CVE-2025-4321

Executive Summary

CVE-2025-4321 describes a Denial of Service (DoS) vulnerability in Bluetooth devices using the Silicon Labs RS9116-WiseConnect SDK. The flaw is triggered when the device receives malformed L2CAP (Logical Link Control and Adaptation Protocol) packets. Successful exploitation causes the Bluetooth stack to enter an unrecoverable state, requiring a hard reset (power cycle) to restore normal operation. No public proof-of-concept (PoC) code, exploit, or patch is currently available. The vulnerability is classified as a classic input validation failure (CWE-20) leading to a denial of service.

This vulnerability is particularly concerning for IoT and embedded systems that rely on the RS9116 module for Bluetooth connectivity, as a remote attacker within Bluetooth range could disrupt device operation without authentication.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It affects a hardware SDK (RS9116-WiseConnect) used in embedded firmware development. The risk is in the supply chain of IoT devices that integrate this module.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official CVSS vector assigned by NVD)
• Attack Vector (AV): Adjacent Network (Bluetooth range)
• Attack Complexity (AC): Low (malformed L2CAP packets are trivial to craft)
• Privileges Required (PR): None
• User Interaction (UI): None
• Scope (S): Unchanged
• Confidentiality (C): None
• Integrity (I): None
• Availability (A): High (complete denial of service)
• Threat Level: High (Remote, unauthenticated DoS with no recovery other than hard reset)

Deep Dive & Mechanics

The RS9116-WiseConnect SDK implements the Bluetooth L2CAP layer, which handles segmentation, reassembly, and multiplexing of higher-layer protocols (e.g., RFCOMM, SDP). The vulnerability lies in the L2CAP packet parsing logic.

When the device receives an L2CAP packet that violates the protocol specification (e.g., invalid length, malformed header, or unexpected channel ID), the SDK's input validation fails to handle the error gracefully. Instead of discarding the packet and continuing normal operation, the firmware enters an undefined state—likely a buffer overflow, null pointer dereference, or infinite loop—that hangs the Bluetooth stack. Because the Bluetooth controller is tightly coupled with the main CPU in many embedded designs, the entire device becomes unresponsive.

The only recovery mechanism is a hardware reset (power cycle), as the firmware does not implement a watchdog timer or error recovery routine for this specific failure mode.

Vulnerable vs. Patched Code

Since no patch is available, a hypothetical fix would involve adding robust input validation and error handling in the L2CAP packet processing function.

Vulnerable Code (Conceptual):

void process_l2cap_packet(uint8_t *packet, uint16_t length) {
    l2cap_header_t *header = (l2cap_header_t *)packet;
    // No length check before accessing header fields
    uint16_t channel_id = header->channel_id;
    uint16_t payload_length = header->length;
    // Directly process payload without validation
    process_payload(channel_id, packet + sizeof(l2cap_header_t), payload_length);
}

Patched Code (Conceptual):

void process_l2cap_packet(uint8_t *packet, uint16_t length) {
    if (length < sizeof(l2cap_header_t)) {
        // Invalid packet, discard
        return;
    }
    l2cap_header_t *header = (l2cap_header_t *)packet;
    uint16_t channel_id = header->channel_id;
    uint16_t payload_length = header->length;
    // Validate payload length against actual packet length
    if (payload_length > (length - sizeof(l2cap_header_t))) {
        // Malformed packet, discard
        return;
    }
    // Validate channel ID
    if (!is_valid_channel(channel_id)) {
        return;
    }
    process_payload(channel_id, packet + sizeof(l2cap_header_t), payload_length);
}

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• A Bluetooth adapter capable of raw L2CAP packet injection (e.g., CSR8510 with BlueZ or internal Bluetooth on Linux).
• Python with pybluez or scapy for crafting packets.
• Target device using RS9116-WiseConnect SDK.

