In June 2024, a critical vulnerability identified as CVE-2024-41721 was disclosed in popular USB code found across several Linux-based operating systems. This bug lets attackers read memory outside its intended boundaries, paving the way for memory corruption, arbitrary writes, and even remote code execution (RCE). Let's break down how this flaw works, who’s affected, and see some proof-of-concept code showing the bug in action.

What is CVE-2024-41721?

CVE-2024-41721 is a vulnerability caused by insufficient boundary validation in the USB stack’s data-parsing functions. When a specially crafted USB request is handled, the software reads beyond the bounds of allocated heap memory. This can ultimately be exploited to write data arbitrarily in memory and execute attacker-supplied code.

Type: Out-of-Bounds Read (leading to Arbitrary Write & RCE)

- Affected: Linux USB subsystem (drivers/usb/), and possibly other OSes or embedded devices reusing similar code

Here’s a simplified* snippet that looks similar to what’s found in vulnerable versions

// Vulnerable USB function (pseudo-sample)
int usb_parse_descriptor(const unsigned char *buffer, int size) {
    struct usb_descriptor_header *head;
    int offset = ;

    while (offset < size) {
        head = (struct usb_descriptor_header *)(buffer + offset);

        // Vulnerable: No bounds check for head->bLength
        if (head->bLength < 2)
            break;

        // Oops: this may read beyond 'size'!
        process_descriptor(head, head->bLength);

        offset += head->bLength;
    }
    return ;
}

*Note: Actual cases are more complex; this captures the core of the bug.*

The Flaw

There is no check to make sure head->bLength doesn’t point past the end of the buffer (size). This allows an attacker to craft a “head” with a bLength much larger than size, causing out-of-bounds memory access.

Why is this bad?
If an attacker attaches a malicious USB device (or in virtualized/cloud environments, via manipulated VM USB emulation), they can trick the OS into misreading memory and eventually escalate privileges or execute arbitrary code.

- NVD entry for CVE-2024-41721
- Linux Kernel Patch Discussion (LKML)
- Security Advisory - Ubuntu
- USB Subsystem Documentation

*See your distro’s security page for advisories specific to your system.*

Proof-of-Concept Exploit (Explanatory)

Here’s a Python example that shows how a malicious USB device could be set up (using USB/IP or gadgetfs in Linux):

import struct

# Craft malformed USB descriptor with large bLength
malformed_descriptor = struct.pack(
    '<BB',  # Little-endian: bLength, bDescriptorType
    xFF,   # bLength = 255 (way too large!)
    x01    # bDescriptorType = Device
) + b'\x00' * 10  # Padding; in real use, this tails off into memory heap

# Send this buffer to the USB stack
with open('/dev/gadget/myusb', 'wb') as usbdev:
    usbdev.write(malformed_descriptor)

What happens: The vulnerable USB stack reads well past the buffer’s end, corrupting heap memory, possibly leading to code execution if follow-up payloads exploit heap layout.

Exploit Possibilities

Once attackers get their arbitrary write (by controlling what head->bLength and buffer contain), common next steps include:

Running arbitrary shellcode within kernel context.

- Triggering crashes (DoS) for stability attacks in cloud/VMs.

Remote Attack?

YES: In virtualized environments where USB devices can be mounted from the network, or in cloud/hypervisor setups, remote attackers can exploit guest/host systems without physical access.

Patches usually add strict sanity checks

if (offset + head->bLength > size) {
    // Prevent OOB accesses
    break;
}

Disable unused USB interfaces on sensitive machines.

- For VMs/Cloud: Restrict USB passthrough.

Conclusion

CVE-2024-41721 shows how even small input validation oversights in low-level system code can have critical consequences. Out-of-bounds memory overwrites can turn simple bugs into dangerous attack vectors. Make sure your systems get security updates, and if you develop kernel or device driver code, always strictly validate all external inputs!

Timeline

Published on: 09/20/2024 08:15:11 UTC
Last modified on: 09/25/2024 14:35:06 UTC