CVE-2026-31773 Bluetooth: SMP: derive legacy responder STK authentication from MITM state
Attackers can bypass Bluetooth security requirements by exploiting incorrect authentication state handling in legacy pairing, allowing unauthenticated connections.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-31773?
CVE-2026-31773 is a high-severity authentication bypass vulnerability in Linux kernel, published on 1 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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.
Weakness: CWE-287
Which versions are affected by CVE-2026-31773?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-31773?
Apply the vendor's patch.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-31773 being exploited?
There is no known exploitation of CVE-2026-31773 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2026-31773, DEBIAN-CVE-2026-31773, GHSA-vm8r-x97v-544j, ROOT-OS-DEBIAN-11-CVE-2026-31773, ROOT-OS-UBUNTU-2204-CVE-2026-31773, ROOT-OS-UBUNTU-2404-CVE-2026-31773 and 1 more.