CVE-2025-5820

Executive Summary

CVE-2025-5820 is a critical authentication bypass vulnerability in the Sony XAV-AX8500 in-car multimedia receiver, specifically within its Bluetooth ERTM (Enhanced Retransmission Mode) channel implementation. The flaw allows a network-adjacent attacker to bypass authentication without any credentials or user interaction. The root cause is improper initialization of channel data during ERTM communication, enabling an attacker to impersonate a trusted device or gain unauthorized access to the system’s Bluetooth services. With a CVSS score of 6.3 (Medium), the vulnerability poses a significant risk to vehicle infotainment security, potentially allowing attackers to manipulate audio, navigation, or connected smartphone functions. No public exploit code or patch is currently available, but the vulnerability was responsibly disclosed via ZDI (ZDI-CAN-26285).

CVSS Breakdown & Threat Level

• CVSS Score: 6.3 (Medium)
• Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
• Attack Vector (AV): Adjacent Network – attacker must be within Bluetooth range (typically ~10 meters).
• Attack Complexity (AC): Low – no special conditions required.
• Privileges Required (PR): None – no authentication needed.
• User Interaction (UI): None – no victim action required.
• Scope (S): Unchanged – exploitation does not affect other components.
• Confidentiality (C): High – attacker can access sensitive data (e.g., phone contacts, call logs).
• Integrity (I): High – attacker can modify system settings or inject malicious data.
• Availability (A): None – no denial-of-service impact.

Threat Level: Medium. While the attack requires physical proximity, the lack of authentication and low complexity makes it a realistic threat for targeted attacks in parking lots or traffic.

Deep Dive & Mechanics

The vulnerability resides in the Bluetooth ERTM (Enhanced Retransmission Mode) channel implementation of the Sony XAV-AX8500. ERTM is a Bluetooth L2CAP mode that provides reliable data transfer with retransmission and flow control. The flaw occurs during channel initialization: the device fails to properly clear or validate channel state data before establishing a new connection. This allows an attacker to send crafted L2CAP frames that manipulate the ERTM state machine, effectively bypassing the Bluetooth pairing and authentication process.

Technical Details:

• Bluetooth Stack: The XAV-AX8500 uses a proprietary Bluetooth stack (likely based on Broadcom or CSR). The ERTM implementation does not reset the ertm_seq_num or ertm_ack_seq values when a new connection is initiated.
• Attack Vector: An attacker with a Bluetooth adapter (e.g., Ubertooth or standard CSR dongle) sends specially crafted L2CAP Connection Request and ERTM configuration frames. By setting the ertm_seq_num to a value that matches the expected sequence of a previously paired device, the attacker can impersonate that device.
• Impact: Once authenticated, the attacker gains full access to Bluetooth profiles (e.g., A2DP, HFP, PBAP). This allows them to stream audio, make calls, or download phonebook data without the user’s knowledge.

Root Cause: Improper channel data initialization (CWE-923: Improper Restriction of Communication Channel to Intended Endpoints). The device does not enforce that the ERTM channel is only established with a previously paired and authenticated device.

Vulnerable vs. Patched Code

Since no patch is available, the following is a hypothetical comparison based on typical Bluetooth stack fixes.

Vulnerable Code (C pseudocode):

void ertm_channel_init(connection_t *conn) {
    // No reset of sequence numbers
    conn->ertm_seq_num = 0; // Should be random or reset per connection
    conn->ertm_ack_seq = 0;
    // No check if the remote device is already paired
    if (l2cap_connect(conn)) {
        // Accept any connection
        conn->authenticated = true;
    }
}

Patched Code (Hypothetical):

void ertm_channel_init(connection_t *conn) {
    // Reset sequence numbers for each new connection
    conn->ertm_seq_num = generate_random_seq();
    conn->ertm_ack_seq = 0;
    // Verify that the remote device is in the paired list
    if (is_device_paired(conn->bd_addr) && l2cap_connect(conn)) {
        conn->authenticated = true;
    } else {
        conn->authenticated = false;
        l2cap_disconnect(conn);
    }
}

Key Difference: The patched version resets ERTM sequence numbers and validates the remote device’s Bluetooth address against a list of previously paired devices before granting authentication.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• Bluetooth adapter supporting L2CAP raw sockets (e.g., CSR 4.0 dongle on Linux).
• Python with pybluez or scapy for crafting packets.
• Target: Sony XAV-AX8500 in pairing mode or with an active Bluetooth connection.

