flawopen.com/Teardowns/cve-2020-0022-android-bluetooth-hci-heap-overflow

● CVE-2020-0022 · CVSS 8.8 · Alta
Investigación · FlawOpen

CVE-2020-0022: Android Fluoride Bluetooth HCI Heap Buffer Overflow Teardown

Análisis técnico del código fuente y mitigaciones de ingeniería para vulnerabilidade: How an unchecked fragmentation boundary mismatch in system/bt/stack/l2cap enabled zero-click remote code execution over Bluetooth.

💡 Explicación en Lenguaje Sencillo (ELI5)

Analogía práctica: 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 Clave y Términos

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.

Análisis de Causa Raíz

La causa raíz se debe a parámetros de límite no validados en sistemas de código abierto, lo que permite la desincronización de estado y la elusión de controles de seguridad.

Flujo de Ataque Paso a Paso

Step 1

Fase de ataque: Attacker in Bluetooth Range

Mecanismo técnico de explotación: The attacker broadcasts crafted ACL packets toward a nearby Android device.

Step 2

Fase de ataque: Sending Incomplete First Fragment

Mecanismo técnico de explotación: The attacker sends an initial L2CAP packet claiming a small total packet length.

Step 3

Fase de ataque: Sending Oversized Continuation Packet

Mecanismo técnico de explotación: The attacker sends a second fragment with an unexpected payload size exceeding the allocation.

Step 4

Corrupción de memoria : Heap Corruption in com.android.bluetooth

Mecanismo técnico de explotación: The reassembly loop overwrites adjacent heap chunks, executing code inside the Bluetooth daemon.

Código Fuente: Vulnerable vs. Seguro

IMPLEMENTACIÓN VULNERABLE
// 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 Y ROBUSTO
// 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 de Seguridad para Ingeniería

Fuentes