Exploitation Steps:

1. Scan for Bluetooth devices: hcitool scan
2. Identify the target's BD_ADDR.
3. Craft a malformed L2CAP packet (e.g., invalid length field or unexpected channel ID).
4. Send the packet to the target using L2CAP connectionless or connection-oriented channel.

Example Python Script (Conceptual):

from scapy.all import *
import sys

target_bdaddr = sys.argv[1]

# Craft a malformed L2CAP packet with invalid length
l2cap_packet = L2CAP_Hdr(len=0xFFFF, cid=0x0001) / Raw(b'A'*100)

# Send via L2CAP (requires raw socket)
sendp(l2cap_packet, iface="hci0", dst=target_bdaddr)

Note: No public exploit code exists. The above is a conceptual example.

Blue Team (Defensive)

Detection:

• Monitor Bluetooth traffic for anomalous L2CAP packets (e.g., packets with length fields exceeding MTU, invalid channel IDs, or repeated malformed packets).
• Implement network-level monitoring using Bluetooth sniffers (e.g., Ubertooth, Adafruit Bluefruit LE Sniffer).

Mitigation:

• Apply firmware update from Silicon Labs when available.
• Implement a watchdog timer in firmware to automatically reset the Bluetooth stack if it becomes unresponsive.
• Add input validation for all L2CAP packets as shown in the patched code section.
• Restrict Bluetooth discovery and pairing to trusted devices only.

Indicators of Compromise (IOCs)

• Network IOCs:
  • L2CAP packets with length field set to 0xFFFF or other invalid values.
  • L2CAP packets with channel ID 0x0000 or reserved values.
  • Repeated L2CAP connection requests from the same source BD_ADDR.
• System IOCs:
  • Device becomes unresponsive to Bluetooth commands.
  • No response to L2CAP ping or echo requests.
  • Device requires hard reset to recover.
• Log IOCs:
  • Firmware logs (if accessible) showing "L2CAP error" or "invalid packet" before crash.

Detection Engineering

YARA Rule

rule CVE_2025_4321_L2CAP_Malformed {
    meta:
        description = "Detects malformed L2CAP packets targeting RS9116 devices"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-4321"
    strings:
        $l2cap_header = { ?? ?? ?? ?? } // L2CAP header (4 bytes)
        $invalid_length = { FF FF } // Length 0xFFFF
        $invalid_cid = { 00 00 } // Channel ID 0x0000
    condition:
        $l2cap_header and ($invalid_length or $invalid_cid)
}

Sigma Rule

title: CVE-2025-4321 - Malformed L2CAP Packet Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects malformed L2CAP packets in Bluetooth traffic that may trigger DoS on RS9116 devices
author: Valters IT
date: 2025-04-10
logsource:
    category: bluetooth
    product: linux
detection:
    selection:
        l2cap_length: 0xFFFF
        l2cap_cid: 0x0000
    condition: selection
falsepositives:
    - Legitimate Bluetooth testing tools
level: high

Suricata Rule

alert bluetooth any any -> any any (msg:"CVE-2025-4321 - Malformed L2CAP Packet Detected"; content:"|FF FF|"; offset:2; depth:2; content:"|00 00|"; offset:4; depth:2; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: If possible, disable Bluetooth on devices using RS9116-WiseConnect SDK until a patch is available.
2. Network segmentation: Place Bluetooth-enabled devices on a separate VLAN or network segment to limit lateral movement.
3. Monitor Bluetooth traffic: Use Bluetooth sniffers to detect anomalous L2CAP packets.

Long-Term Fixes:

1. Apply firmware patch: Contact Silicon Labs for an updated SDK that includes proper L2CAP input validation.
2. Implement watchdog timer: Add a hardware or software watchdog to reset the Bluetooth stack if it hangs.
3. Code review: Review all L2CAP packet handling code for similar input validation issues.

Workarounds:

• If firmware cannot be updated, consider using a different Bluetooth module that is not affected.
• Implement a host-side filter that drops malformed L2CAP packets before they reach the Bluetooth controller (if possible).

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Analyst