Bug 2464441 (CVE-2026-31773)

Summary: CVE-2026-31773 kernel: Bluetooth: SMP: derive legacy responder STK authentication from MITM state
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's Bluetooth Security Manager Protocol (SMP). The system incorrectly labels a Short Term Key (STK) as authenticated during legacy pairing, even when Man-in-the-Middle (MITM) protection was not established. This misrepresentation of the key's authentication status could allow an attacker to bypass expected security measures, potentially leading to unauthorized access or a downgrade of the Bluetooth connection's security.
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Description OSIDB Bzimport 2026-05-01 15:06:59 UTC 
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.
Comment 1 Keith Grant 2026-05-01 23:12:31 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026050149-CVE-2026-31773-e287@gregkh/T