A DHCP server, or anyone on the LAN who answers a DISCOVER first, can make the client read about a
kilobyte past the end of the received message. The option walk keeps a pointer and an offset in step, and the only bound check uses the offset:
```c
/* addons/dhcp/nxd_dhcp_client.c:7538, 7572 */
while (i < length - 1)
{
...
size = *(++data); /* data moves 1: type -> length byte */
data += size + 1; /* data moves size + 1 more */
i += size + 1; /* i moves only size + 1 */
}
```
A TLV option occupies size + 2 bytes. `data` is advanced by size + 2 in total, `i` by size + 1, so
the offset falls one byte behind the real read position for every option the walk skips. After
enough skipped options the check `i < length - 1` still holds while `data` is already past the end
of the message, and the subsequent read of the type and length bytes comes from whatever follows.
A single OFFER carrying a long run of skippable options is enough:
```
ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 1 at 0x61b000000794 thread T5
#0 _nx_dhcp_search_buffer addons/dhcp/nxd_dhcp_client.c:7541
#1 _nx_dhcp_get_option_value addons/dhcp/nxd_dhcp_client.c:7082
0x61b000000794 is located 164 bytes to the right of 1648-byte region
```
A well formed OFFER through the same path is handled normally, the client records the offer and
moves to REQUESTING, so the difference is the option layout rather than the harness. The read runs in the DHCP client thread while the client is still unconfigured, so it happens on
every boot in reach of a hostile DHCP responder. The values read are used to configure the
interface, which is how the disclosed bytes become observable.
Advance `i` by size + 2, or derive the bound from `data` rather than keeping a second counter.
Attack Parameters
Impact Assessment
CVSS Vector v4.0