CVE-2025-5476

Executive Summary

CVE-2025-5476 is a critical authentication bypass vulnerability in the Sony XAV-AX8500 in-car multimedia receiver. The flaw resides in the Bluetooth implementation, specifically within the handling of ACL-U (Asynchronous Connection-Less - Unicast) links. Due to improper isolation of L2CAP (Logical Link Control and Adaptation Protocol) channels, an unauthenticated, network-adjacent attacker can bypass the device's authentication mechanisms. This allows the attacker to interact with the system without proper credentials, potentially leading to unauthorized control of media playback, navigation, or other connected functions. The vulnerability was reported through the Zero Day Initiative (ZDI-CAN-26284) and currently has no known patch or public exploit code. The CVSS score of 6.3 reflects a medium severity, but the ease of exploitation (no authentication required, low attack complexity) elevates the risk for users in close physical proximity.

Supply Chain & Package Security

This vulnerability is not related to any open-source package manager (NPM, PyPI, Go, Ruby). It is a hardware/firmware-level issue in a proprietary Sony product. No supply chain implications are identified.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• Base Score: 6.3 (Medium)
• Attack Vector (AV): Adjacent Network (AV:A) – The attacker must be within Bluetooth range of the device.
• Attack Complexity (AC): Low (AC:L) – No special conditions or race conditions are required.
• Privileges Required (PR): None (PR:N) – No prior authentication needed.
• User Interaction (UI): None (UI:N) – No user action required for exploitation.
• Scope (S): Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality (C): Low (C:L) – Limited information disclosure possible.
• Integrity (I): Low (I:L) – Limited ability to modify data.
• Availability (A): Low (A:L) – Limited impact on device availability.

Threat Level: Medium. While the CVSS score is moderate, the lack of required authentication and the potential for lateral movement within the vehicle's network (if connected) make this a significant concern for targeted attacks.

Deep Dive & Mechanics

The vulnerability is rooted in the Bluetooth stack's implementation of ACL-U links. ACL-U links are used for asynchronous data transfer over Bluetooth. The L2CAP layer is responsible for multiplexing multiple logical channels over a single ACL link. In the Sony XAV-AX8500, the L2CAP channel isolation is flawed. This means that an attacker can create a malicious L2CAP channel that is not properly separated from legitimate channels used for authentication.

Technical Breakdown:

1. Bluetooth Pairing Process: Normally, a device must go through a pairing process (e.g., Secure Simple Pairing) to establish a trusted connection. This involves authentication and encryption key exchange.
2. ACL-U Link Exploitation: The attacker, within Bluetooth range, initiates an ACL-U connection to the Sony device. Due to the lack of L2CAP isolation, the attacker can send crafted L2CAP packets that are interpreted by the device as coming from an already-authenticated channel.
3. Authentication Bypass: The device's firmware fails to verify the origin of the L2CAP channel. It processes the attacker's commands as if they were from a trusted source, effectively bypassing the authentication step.
4. Impact: The attacker can then send commands to control the head unit, such as changing audio sources, accessing stored contacts, or potentially injecting malicious data into the vehicle's infotainment system.

Vulnerable vs. Patched Code

No patched code is available. The vulnerable code is within the proprietary firmware of the Sony XAV-AX8500. The following is a conceptual representation of the flaw.

Vulnerable Code (Conceptual C-like pseudocode):

// Vulnerable L2CAP channel handler
void handle_l2cap_channel(l2cap_packet *pkt) {
    // No check on channel origin or authentication state
    if (pkt->channel_id == AUTH_CHANNEL_ID) {
        process_authenticated_command(pkt->data);
    } else {
        process_unauthenticated_command(pkt->data);
    }
}

Patched Code (Conceptual C-like pseudocode):

// Patched L2CAP channel handler
void handle_l2cap_channel(l2cap_packet *pkt, connection_state *conn) {
    // Verify the channel is properly isolated and authenticated
    if (pkt->channel_id == AUTH_CHANNEL_ID && conn->is_authenticated) {
        process_authenticated_command(pkt->data);
    } else {
        // Reject or process with limited privileges
        reject_packet(pkt);
    }
}

The patch would require adding a check to ensure that the L2CAP channel is associated with an authenticated connection before processing privileged commands.

