CVE-2026-74544
Received Received - Intake

Linux Kernel cls_u32 Shift-Out-of-Bounds

Publication date: 2026-08-15

Last updated on: 2026-08-15

Assigner: kernel.org

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().

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Meta Information

Published
2026-08-15
Last Modified
2026-08-15
Generated
2026-08-15
AI Q&A
2026-08-15
EPSS Evaluated
N/A
NVD
CVE-2026-74544
EUVD
EUVD-2026-59737

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
linux linux_kernel *

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Exploitability

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CWE ID Description
CWE-UNKNOWN

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Executive Summary

This is a Linux kernel vulnerability where an unprivileged user can trigger a shift-out-of-bounds error in the net/sched cls_u32 classifier. The issue occurs when a user-provided offshift value (0-255) is not validated during filter creation. Later, when processing packets, the kernel performs a bitwise shift operation with an invalid shift value, leading to undefined behavior.

Detection Guidance

This vulnerability involves a shift-out-of-bounds issue in the Linux kernel's cls_u32 classifier. Detection requires checking kernel logs for UBSAN warnings related to net/sched/cls_u32.c. Monitor for messages like 'shift-out-of-bounds in net/sched/cls_u32.c' using dmesg or journalctl.

Impact Analysis

An attacker with access to user or network namespaces could exploit this to cause undefined behavior in the kernel, potentially leading to crashes, data corruption, or privilege escalation. Systems using vulnerable Linux kernel versions may experience instability or security breaches.

Mitigation Strategies

Apply the kernel patch that restricts offshift values to less than 16 during filter creation in u32_change(). Update to a fixed kernel version where this issue is resolved. If immediate patching is not possible, disable unprivileged user namespaces or restrict access to network namespaces.

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