CVE-2026-102715

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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/* 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:

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==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.

Weakness

The product writes data past the end, or before the beginning, of the intended buffer.

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer.

• Be wary that a language’s interface to native code may still be subject to overflows, even if the language itself is theoretically safe.

• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• Examples include the Safe C String Library (SafeStr) by Messier and Viega [REF-57], and the Strsafe.h library from Microsoft [REF-56]. These libraries provide safer versions of overflow-prone string-handling functions.

• Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking.

• D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.

• Consider adhering to the following rules when allocating and managing an application’s memory:

• Run or compile the software using features or extensions that randomly arrange the positions of a program’s executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.

• Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as “rebasing” (for Windows) and “prelinking” (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking.

• For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].

• Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment.

• For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].

References

• https://github.com/eclipse-threadx/netxduo/security/advisories/GHSA-2gf7-5224-5vrj