CVE-2025-21704 - Linux Kernel usb:cdc-acm Notification Fragmentation Heap Corruption Explained

Recently, security experts have resolved a serious memory corruption vulnerability in the Linux kernel’s USB CDC ACM (Abstract Control Model) serial driver. Labeled CVE-2025-21704, this flaw can let attackers potentially corrupt kernel memory from a malicious USB device, especially when someone (or something like ModemManager) has opened /dev/ttyACM*. This is big news for Linux users, embedded device makers, and anyone trusting USB device connections.

Let’s break down what happened, why it matters, show a snippet of the fix, and discuss how this might be exploited in the real world.

The Problem: Buffer Size Checking in usb: cdc-acm

The USB CDC ACM driver is what lets Linux talk to modems and some dev boards over USB (“serial over USB”). It handles stuff like /dev/ttyACM.

When USB devices want to send a “notification” (to signal things like “I have new data” or “something changed!”), these notifications are passed to the kernel—a piece at a time if they’re too big (fragmented).

The bug happened when these notification fragments were not checked for their actual size before processing. If the first “fragment” was too short, the kernel might read part of the notification header from memory it shouldn’t, or—worse—let fragment index math wrap around, letting someone overwrite critical memory and potentially escalate privileges.

The Root Cause

From the kernel commit log (reference here):

> If the first fragment is shorter than struct usb_cdc_notification, we can't calculate expected_size. Log an error and discard the notification instead of reading lengths from memory outside the received data, which can lead to memory corruption when the expected_size decreases between fragments, causing expected_size - acm->nb_index to wrap.

Older code did not check for too-short fragments before accessing their length, letting attackers (with control over the USB device) submit maliciously-crafted fragments and force the kernel to miscompute buffer lengths and write unexpectedly.

When Is This Reached?

Good news: The affected function (acm_ctrl_irq()) only runs after something has opened the ACM device (/dev/ttyACM*). Usually, only modem-related software does this—but many Linux systems run ModemManager by default, and it’ll open almost anything that looks suspiciously modem-like.

So: Plug in a malicious USB device, and it can talk to the kernel.

Exploit Scenario

Attacker prerequisites:

Malicious firmware, or a device that fakes CDC ACM notifications.

- Target runs ModemManager or something that will open /dev/ttyACM*.

Attack steps

1. Device sends a first fragmented notification with a length smaller than struct usb_cdc_notification.

Kernel reads this incomplete notification and calculates expected sizes based on bogus memory.

3. The math wraps, and the driver writes/reads outside its buffer.
4. Heap corruption occurs—possibly overwritten kernel function pointers, panic, or, with cleverness, escalation.

*In short: plug in a tricky USB device, and if lucky and with precision, the kernel gets “owned.”*

Code Snippet: The Patch

Let’s see the fix—in simple C—from commit 125003fb60a2:

if (len < sizeof(struct usb_cdc_notification)) {
    dev_err(&acm->data->dev, "short ctrl irq notification received: %d bytes\n", len);
    return;
}

Old behavior:

Didn't check if len was big enough before accessing notification data.

Fixed behavior:
- Now, if the fragment is shorter than the notification struct, the kernel logs and discards the data—never misreads lengths or writes out of bounds.

Original References

- CVE-2025-21704 at cve.org (pending full details as of writing)
- Kernel patch commit: usb: cdc-acm: Check control transfer buffer size before access (commit 125003fb60a2)
- Linux source documenting cdc-acm.c
- ModemManager project

Mitigation

- Update your kernel: Ensure you’re running a version with the patch described above (check your distro’s security advisories).

Limit USB devices: Don’t plug in unknown devices. Disable ModemManager if not used.

- Kernel lockdown: Enable kernel lockdown and USB device restrictions on desktop and server systems.

Conclusion

CVE-2025-21704 is a classic modern Linux bug: a tiny forgotten length check in code for an obscure-yet-widespread device, only tractable if you trust USB not to be evil. As USB attacks grow, these “minor” checks are a big deal—and proof that even ancient kernel code can, with one commit, open a path for an attacker.

If your work involves USB hosts, embedded hardware, kiosks, or simply Linux machines where security matters: patch promptly, restrict unnecessary USB access, and beware the blinking cable.

Stay safe, and watch those kernel updates!

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*Author: [your_name]*
*Date: YYYY-MM-DD*

Timeline

Published on: 02/22/2025 10:15:11 UTC
Last modified on: 05/04/2025 07:19:21 UTC