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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.
Published 2026-09-29 · last modified 2026-09-29
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