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 store

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:

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 2 
 3 
 4/* addons/mdns/nxd_mdns.c:11436, 11443, 11447 */
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 6 
 7 
 8memory_len = ((memory_len & 0xFFFFFFFC) + 8) & 0xFFFFFFFF;
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10 
11 
12...
13 
14 
15 
16len = *((USHORT*)(p - 2));           /* slot size, not string length */
17 
18 
19 
20if ((len == memory_len) && ... _nx_mdns_name_match(start, memory_ptr, memory_size) ...)
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22 

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:

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 3 
 4==87491==ERROR: AddressSanitizer: heap-buffer-overflow
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 6 
 7 
 8WRITE of size 1 at 0x611000000124 thread T5
 9 
10    #0 _nx_mdns_name_string_encode  addons/mdns/nxd_mdns.c:13096
11    #1 _nx_mdns_packet_rr_add       addons/mdns/nxd_mdns.c:8911
12 
13 
140x611000000124 is 0 bytes to the right of 228-byte region
15 
16 

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.