CVE-2026-19938 Use-after-free of an ATT PDU buffer when signing an outgoing Bluetooth ATT Signed Write Command fails
A use-after-free in Zephyr's Bluetooth ATT stack allows attackers to corrupt memory and potentially execute code by triggering a signing failure on unencrypted connections.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-19938?
CVE-2026-19938 is a medium-severity use after free vulnerability in zephyrproject zephyr, published on 12 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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.
Weakness: CWE-416
Which versions are affected by CVE-2026-19938?
How do I fix CVE-2026-19938?
No fix has been published for CVE-2026-19938 yet. Limit exposure of the affected software until one is available.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-19938 being exploited?
There is no known exploitation of CVE-2026-19938 in the wild and no public exploit code.
Sources
Also known as GHSA-p3r4-qw98-j59m.