In late 2022, security researchers disclosed CVE-2022-42896, a set of dangerous use-after-free vulnerabilities in the Linux kernel's Bluetooth subsystem, specifically in the l2cap_core.c code. Attackers in Bluetooth range could exploit these bugs to leak kernel memory—or even execute code—on nearby Linux devices with Bluetooth turned on.

This post aims to explain what happened, how the bug could be exploited, and how you can protect your systems. We’ll keep the language straightforward, provide code snippets, and share the best sources for further reading.

What’s the Problem?

The flaws were found in the functions l2cap_connect and l2cap_le_connect_req in Linux's net/bluetooth/l2cap_core.c. These handle incoming Bluetooth connections.

The issue is a classic use-after-free: an object in memory gets freed, but the program keeps using its pointer. If an attacker times their actions right, it’s possible to read from (leak memory) or write to (execute code) parts of memory they shouldn’t be able to touch.

Who’s Affected?

- All Linux devices with Bluetooth enabled before the fix was applied—and that includes laptops, desktops, IoT devices, and many embedded systems.

The Vulnerable Code

Let’s take a look at a simplified version of the code involved. The following is not vulnerable, but highlights how the use-after-free can happen:

/* BAD: Use-After-Free (simplified) */
void l2cap_connect(...) {
    struct l2cap_chan *chan = l2cap_chan_create();
    ...
    if (some_error) {
        l2cap_chan_free(chan);  // Frees chan
        // Uh-oh! We keep using 'chan' after freeing
        chan->state = L2CAP_FAILED;  // Use-after-free!
    }
}

After l2cap_chan_free(chan), the pointer chan is dangling; using it is unsafe. If a remote attacker carefully crafts a Bluetooth connection, they can trigger this sequence and possibly control what memory is re-allocated in its place.

The bug is triggered, freeing a struct used to track the connection.

3. The system continues to use the freed memory. If the attacker is lucky—and skillful—they can influence what gets allocated at that memory spot.

Kernel memory leaks: Attackers trick the system into sending back sensitive memory.

5. Code execution (harder): By manipulating memory just right, an attacker can get the system to run code of their choosing.

Note: Exploits only work if Bluetooth is enabled and the device is within wireless range of the attacker.

While I won’t include weaponized PoC code, a sketch of how an attacker might start

# Python pseudocode only; real exploits are much more complex!

from bluetooth import BluetoothSocket, L2CAP

sock = BluetoothSocket(L2CAP)
sock.connect(("AA:BB:CC:DD:EE:FF", x1001)) # Victim's MAC, L2CAP PSM

# Send crafted packets that trigger the error handling paths.
sock.send(malicious_payload)

Fixing It: Patch Details

The bug was fixed in commit 711f8c3fb3db61897080468586b970c87c61d9e4 (late October 2022).

Here’s how the patch (simplified) fixes the problem

if (some_error) {
    l2cap_chan_free(chan);  // Free chan
    chan = NULL;            // NULL the pointer
    return;                 // Exit before using 'chan'
}


Now, the code never uses the freed pointer.

What Should You Do?

1. Upgrade your Linux kernel: Make sure you’re running a version *after* the 711f8c3fb3db61897080468586b970c87c61d9e4 patch. Most distributions fixed this in their late 2022 or early 2023 kernel releases.

Turn off Bluetooth when not needed: Especially on servers or IoT devices.

3. Monitor for suspicious Bluetooth activity: Unusual connection attempts or unexpected Bluetooth traffic can be a red flag.

- Linux commit fixing CVE-2022-42896 (Github)
- CVE details on NVD
- Bluetomcat project analysis (Mailing list post)
- Kernel issue discussion

Final Thoughts

CVE-2022-42896 is a powerful reminder that kernel bugs—especially those in wireless stacks—can have far-reaching consequences. Exploits can happen quietly, as long as your system is somewhere an attacker can reach via Bluetooth.

If your Linux device runs Bluetooth, make sure it’s up to date. Patch, reboot, and stay safe.

Timeline

Published on: 11/23/2022 15:15:00 UTC
Last modified on: 03/01/2023 20:15:00 UTC