of gpsd versions prior to commit "ffa1d6f40bca0b035fc7f5e563160ebb67199da7". When parsing a NAVCOM packet, the payload length is calculated using "lexer->length = (size_t)c - 4" without checking if the input byte "c" is less than 4. This results in an unsigned integer underflow, setting "lexer->length" to a very large value (near "SIZE_MAX"). The parser then enters a loop attempting to consume this massive number of bytes, causing 100% CPU utilization and a Denial of Service (DoS) condition.