Any host on the LAN can send two mDNS records and make the responder write past the end of its
transmit packet.
The string table stores each name in a slot rounded up to a multiple of four:
“`c
/* addons/mdns/nxd_mdns.c:11436, 11443, 11447 */
memory_len = ((memory_len & 0xFFFFFFFC) + 8) & 0xFFFFFFFF;
…
len = *((USHORT*)(p – 2)); /* slot size, not string length */
if ((len == memory_len) && … _nx_mdns_name_match(start, memory_ptr, memory_size) …)
“`
The lookup that decides whether an incoming name is already stored compares the rounded slot size,
so names of 12, 13, 14 and 15 characters share one bucket. A second name in the bucket is answered
with the pointer to the first, and the record then carries a string up to three bytes longer than
the length the caller accounted for. `_nx_mdns_packet_rr_add` (nxd_mdns.c:8911) sizes its only
bound check from that stale length, and `_nx_mdns_name_string_encode` writes the real string.
Two PTR records are enough, both ordinary mDNS responses to a `_http._tcp` query, with owner names
whose lengths fall in the same bucket:
“`
==87491==ERROR: AddressSanitizer: heap-buffer-overflow
WRITE of size 1 at 0x611000000124 thread T5
#0 _nx_mdns_name_string_encode addons/mdns/nxd_mdns.c:13096
#1 _nx_mdns_packet_rr_add addons/mdns/nxd_mdns.c:8911
0x611000000124 is 0 bytes to the right of 228-byte region
“`
The overflow is one to three bytes of attacker-influenced name data past `nx_packet_data_end`. In a
normal pool that lands in the next packet in the same pool rather than in a redzone, so the visible
effect is a corrupted neighbouring packet or a corrupted pool free list rather than a clean crash.
Compare the slot size against the stored string length before declaring a match, or keep the
string length in the slot header and return it to the caller so the encoder and the bound check
agree.
