CVE-2026-31773: Bluetooth: SMP: derive legacy responder STK authentication from MITM state

Severity: high (CVSS 8.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-01 · updated 2026-10-19
Weakness: CWE-287

Description

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: SMP: derive legacy responder STK authentication from MITM state The legacy responder path in smp_random() currently labels the stored STK as authenticated whenever pending_sec_level is BT_SECURITY_HIGH. That reflects what the local service requested, not what the pairing flow actually achieved. For Just Works/Confirm legacy pairing, SMP_FLAG_MITM_AUTH stays clear and the resulting STK should remain unauthenticated even if the local side requested HIGH security. Use the established MITM state when storing the responder STK so the key metadata matches the pairing result. This also keeps the legacy path aligned with the Secure Connections code, which already treats JUST_WORKS/JUST_CFM as unauthenticated.

Affected products

Linux kernel: from 3.15.5 before 3.16; from 3.16.1 before 5.10.253; from 5.11 before 5.15.203; from 5.16 before 6.1.168; from 6.2 before 6.6.134; from 6.7 before 6.12.81 — fixed in 3.16, 5.10.253, 5.15.203, 6.1.168, 6.12.81, 6.18.22

How to fix

Upgrade to a fixed version: Linux kernel 3.16, 5.10.253, 5.15.203, 6.1.168. Check the vendor advisory for mitigations.

References

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