CVE-2026-90076 net/sched: fq: add overflow bounds to quantum and initial quantum
A local attacker with CAP_NET_ADMIN can cause a kernel soft lockup by configuring a network device with an excessively large MTU, leading to an integer overflow in the fq scheduler.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-90076?
CVE-2026-90076 is a vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
net/sched: fq: add overflow bounds to quantum and initial quantum
fq_init() computes quantum = 2 * psched_mtu() and initial_quantum = 10 * psched_mtu() with no overflow check. A device with a huge MTU (e.g. dummy with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return 0x80000000; the 2 * and 10 * multiplications wrap to 0 in 32-bit arithmetic, so q->quantum == 0. Then in fq_dequeue() the credit-refill loop adds 0 to f->credit (which stays <= 0) and goto begin loops forever under the qdisc lock, creating a soft lockup.
Clamp psched_mtu() to [1, 1 << 20] before multiplying so the product cannot wrap, then cap the result at 1 << 20, matching the bound already enforced on TCA_FQ_QUANTUM in fq_change().
Conditions to recreate the bug: a device whose MTU (plus hard_header_len) is large enough that 2 * psched_mtu() wraps (e.g. a dummy device with max_mtu == 0 accepting MTU 2147483634). Requires CAP_NET_ADMIN in a user namespace.
Which versions are affected by CVE-2026-90076?
How do I fix CVE-2026-90076?
Apply the vendor's patch.
Is CVE-2026-90076 being exploited?
There is no known exploitation of CVE-2026-90076 in the wild and no public exploit code.
Sources
Also known as AZL-102791, BELL-CVE-2026-90076, DEBIAN-CVE-2026-90076, GHSA-6j38-2pg6-m9mv, UBUNTU-CVE-2026-90076.