Zephyr Project up to 4.4.x Bluetooth Host ATT att.c chan_send double free

Summaryinfo

A vulnerability labeled as critical has been found in Zephyr Project Zephyr up to 4.4.x. This affects the function chan_send of the file subsys/bluetooth/host/att.c of the component Bluetooth Host ATT. Executing a manipulation can lead to double free. The identification of this vulnerability is CVE-2026-19938. The attack may be launched remotely. There is no exploit available. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in Zephyr Project Zephyr up to 4.4.x. It has been declared as critical. This vulnerability affects the function chan_send of the file subsys/bluetooth/host/att.c of the component Bluetooth Host ATT. The manipulation with an unknown input leads to a double free vulnerability. The CWE definition for the vulnerability is CWE-415. The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations. As an impact it is known to affect integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-19938 since 08/15/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available.

Upgrading to version 4.5.0 eliminates this vulnerability. Applying the patch ba8627d91c8ce15c26224cbc7f7f92bb66f56b03 is able to eliminate this problem. The best possible mitigation is suggested to be upgrading to the latest version.

Once again VulDB remains the best source for vulnerability data.

Productinfo

Type

• Project Management Software

Vendor

• Zephyr Project

Name

• Zephyr

Version

• 4.0
• 4.1
• 4.2
• 4.3
• 4.4

CPE 2.3info

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CPE 2.2info

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