CVE-2018-9475 - Bluetooth Headset Vulnerability in Android (Out of Bounds Stack Write Exploit Explained)

A critical vulnerability—CVE-2018-9475—was discovered in the Android Bluetooth stack that can let an attacker escalate their privileges remotely over Bluetooth. This happens due to a missing bounds check in the function *HeadsetInterface::ClccResponse* inside the btif_hf.cc file. Let’s walk through what this bug is, how it’s exploited, and what you can do about it.

What is CVE-2018-9475?

This vulnerability exists in Android's Bluetooth Hands-Free Profile (HFP) interface, specifically in code handling SIP (Single Inbound Path) calls. When certain Bluetooth commands are processed, the function ClccResponse does not check if the index being referenced is within valid bounds. This can result in writing data outside the buffer allocated on the stack, a classic out-of-bounds (OOB) write.

Why is that dangerous?

Attackers within Bluetooth range can exploit this automatically, without your interaction. By sending specially crafted packets, they cause your device to overwrite part of its stack memory, which can lead to code execution or device crash. All that’s required is that you have SIP calls enabled via Bluetooth—a situation not uncommon in modern cars, headsets, or conference devices.

No special app installation is needed, and users are often unaware an attack is happening.

To help you visualize the issue, here’s a simplified snippet from btif_hf.cc

#define BTIF_HF_MAX_CALLS 7

void HeadsetInterface::ClccResponse(
    int index, // Call list index from remote
    ... // other parameters
) {
    // BAD: No bounds check!
    clcc_entries[index].state = CALL_STATE_ACTIVE;
    // ... more assignments
}

The problem is that index is taken directly, maybe from a Bluetooth packet. If an attacker sends index=8 (but the array only allows up to 6 or 7), you get a stack write outside the allocated memory. That’s an easy place for attackers to push malicious data.

Attacker's Requirements

- Be within Bluetooth range (33ft/10m)

Exploit Steps

1. Establish Connection: Attacker pairs with the victim or tricks the device into connecting via Bluetooth Hands-Free Profile.
2. Send Malicious CLCC Packet: Attacker sends a *CLCC* response with an out-of-bounds index (e.g., 99).
3. Trigger OOB Write: The device processes the packet, passing the bad index into clcc_entries[], corrupting stack memory.
4. Gain Control/Crash: This can leak sensitive data, crash the Bluetooth daemon, or—if combined with other bugs—allow code execution (remote privilege escalation).

Here’s a pseudocode example for the Bluetooth attacker

# WARNING: Don't run this.
# Intended for educational purposes, showing attack logic.
attacker_packet = create_clcc_response(index=99, ...other params...)
bluetooth_socket.send(attacker_packet)

References & Technical Details

- Android Security Bulletin (Look for "CVE-2018-9475")
- AOSP Commit Fix (Relevant code patch)
- NIST National Vulnerability Database entry

The official patch *bounds checks* all received indices

if (index <  || index >= BTIF_HF_MAX_CALLS) {
    log("Invalid index in CLCC: %d", index);
    return;
}
clcc_entries[index].state = CALL_STATE_ACTIVE;

Bottom Line

CVE-2018-9475 is a stark reminder that small mistakes (like forgetting a bounds check) can lead to heavy remote vulnerabilities. If your phone or device is affected, patch as soon as possible and use Bluetooth cautiously in unknown environments.

Stay safe, stay updated!

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Timeline

Published on: 11/20/2024 18:15:19 UTC
Last modified on: 12/18/2024 15:13:24 UTC