CVE-2021-42739

What it actually does

Buffer overflow in the firewire subsystem's AVC handling code allows out-of-bounds writes in kernel memory when processing malformed PMT messages.

Why it matters to the business

On systems with FireWire hardware and the affected kernel, attackers with local administrative access could corrupt kernel memory, potentially leading to system crashes or unauthorized code execution with full privileges; widespread Linux deployments increase exposure across servers and embedded devices.

How an attack unfolds

An attacker with local root privileges crafts a malicious AVC CA PMT message and sends it through the FireWire interface to trigger the bounds checking failure, resulting in a buffer overflow that can be leveraged for kernel-level exploitation.

How to fix it

Update the Linux kernel to a version containing commit 35d2969ea3c7d32aee78066b1f3cf61a0d935a4e or later, which addresses the bounds checking issue in firedtv-avc.c and firedtv-ci.c.

How to confirm the fix worked

Verify the kernel version is updated beyond 5.14.13 and check for the presence of the fix commit in the source tree; test FireWire functionality with valid inputs.

Typical attack path

1. Gain local access to the target system
2. Obtain high privileges (root)
3. Interact with FireWire device to send crafted AVC messages
4. Trigger buffer overflow for memory corruption
5. Escalate to full kernel control or denial of service

How to detect it

Examine kernel logs for signs of buffer overflows or crashes related to firedtv modules; look for unusual FireWire activity in system logs.

What your security team should do

• Prioritize kernel updates on systems using FireWire media devices
• Review and restrict local access where possible
• Monitor for kernel updates from distributions
• Disable FireWire if not needed