CVE-2020-26556: Bluetooth Core Auth Bypass Vulnerability

CVE-2020-26556 Overview

CVE-2020-26556 is a cryptographic vulnerability in the Bluetooth Mesh profile versions 1.0 and 1.0.1 that affects the Mesh Provisioning protocol. The flaw permits a nearby attacker with adjacent network access to exploit an insufficiently random AuthValue during the provisioning procedure. By leveraging a Malleable Commitment weakness, an attacker capable of conducting a successful brute-force attack before the provisioning procedure times out can complete authentication and gain unauthorized access to the mesh network.

Critical Impact

A successful exploitation allows an attacker within Bluetooth range to bypass authentication during mesh provisioning, potentially gaining full access to the Bluetooth Mesh network and all connected devices.

Affected Products

• Bluetooth Mesh Profile 1.0.0
• Bluetooth Mesh Profile 1.0.1
• Bluetooth Core Specification (various versions)

Discovery Timeline

• 2021-05-24 - CVE CVE-2020-26556 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-26556

Vulnerability Analysis

The vulnerability exists in the Mesh Provisioning protocol implementation within the Bluetooth Mesh profile. During the provisioning process, devices exchange authentication values (AuthValue) to establish secure communication. The weakness stems from the use of an insufficiently random AuthValue combined with a Malleable Commitment cryptographic flaw.

The Malleable Commitment issue allows an attacker to manipulate or predict commitment values during the authentication exchange. When combined with a weak or predictable AuthValue, this creates a window of opportunity where a brute-force attack becomes feasible within the provisioning timeout period.

The attack requires adjacent network access, meaning the attacker must be within Bluetooth communication range of the target device. While this limits remote exploitation, it presents significant risks in environments with high-value Bluetooth Mesh deployments such as building automation systems, industrial IoT networks, and smart home ecosystems.

Root Cause

The root cause of this vulnerability is twofold: insufficient entropy in the AuthValue generation process (CWE-307: Improper Restriction of Excessive Authentication Attempts) and the cryptographic weakness of Malleable Commitment in the provisioning protocol. The protocol design does not adequately protect against brute-force attempts during the limited provisioning window, and the commitment scheme allows for manipulation that can accelerate authentication bypass attacks.

Attack Vector

The attack requires an adversary to be within adjacent network proximity (Bluetooth range, typically 10-100 meters depending on device class). The attacker must:

1. Monitor the Bluetooth Mesh provisioning process between a provisioner and an unprovisioned device
2. Capture the commitment values exchanged during authentication
3. Exploit the Malleable Commitment weakness to manipulate or predict valid authentication responses
4. Conduct a brute-force attack against the insufficiently random AuthValue
5. Complete the attack before the provisioning procedure times out

The attack does not require any privileges or user interaction, though it does require high attack complexity due to the time-sensitive nature of the brute-force component.

No verified code examples are available for this vulnerability. The exploitation involves manipulating cryptographic commitment values and brute-forcing authentication material during the Bluetooth Mesh provisioning handshake. For detailed technical analysis, refer to the CERT Vulnerability Advisory #799380 which provides comprehensive information on the attack methodology.

Detection Methods for CVE-2020-26556

Indicators of Compromise

• Unusual or repeated Bluetooth Mesh provisioning attempts detected in proximity to protected devices
• Multiple failed provisioning authentication exchanges followed by successful provisioning from unknown devices
• Unexpected devices appearing in the Bluetooth Mesh network topology
• Anomalous timing patterns in provisioning protocol exchanges suggesting brute-force activity

Detection Strategies

• Deploy Bluetooth spectrum monitoring tools to detect suspicious provisioning activity within your environment
• Implement logging and alerting for all Bluetooth Mesh provisioning events, particularly failed and repeated attempts
• Maintain an inventory of authorized provisioning devices and alert on provisioning from unknown sources
• Use network security monitoring solutions capable of analyzing Bluetooth protocol traffic for anomalies

Monitoring Recommendations

• Enable verbose logging on Bluetooth Mesh provisioner devices to capture detailed provisioning event data
• Implement physical security controls to limit adversary access to areas where Bluetooth Mesh provisioning occurs
• Regularly audit the Bluetooth Mesh network for unauthorized or unexpected device additions
• Consider deploying dedicated Bluetooth security monitoring in high-security environments

How to Mitigate CVE-2020-26556

Immediate Actions Required

• Review and identify all Bluetooth Mesh deployments running affected profile versions 1.0.0 and 1.0.1
• Ensure provisioning activities are conducted only in secure, physically controlled environments
• Use strong, high-entropy AuthValues during provisioning procedures when possible
• Limit provisioning windows and disable provisioning mode when not actively adding devices
• Contact device vendors to inquire about firmware updates addressing this vulnerability

Patch Information

The Bluetooth SIG has been notified of this vulnerability. Organizations should consult the Bluetooth Security Reporting Guide for official guidance and monitor for updated Bluetooth Mesh profile specifications. Device manufacturers may release firmware updates that implement improved AuthValue generation or enhanced commitment schemes. Check with your specific device vendors for available patches.

Workarounds

• Conduct all Bluetooth Mesh provisioning in RF-shielded or physically secured locations to prevent adjacent network attacks
• Implement out-of-band (OOB) provisioning methods where supported, which provide stronger authentication than in-band methods
• Reduce provisioning timeout values where configurable to minimize the brute-force attack window
• Consider network segmentation to limit the impact of unauthorized device access to the mesh network

When provisioning new devices, consider implementing physical access controls:

# Example: Document and control provisioning sessions
# Maintain provisioning logs with timestamps and operator identification
# Provisioning should only occur in designated secure areas
# Log format recommendation:
echo "$(date -u +%Y-%m-%dT%H:%M:%SZ) | Operator: $USER | Device: $DEVICE_ID | Location: SECURE_ROOM_A" >> /var/log/bluetooth_mesh_provisioning.log