CVE-2026-102715: mDNS string-cache lookup matches on slot size, so a peer name aliases a shorter one and the response encoder writes past the packet

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/nxdmdns.c:11436, 11443, 11447 /

memorylen = ((memorylen & 0xFFFFFFFC) + 8) & 0xFFFFFFFF;

...

len = ((USHORT)(p - 2)); / slot size, not string length /

if ((len == memorylen) && ... nxmdnsnamematch(start, memoryptr, memorysize) ...)

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. nxmdnspacketrradd (nxdmdns.c:8911) sizes its only

bound check from that stale length, and nxmdnsnamestringencode 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 nxmdnsnamestringencode addons/mdns/nxdmdns.c:13096 #1 nxmdnspacketrradd addons/mdns/nxdmdns.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 nxpacketdataend. 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.