Bluetooth Classic RFCOMM: integer underflow in the MCC TEST command handler leads to out-of-bounds read disclosed to the peer and out-of-bounds write

Summary

CVE	CVE-2026-77685
State	PUBLISHED
Assigner	zephyr
Source Priority	CVE Program / NVD first with legacy fallback
Published	2026-11-10 22:20:53 UTC
Updated	2026-11-10 22:20:53 UTC
Description	The RFCOMM multiplexer control-channel handler in subsys/bluetooth/host/classic/rfcomm.c computed the echo length for a BT_RFCOMM_TEST command as buf->len - 1 and passed it to rfcomm_send_test(), whose length parameter is a uint8_t. A malformed but FCS-valid UIH frame on DLCI 0 can leave buf->len == 0 after rfcomm_handle_msg() pulls the two-byte MCC header, so buf->len - 1 underflows and narrows to 255. rfcomm_send_test() then copies 255 bytes out of a receive buffer that holds no remaining valid data, and appends them to an outgoing L2CAP PDU allocated with only CONFIG_BT_L2CAP_TX_MTU (253 by default for CONFIG_BT_CLASSIC) bytes of tailroom. The source read runs past the end of the ACL receive buffer into adjacent buffers of the same net_buf pool, and the destination copy overruns the transmit buffer by about eight bytes; net_buf_add_mem() performs no runtime bounds check in ordinary builds, where the underlying __ASSERT_NO_MSG() is compiled out. The trigger is a single five-byte RFCOMM frame on the multiplexer control channel, which is processed as soon as the L2CAP channel for PSM 3 is established — before any DLC exists and independent of RFCOMM session state. bt_rfcomm_init() registers that L2CAP server with .sec_level = BT_SECURITY_L1, which l2cap_br_conn_security() accepts without any link key from a peer that does not claim host Secure Simple Pairing support, so an unpaired BR/EDR device in radio range of a connectable target with CONFIG_BT_RFCOMM enabled (SPP, HFP, HSP, GOEP and similar profiles all enable it) can reach the defect. The resulting 255-byte TEST response is transmitted back to the attacker, disclosing roughly 255 bytes of adjacent stack/pool memory — including the contents of neighbouring receive buffers, which an attacker can prime with earlier packets — and the accompanying out-of-bounds write corrupts memory following the transmit buffer, which can crash the Bluetooth host. The request is trivially repeatable, so both the disclosure and the corruption can be driven at will. Two fixes are needed, and the first on its own does not close the out-of-bounds write. Fix 1 (0040ef781e1d, #115768) strips the FCS byte in rfcomm_recv() and echoes buf->len, rejecting anything over UINT8_MAX, which removes the underflow and the over-read. But rfcomm_make_uih_msg() still allocates the response with bt_l2cap_create_pdu() and never compares the requested length with the buffer's tailroom, so a well-formed TEST command carrying a pattern of about 248 bytes or more still builds a response (3-byte frame header, 2-byte message header, pattern, FCS) larger than the 253 bytes available. A pattern that long can only be received when the RFCOMM L2CAP receive MTU (CONFIG_BT_RFCOMM_L2CAP_MTU, which defaults to CONFIG_BT_BUF_ACL_RX_SIZE) exceeds CONFIG_BT_L2CAP_TX_MTU. The plain BR/EDR defaults (200 against 253) cannot reach it, but from v4.3.0 enabling CONFIG_BT_GOEP raises CONFIG_BT_BUF_ACL_RX_SIZE to 264 (265 from v4.4.0), so GOEP builds with only fix 1 applied still overrun the transmit buffer by up to eight bytes. This includes v4.3.2, the last v4.3 release, which carries fix 1 but will not receive fix 2. Fix 2 (214530895dd3, #118293) checks the tailroom on every RFCOMM PDU allocation and closes this residual write.

Risk And Classification

Primary CVSS: v3.1 8.3 HIGH from [email protected]

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:H

Version	Source	Type	Score	Severity	Vector
3.1	[email protected]	Secondary	8.3	HIGH	`CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:H`
3.1	CNA	CVSS	8.3	HIGH	`CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:H`

CVSS v3.1 Breakdown

Attack Vector

Adjacent

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

Low

Availability

High

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:H

Vendor Declared Affected Products

Source	Vendor	Product	Version	Platforms
CNA	Zephyrproject	Zephyr	affected 1.7.0 4.5.0 semver	Not specified

References

Reference	Source	Link	Tags
github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-mhhp-8w67-...	[email protected]	github.com
github.com/zephyrproject-rtos/zephyr/commit/214530895dd327ef34468fb25e13...	[email protected]	github.com
github.com/zephyrproject-rtos/zephyr/commit/0040ef781e1d705066045850782c...	[email protected]	github.com
CVE Program record	CVE.ORG	www.cve.org	canonical
NVD vulnerability detail	NVD	nvd.nist.gov	canonical, analysis

No vendor comments have been submitted for this CVE.

There are currently no legacy QID mappings associated with this CVE.