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:1143…

Overview

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:

/* 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.

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.