CVE-2026-19938

The Bluetooth host's ATT transmit path releases a buffer it does not own when signing fails. chan send() in subsys/bluetooth/host/att.c documents (and every other error exit implements) the contract that on failure the caller retains ownership of the net buf; the BT ATT OP SIGNED WRITE CMD branch instead called net buf unref(buf) when bt smp sign() returned an error and then returned that error to the caller. The callers act on the contract: process queue() re-queues the buffer with k queue prepend(), bt att chan send rsp() puts it back on chan->tx queue, bt att chan req send() restores it into the request, and att req retry() unreferences it — so the already-freed buffer was re-queued, later dereferenced, and eventually freed again.

An ATT Signed Write Command is produced when an application calls bt gatt write without response cb() with sign set on an unencrypted connection. bt smp sign() fails whenever no local CSRK exists for the peer (-ENOENT), and whether one exists is negotiated with the peer: smp distribute keys() stores a local CSRK only when the peer's pairing key-distribution field requests the signing key and bonding was agreed. A peer can therefore pair without accepting the CSRK, pair with No Bonding, remain unpaired, or keep the link unencrypted, and the victim's next signed write takes the failing path. Because struct net buf's node field is simultaneously the linkage word for the ATT TX FIFO, for the deferred-destroy list used by the att pool destroy callback att tx destroy(), and for the pool free list, the mishandled buffer corrupts these lists and can be re-issued by bt att create pdu() to a new PDU while the ATT TX path still holds it.

The same branch also saved the net buf simple state only after signing, so a buffer restored after a bt l2cap send pdu() failure kept its 12-octet signature; a subsequent retry re-signed it and appended a second signature with net buf add(buf, 12) to a buffer that bt att create pdu() sized for exactly one, writing past the end of the pooled buffer in builds where the net buf tailroom assertion is compiled out.

Impact is host-side memory corruption in the Bluetooth stack: list corruption, a pooled ATT buffer with two owners (wrong PDU contents transmitted, or a double free when the queues are drained at disconnect), and a bounded out-of-bounds write on the retry path — typically observed as a crash or assertion failure. Exposure requires the deprecated CONFIG BT SIGNING to be enabled and an application that actually issues signed writes; in that configuration the triggering condition is under the remote peer's control.