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

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:

 1 
 2 
 3 
 4/* addons/dhcp/nxd_dhcp_client.c:7538, 7572 */
 5 
 6 
 7 
 8while (i < length - 1)
 9 
10 
11 
12{
13 
14    ...
15    size = *(++data);      /* data moves 1: type -> length byte */
16    data += size + 1;      /* data moves size + 1 more          */
17    i += size + 1;         /* i moves only size + 1             */
18 
19 
20}
21 
22 

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:

 1 
 2 
 3 
 4ERROR: AddressSanitizer: heap-buffer-overflow
 5 
 6 
 7 
 8READ of size 1 at 0x61b000000794 thread T5
 9 
10    #0 _nx_dhcp_search_buffer     addons/dhcp/nxd_dhcp_client.c:7541
11    #1 _nx_dhcp_get_option_value  addons/dhcp/nxd_dhcp_client.c:7082
12 
13 
140x61b000000794 is located 164 bytes to the right of 1648-byte region
15 
16 

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.