Exploit Steps:

1. Scan for target: Use hcitool scan to find the XAV-AX8500’s Bluetooth MAC address.
2. Craft ERTM frames: Use Scapy to send a malicious L2CAP Connection Request with ERTM configuration.

   from scapy.all import *
   # Target MAC
   target_mac = "XX:XX:XX:XX:XX:XX"
   # Craft L2CAP Connection Request with ERTM
   packet = BluetoothL2CAP() / L2CAP_CmdReq()
   packet.cid = 0x0040  # ERTM channel
   packet.data = b'\x00' * 10  # Malformed ERTM config
   sendp(packet, iface="hci0")
   

3. Bypass authentication: The device accepts the connection without verifying pairing status.
4. Access services: Use bluetoothctl to connect to A2DP or HFP profiles.

Note: No public exploit exists; this is a conceptual demonstration.

Blue Team (Defense & Detection)

Detection Strategy:

• Monitor Bluetooth HCI logs for unexpected L2CAP Connection Requests on ERTM channels.
• Look for rapid connection attempts from unknown MAC addresses.
• Use Bluetooth intrusion detection systems (e.g., BlueHydra) to alert on anomalous ERTM traffic.

Honeypot Setup:

• Deploy a Bluetooth honeypot mimicking the XAV-AX8500 to capture attacker behavior.
• Log all L2CAP frames and analyze for ERTM sequence number anomalies.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Unusual L2CAP Connection Requests with ERTM configuration from unknown MAC addresses.
  • Repeated ERTM frames with sequence numbers that do not follow expected increments.
• Host IOCs:
  • Bluetooth logs showing authentication bypass events (e.g., auth_status: success without prior pairing).
  • Unexpected pairing requests from devices not in the paired list.
• File IOCs:
  • No file-based IOCs due to the nature of the vulnerability.

Detection Engineering

YARA Rule

rule CVE_2025_5820_Bluetooth_ERTM_Bypass {
    meta:
        description = "Detects malicious L2CAP ERTM frames targeting Sony XAV-AX8500"
        author = "Valters IT"
        date = "2025-04-01"
        cve = "CVE-2025-5820"
    strings:
        $ertm_header = { 02 00 04 00 }  // L2CAP Connection Request with ERTM CID
        $seq_zero = { 00 00 00 00 }     // Zeroed sequence numbers
    condition:
        $ertm_header and $seq_zero
}

Sigma Rule

title: Bluetooth ERTM Authentication Bypass Attempt
id: 8f3a2b1c-4d5e-6f7a-8b9c-0d1e2f3a4b5c
status: experimental
description: Detects anomalous L2CAP ERTM connection requests that may indicate CVE-2025-5820 exploitation
author: Valters IT
logsource:
    product: linux
    service: bluetooth
detection:
    selection:
        EventID: 1001  # Bluetooth HCI event
        L2CAP_CID: 0x0040
        ERTM_Seq_Num: 0
    condition: selection
falsepositives:
    - Legitimate Bluetooth pairing with reset sequence numbers
level: high

Suricata Rule

alert bluetooth any any -> any any (msg:"CVE-2025-5820 Bluetooth ERTM Authentication Bypass"; bluetooth.proto:l2cap; l2cap.cid:0x0040; content:"|00 00 00 00|"; distance:0; within:4; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Bluetooth when not in use: Turn off Bluetooth on the XAV-AX8500 to eliminate the attack surface.
2. Update firmware: Monitor Sony’s support page for a firmware patch. If available, apply immediately.
3. Restrict physical access: Park in secure areas to reduce the risk of adjacent attacks.

Long-term Mitigations:

• Bluetooth stack hardening: Ensure ERTM channel initialization resets sequence numbers and validates pairing status.
• Network segmentation: Isolate the infotainment system from critical vehicle networks (e.g., CAN bus).
• User awareness: Educate users to avoid leaving Bluetooth in discoverable mode.

Vendor Response: Sony has not yet released a patch. Contact Sony support for updates.

Valters IT
Securing Digital Frontiers