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CVE-2026-102713
The TFTP server accepts a DATA datagram of any size. The dispatcher rejects datagrams shorter than
four bytes (nxd_tftp_server.c:1037) and nothing anywhere checks an upper bound, in particular not
against the protocol maximum of 4 + NX_TFTP_FILE_TRANSFER_MAX. Two things follow from that one
missing check, both reachable before any authentication because TFTP has none.
The handler passes nx_packet_length - 4 straight to FileX:
/* addons/tftp/nxd_tftp_server.c:1863, 1889 */
status = nx_packet_copy(packet_ptr, &temp_ptr,
server_ptr -> nx_tftp_server_packet_pool_ptr, NX_WAIT_FOREVER);
...
fx_file_write(&(client_request_ptr -> nx_tftp_client_request_file),
packet_ptr -> nx_packet_prepend_ptr + 4,
packet_ptr -> nx_packet_length - 4);
nx_packet_length is the length of a chain, not of one contiguous buffer, so FileX copies past the
end of the first packet:
ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 1280 at 0x621000001108 thread T5
#0 __interceptor_memcpy
#1 _fx_utility_memory_copy filex/common/src/fx_utility_memory_copy.c:78
0x621000001108 is 0 bytes to the right of 4104-byte region
Those bytes are written into the file the attacker is uploading, and a TFTP read request hands them
back, so this is a memory disclosure with a convenient retrieval channel.
The same datagram also wedges the server. nx_packet_copy at :1863 needs
ceil(nx_packet_length / pool_payload) packets and asks for them with NX_WAIT_FOREVER, so when the
attacker sizes the datagram beyond what the pool holds, the server thread suspends and never
returns. A liveness probe after one such datagram times out with the pool at 0 of 12 packets and
the server thread suspended, and no later client is served.
Reject nx_packet_length > 4 + NX_TFTP_FILE_TRANSFER_MAX in the DATA branch before either call,
and use a bounded wait rather than NX_WAIT_FOREVER for the copy.
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The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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