CVE-2023-3776

What it actually does

Use-after-free in net/sched cls_fw: tcf_change_indev failure in fw_set_parms leaves a dangling reference after refcount manipulation, allowing attacker-controlled reuse of freed kernel memory for arbitrary read/write and privilege escalation.

Why it matters to the business

Local users on affected Linux systems can obtain full root access, leading to complete host compromise, data theft, or persistence; kernel-wide impact affects all containers and processes on the machine.

How an attack unfolds

An attacker with a local account crafts traffic or netfilter rules that trigger tcf_change_indev failure, increments then decrements the reference counter to zero, forces the subsequent free, and re-allocates the memory with attacker-controlled data to hijack kernel control flow.

How to fix it

Upgrade the Linux kernel past commit 0323bce598eea038714f941ce2b22541c46d488f or apply the equivalent backported fixes listed for the affected distributions; rebuild and reboot with the patched kernel.

How to confirm the fix worked

Verify the running kernel contains commit 0323bce598eea038714f941ce2b22541c46d488f or later; run regression tests exercising cls_fw filter creation and deletion paths; confirm absence of the vulnerable tcf_bind_filter logic via source inspection or kernel symbol checks.

Typical attack path

1. Obtain local shell on target Linux host
2. Trigger cls_fw filter creation that causes tcf_change_indev to fail
3. Manipulate reference counter via repeated bind/unbind operations
4. Force use-after-free by returning from fw_set_parms
5. Reclaim freed object with malicious payload
6. Escalate to root via corrupted kernel structures

How to detect it

Review kernel logs for oops/panic messages referencing cls_fw or tcf_bind_filter around the time of suspected activity; inspect audit logs for unexpected root transitions or netfilter rule manipulations performed by non-admin users.

What your security team should do

• Patch all affected kernels to the fixed versions immediately
• Restrict local user accounts and enforce least-privilege access
• Monitor for unusual kernel oops or privilege-escalation attempts
• Consider enabling kernel lockdown or seccomp filters for untrusted workloads