Linux kernel Bluetooth SMP responder MITM enforcement flaw
CVE-2026-43334 is a vulnerability in the Linux kernel Bluetooth Security Manager Protocol (SMP) pairing logic. In smp_cmd_pairing_req(), the responder constructs the pairing response using the initiator's auth_req before enforcing the local BT_SECURITY_HIGH requirement. As a result, if the initiator omits the SMP_AUTH_MITM bit, the responder's pairing response can also omit SMP_AUTH_MITM even when the local device requires man-in-the-middle protection. Later, tk_request() may observe an auth value without SMP_AUTH_MITM and select the JUST_CFM pairing method, creating a mismatch between the responder's intended security policy and the pairing method actually selected. The upstream fix verifies that MITM is achievable from the devices' IO capabilities when HIGH security is required, and forces SMP_AUTH_MITM into both rsp.auth_req and auth before the pairing response is built, keeping responder authentication bits and method selection consistent.
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