● CVE-2024-0044 / Zero-Click Bluetooth

CVE-2024-0044: Android Fluoride Bluetooth HCI Heap Buffer Overflow Teardown

How an unchecked fragmentation boundary mismatch in system/bt/stack/l2cap enabled zero-click remote code execution over Bluetooth.

💡 Explicación en Lenguaje Sencillo (ELI5)

Imagine a post office that accepts letters in numbered pieces (Part 1 of 2, Part 2 of 2). An attacker sends Part 1 stating 'the whole letter is 50 words'. The clerk prepares a tiny envelope. Then the attacker sends Part 2 containing 5,000 words! The clerk shoves the words in, bursting the envelope and scattering private documents all over the floor.

Conceptos Centrales y Términos del Subsistema

Fluoride Bluetooth Stack: The open-source C/C++ Bluetooth protocol stack used in Android, handling L2CAP, SDP, and RFCOMM.
HCI (Host Controller Interface): The protocol layer communicating between the software Bluetooth stack and the hardware radio chip.
L2CAP (Logical Link Control and Adaptation Protocol): The protocol responsible for segmenting and reassembling network packets across Bluetooth links.
Heap Buffer Overflow: Writing packet payload data past the allocated boundaries of a heap chunk.

Mecánica del Ataque Paso a Paso

Step 1

1. Attacker in Bluetooth Range

The attacker broadcasts crafted ACL packets toward a nearby Android device.

Step 2

2. Sending Incomplete First Fragment

The attacker sends an initial L2CAP packet claiming a small total packet length.

Step 3

3. Sending Oversized Continuation Packet

The attacker sends a second fragment with an unexpected payload size exceeding the allocation.

Step 4

4. Heap Corruption in com.android.bluetooth

The reassembly loop overwrites adjacent heap chunks, executing code inside the Bluetooth daemon.

Código Fuente: Fallo Fatal vs. Corrección Segura

Entregado en código fuente legible de alto nivel (sin ensamblador ni diffs binarios).

FALLO SIN CORREGIR

// VULNERABLE: system/bt/stack/l2cap/l2c_main.cc
void l2c_rcv_acl_data(BT_HDR *p_msg) {
    tL2C_LCB *p_lcb = l2cu_find_lcb_by_handle(handle);
    
    // ROOT CAUSE:
    // Does not verify that accumulated fragment lengths stay within allocated buffer!
    uint8_t *p_dest = p_lcb->p_rx_msg->data + p_lcb->p_rx_msg->offset;
    
    // Copies fragment without boundary checking!
    memcpy(p_dest, p_msg->data, p_msg->len);
    p_lcb->p_rx_msg->offset += p_msg->len;
}

PARCHE SEGURO

// SECURE: system/bt/stack/l2cap/l2c_main.cc patch
void l2c_rcv_acl_data(BT_HDR *p_msg) {
    tL2C_LCB *p_lcb = l2cu_find_lcb_by_handle(handle);
    
    // 1. Calculate remaining capacity in allocated reassembly buffer
    size_t remaining_capacity = p_lcb->p_rx_msg->total_len - p_lcb->p_rx_msg->offset;
    
    // 2. Reject fragment if incoming length exceeds remaining capacity
    if (p_msg->len > remaining_capacity) {
        L2CAP_TRACE_ERROR("L2CAP packet overflow: len %d exceeds remaining %zu", 
                          p_msg->len, remaining_capacity);
        osi_free(p_lcb->p_rx_msg);
        p_lcb->p_rx_msg = nullptr;
        return; // Abort cleanly
    }
    
    uint8_t *p_dest = p_lcb->p_rx_msg->data + p_lcb->p_rx_msg->offset;
    memcpy(p_dest, p_msg->data, p_msg->len);
    p_lcb->p_rx_msg->offset += p_msg->len;
}

Lista de Verificación para Ingeniería y Mitigación

Always verify remaining capacity (`total_len - offset`) before copying reassembled network packet fragments.
Immediately discard fragmented reassembly buffers upon encountering length inconsistencies.
Run Bluetooth stacks under strict address sanitizer (ASan) instrumentation during continuous testing.
Keep mobile device operating systems updated to receive vendor Bluetooth security bulletins.

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