CVE-2026-74544 net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds
An unprivileged user can trigger undefined behavior and potential denial of service in the Linux kernel by providing an invalid offshift value to the cls_u32 classifier.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-74544?
CVE-2026-74544 is a high-severity vulnerability in Linux, published on 15 Aug 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: cls_u32: validate offshift to prevent shift-out-of-bounds
u32_change() copies the user-provided tc_u32_sel.offshift (unsigned char, 0-255) into the kernel knode object without bounds validation. When a packet later hits u32_classify() with TC_U32_VAROFFSET set, it evaluates `ntohs(offmask & *data) >> offshift` where the left operand is a 16-bit value promoted to a 32-bit int. Any offshift >= 32 is undefined behavior per C11 6.5.7p3, triggerable by an unprivileged user via user/network namespaces.
UBSAN: shift-out-of-bounds in net/sched/cls_u32.c:236:43 shift exponent 32 is too large for 32-bit type int
Fix this by rejecting offshift >= 16 during filter creation in u32_change().
Which versions are affected by CVE-2026-74544?
How do I fix CVE-2026-74544?
Apply the vendor's patch.
Is CVE-2026-74544 being exploited?
There is no known exploitation of CVE-2026-74544 in the wild and no public exploit code.
Sources
Also known as AZL-96395, BELL-CVE-2026-74544, DEBIAN-CVE-2026-74544, GHSA-p37c-4w2w-mm9f, UBUNTU-CVE-2026-74544.