Recently, security researchers spotted a vulnerability in the Linux kernel Bluetooth subsystem that exposed systems to possible memory errors and crashes. The issue, now tracked as CVE-2024-56590, involved the improper handling of Bluetooth ACL packets. In this post, we’ll break down what went wrong, see the kernel bug fix, look at a simple exploit example, and help you understand the risk in clear, plain language.

What is CVE-2024-56590 All About?

When your computer or phone communicates wirelessly over Bluetooth, it relies on something called the Host Controller Interface (HCI). This module processes packets (little bundles of data) moving to and from Bluetooth devices.

The bug hid in the hci_core part of the Linux kernel. Specifically, the function handling “ACL data packets” (these carry actual Bluetooth payloads) wasn’t checking if the data was long enough before treating it as a complete header.

- Translation: The kernel code could try to read memory *outside* of a valid data area. This might lead to a system crash (kernel panic) or, in worst cases, let an attacker manipulate system memory—sometimes even running code.

Let’s look at how this happened. Here’s a simplified view (C language) of the bad code

// Vulnerable code (before the fix)
static void hci_acldata_packet(struct hci_dev *hdev, struct sk_buff *skb)
{
    struct hci_acl_hdr *hdr = (void *) skb->data;
    __u16 handle = __le16_to_cpu(hdr->handle);
    __u16 dlen = __le16_to_cpu(hdr->dlen);
    //... more processing ...
}

What’s wrong?
There’s *no check* that skb->len (data length) is at least as long as struct hci_acl_hdr. If an attacker can send a short (tiny) packet, hdr points into nowhere.

The fixed version looks like this

// Fixed code (after the bug fix)
static void hci_acldata_packet(struct hci_dev *hdev, struct sk_buff *skb)
{
    if (skb->len < HCI_ACL_HDR_SIZE) {
        /* drop invalid packet */
        return;
    }
    struct hci_acl_hdr *hdr = (void *) skb->data;
    __u16 handle = __le16_to_cpu(hdr->handle);
    __u16 dlen = __le16_to_cpu(hdr->dlen);
    //... more processing ...
}

Translation:
Now the code checks if the packet is big enough before reading header fields!

How Could Attackers Exploit It?

If your device listens for Bluetooth packets (for example, a laptop or phone with Bluetooth enabled), an attacker in range could:

The kernel code tries to read memory from that packet, but the data isn’t there.

3. This could crash your device (DoS), or worse—corrupt memory and *possibly* allow further exploitation.

Note:
Because the bug is about a “read past buffer,” it's mostly a denial-of-service risk, but in some situations with other bugs, people could chain it for deeper attacks.

Demo Exploit (Conceptual - For Education Only!)

Most users *can’t* test this directly without custom Bluetooth hardware or software that lets them craft malicious ACL packets. But for hackers using tools like BlueZ's btproxy or Scapy, you could send a raw, short ACL packet:

# Example Python with Scapy (needs root and custom extensions)
from scapy.all import *
from scapy.layers.bluetooth import Bluetooth_HCI_Hdr

# Craft an empty ACL packet (less than HCI_ACL_HDR_SIZE)
pkt = Bluetooth_HCI_Hdr()/Raw(b"\x00\x00")  # too short!
sendp(pkt, iface="hci")

*This is just for demonstration; actual exploitability will vary.*

Has It Been Exploited in the Wild?

As of June 2024, there are no public reports of wild attacks. However, the issue is *dangerous enough* to update your kernel when possible—especially if you run public-facing or multi-user Linux systems with Bluetooth enabled.

Upstream Linux Patch:

Bluetooth: hci_core: Fix not checking skb length on hci_acldata_packet (git.kernel.org)
- CVE Description at NVD
- Linux Bluetooth subsystem docs

Conclusion

CVE-2024-56590 is a good reminder that even small mistakes in core code (like length checks) can lead to big vulnerabilities. Thanks to the community, this bug is patched—but keep your systems updated!

Timeline

Published on: 12/27/2024 15:15:18 UTC
Last modified on: 05/04/2025 09:59:11 UTC