Exploitation & Lab Setup

Red Team

Lab Setup:

• Hardware: Sony XAV-AX8500 head unit (powered and in pairing/discoverable mode), a laptop with Bluetooth adapter (e.g., CSR 4.0 or better), and a Faraday cage or shielded room to minimize interference.
• Software: Kali Linux, BlueZ stack, Scapy (for packet crafting), and custom Python scripts.

Exploitation Steps:

1. Scan for Devices:

   sudo hcitool scan
   

   Identify the MAC address of the Sony XAV-AX8500.

2. Initiate ACL-U Connection:

   # Using PyBluez or similar
   import bluetooth
   target_mac = "XX:XX:XX:XX:XX:XX"
   sock = bluetooth.BluetoothSocket(bluetooth.L2CAP)
   sock.connect((target_mac, 0x1001))  # PSM for L2CAP
   

3. Craft Malicious L2CAP Packet:

   # Craft a packet that mimics an authenticated channel
   from scapy.all import *
   # Build L2CAP header with a spoofed channel ID
   pkt = L2CAP_Hdr(len=10, cid=0x0040) / Raw(b'\x00'*10)  # 0x0040 is a reserved channel ID
   sock.send(bytes(pkt))
   

4. Send Commands: After bypassing authentication, send commands to control the head unit (e.g., change source, play media).

Note: This is a conceptual exploit. Actual exploitation requires reverse engineering the proprietary protocol.

Blue Team

Detection Steps:

1. Monitor Bluetooth Connections: Use hcidump or btmon to capture Bluetooth HCI events. Look for unexpected ACL-U connections from unknown devices.

   sudo btmon -w bt_trace.log
   

2. Analyze L2CAP Channels: Filter for unusual L2CAP channel IDs or PSM values that are not standard (e.g., PSM 0x1001 is not standard for SDP or RFCOMM).

3. Network Segmentation: Ensure the head unit is on a separate VLAN from critical vehicle systems (e.g., CAN bus). Use a firewall to restrict outbound connections from the head unit.

4. Firmware Integrity: Regularly check for firmware updates from Sony. If a patch becomes available, apply it immediately.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Unusual Bluetooth ACL-U connections from unknown MAC addresses.
  • L2CAP packets with non-standard channel IDs (e.g., 0x0040, 0x1001).
  • Repeated failed pairing attempts followed by successful command execution.

• Host IOCs (on the head unit):

  • Unexpected changes in audio source or volume.
  • Unauthorized access to contacts or phone logs.
  • System logs showing authentication bypass events (if logging is enabled).

• File IOCs:

  • No specific file-based IOCs due to the nature of the vulnerability.

Detection Engineering

YARA Rule

rule CVE_2025_5476_Bluetooth_ACL_U_Exploit {
    meta:
        description = "Detects potential exploitation of CVE-2025-5476 via crafted L2CAP packets"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-5476"
    strings:
        $l2cap_header = { 10 00 40 00 }  // L2CAP length=10, CID=0x0040
        $acl_u_pattern = { 02 00 20 00 }  // ACL-U packet type indicator
    condition:
        $l2cap_header at 0 and $acl_u_pattern
}

Sigma Rule

title: CVE-2025-5476 Bluetooth Authentication Bypass
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects anomalous Bluetooth L2CAP connections indicative of CVE-2025-5476 exploitation
author: Valters IT
date: 2025-04-10
logsource:
    product: linux
    service: bluetooth
detection:
    selection:
        EventID: 1001  # Example: Bluetooth connection event
        PSM: 0x1001    # Non-standard PSM
        ChannelID: 0x0040  # Suspicious channel ID
    condition: selection
falsepositives:
    - Legitimate Bluetooth debugging tools
level: high

Suricata Rule

alert bluetooth any any -> any any (msg:"CVE-2025-5476 Bluetooth L2CAP Authentication Bypass"; bluetooth.proto:l2cap; content:"|10 00 40 00|"; depth:4; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Bluetooth: If Bluetooth is not required, disable it in the head unit's settings.
2. Update Firmware: Monitor Sony's support page for firmware updates. Apply immediately when available.
3. Physical Security: Park in secure, private locations to reduce the risk of close-proximity attacks.

Long-Term Mitigations:

1. Network Segmentation: Isolate the head unit from critical vehicle systems (e.g., CAN bus) using a gateway or firewall.
2. Bluetooth Security: Use Bluetooth 5.0+ devices that support LE Secure Connections with stronger encryption.
3. Vendor Coordination: Contact Sony support to request a patch and report the vulnerability if not already done.

Valters IT Signature This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.