CVE-2026-12363: Out-of-bounds write in LoRaWAN fragmented transport from a fragment index of 0

The LoRaWAN Fragmented Data Block Transport service (subsys/lorawan/services/fragtransport.c) does not validate the fragment counter in a received DATAFRAGMENT command before forwarding it to the configured decoder. In fragtransportpackagecallback() the value fragcounter = hdr->fragindexn & 0x3FFF is taken directly from the downlink payload and passed to the decoder, which derives an array index and flash offset as fragcounter - 1. DataFragment fragments are 1-indexed, so a fragcounter of 0 underflows that arithmetic.

With the default Semtech/LoRaMAC-node decoder, this reaches FragDecoder.FragNbMissingIndex[fragCounter - 1] = 0; in FragDecoderProcess(), where fragCounter - 1 evaluates to -1 and writes a uint16t zero out of bounds, just before the array and into the adjacent MatrixM2B recovery-matrix state of the static decoder object (CWE-787). A companion write derives a wild flash offset, but that path is rejected by the flashareawrite() bounds check. The in-tree low-memory decoder (fragdec()) is not corrupted: its out-of-range bit-array and flash accesses are caught by sysbitarray and flasharea bounds checks.

The handler is the registered downlink callback for the fragmentation transport port, reachable whenever an active fragmentation session exists, so the triggering byte is attacker-influenceable LoRaWAN/FUOTA network input. Triggering it requires authenticated downlinks (LoRaWAN MAC session keys or a malicious/compromised network or FUOTA server) and an active fragmentation session. The impact is contained: corruption of decoder state and denial of the firmware-update (FUOTA) session rather than controllable memory corruption or code execution. The fix adds a transport-layer check that rejects fragcounter == 0, closing the defect for both decoder backends.