CVE-2026-90832: Out-of-bounds read in Bluetooth Host periodic advertising report reassembly due to unvalidated report length

bthcileperadvreportcommon() in subsys/bluetooth/host/scan.c handles the HCILEPeriodicAdvertisingReport (and v2) event. It verified that the fixed event header was present and that the controller-declared DataLength fits the tailroom of the periodic-sync reassembly buffer, but never checked that the event actually carried as many data bytes as evt->length claims. Both reassembly paths — DATASTATUSPARTIAL, and DATASTATUSCOMPLETE while a chain is already in progress — then called netbufsimpleaddmem(&peradvsync->reassembly, buf->data, evt->length), copying up to 255 bytes starting past the end of the event's real payload.

The DataLength field originates in the HCI controller, so the flaw is reachable in host-only/split builds where the controller is a separate part reached over H4 UART, SPI, USB or an inter-core IPC transport: a malicious, compromised or buggy controller (or an attacker able to inject on that link) can send a report declaring a length larger than the event payload. The generic event dispatcher in subsys/bluetooth/host/hcicore.c only enforces a per-event minimum length and never cross-checks the subevent's internal length field, so nothing else catches it. Builds using Zephyr's own in-image controller are not affected, because the controller always emits a length matching the bytes it appends.

Exploitation additionally requires CONFIGBTPERADVSYNC with CONFIGBTPERADVSYNCBUFSIZE > 0 (the option defaults to 0, which compiles the reassembly block out; LE Audio broadcast-sink and Public Broadcast Profile applications that reassemble fragmented periodic advertising data set it), and an established periodic advertising sync.

The consequence is an out-of-bounds read of adjacent HCI RX-pool memory — including stale contents of previously received HCI events — which is copied into the reassembly buffer and then delivered to every registered btleperadvsynccb.recv listener as if it were received advertising data, so the disclosed memory becomes application-visible and may be forwarded off-device. The write side remains in bounds because the tailroom check uses the same length value, so there is no memory corruption; impact is limited to information disclosure and undefined behaviour on the over-read. The fix drops any report whose declared length exceeds the remaining event payload, matching the guard already present in the extended and legacy advertising report handlers.