Linux Kernel up to 6.19.11 fw_classify null pointer dereference

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Summaryinfo

A vulnerability was found in Linux Kernel up to 6.1.167/6.6.133/6.12.80/6.18.21/6.19.11 and classified as critical. The impacted element is the function fw_classify. The manipulation results in null pointer dereference. This vulnerability is cataloged as CVE-2026-31421. There is no exploit available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability was found in Linux Kernel up to 6.1.167/6.6.133/6.12.80/6.18.21/6.19.11. It has been classified as critical. Affected is the function fw_classify. The manipulation with an unknown input leads to a null pointer dereference vulnerability. CWE is classifying the issue as CWE-476. A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit. This is going to have an impact on availability. CVE summarizes:

In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_fw: fix NULL pointer dereference on shared blocks The old-method path in fw_classify() calls tcf_block_q() and dereferences q->handle. Shared blocks leave block->q NULL, causing a NULL deref when an empty cls_fw filter is attached to a shared block and a packet with a nonzero major skb mark is classified. Reject the configuration in fw_change() when the old method (no TCA_OPTIONS) is used on a shared block, since fw_classify()'s old-method path needs block->q which is NULL for shared blocks. The fixed null-ptr-deref calling stack: KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f] RIP: 0010:fw_classify (net/sched/cls_fw.c:81) Call Trace: tcf_classify (./include/net/tc_wrapper.h:197 net/sched/cls_api.c:1764 net/sched/cls_api.c:1860) tc_run (net/core/dev.c:4401) __dev_queue_xmit (net/core/dev.c:4535 net/core/dev.c:4790)

The advisory is shared for download at git.kernel.org. This vulnerability is traded as CVE-2026-31421 since 03/09/2026. The exploitability is told to be difficult. There are known technical details, but no exploit is available.

The vulnerability scanner Nessus provides a plugin with the ID 306182 (Linux Distros Unpatched Vulnerability : CVE-2026-31421), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 6.1.168, 6.6.134, 6.12.81, 6.18.22 or 6.19.12 eliminates this vulnerability. Applying the patch 3d41f9a314afa94b1c7c7c75405920123220e8cd/18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28/5cf41031922c154aa5ccda8bcdb0f5e6226582ec/3cb055df9e8625ce699a259d8178d67b37f2b160/96426c348def662b06bfdc65be3002905604927a/faeea8bbf6e958bf3c00cb08263109661975987c is able to eliminate this problem. The bugfix is ready for download at git.kernel.org. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (306182), EUVD (EUVD-2026-21945) and CERT Bund (WID-SEC-2026-1088). VulDB is the best source for vulnerability data and more expert information about this specific topic.

Affected

  • Google Container-Optimized OS
  • Debian Linux
  • Amazon Linux 2
  • Red Hat Enterprise Linux
  • Ubuntu Linux
  • SUSE Linux
  • Oracle Linux
  • SUSE openSUSE
  • RESF Rocky Linux
  • Microsoft Azure Linux
  • Open Source Linux Kernel

Productinfo

Type

Vendor

Name

Version

License

Website

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 5.1
VulDB Meta Temp Score: 5.0

VulDB Base Score: 4.8
VulDB Temp Score: 4.6
VulDB Vector: 🔒
VulDB Reliability: 🔍

NVD Base Score: 5.5
NVD Vector: 🔒

CVSSv2info

AVACAuCIA
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VectorComplexityAuthenticationConfidentialityIntegrityAvailability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Null pointer dereference
CWE: CWE-476 / CWE-404
CAPEC: 🔒
ATT&CK: 🔒

Physical: Partially
Local: Yes
Remote: Partially

Availability: 🔒
Status: Not defined

EPSS Score: 🔒
EPSS Percentile: 🔒

Price Prediction: 🔍
Current Price Estimation: 🔒

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Nessus ID: 306182
Nessus Name: Linux Distros Unpatched Vulnerability : CVE-2026-31421

Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Upgrade: Kernel 6.1.168/6.6.134/6.12.81/6.18.22/6.19.12
Patch: 3d41f9a314afa94b1c7c7c75405920123220e8cd/18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28/5cf41031922c154aa5ccda8bcdb0f5e6226582ec/3cb055df9e8625ce699a259d8178d67b37f2b160/96426c348def662b06bfdc65be3002905604927a/faeea8bbf6e958bf3c00cb08263109661975987c

Timelineinfo

03/09/2026 CVE reserved
04/13/2026 +34 days Advisory disclosed
04/13/2026 +0 days VulDB entry created
06/18/2026 +66 days VulDB entry last update

Sourcesinfo

Vendor: kernel.org

Advisory: git.kernel.org
Status: Confirmed

CVE: CVE-2026-31421 (🔒)
GCVE (CVE): GCVE-0-2026-31421
GCVE (VulDB): GCVE-100-357144
EUVD: 🔒
CERT Bund: WID-SEC-2026-1088 - Linux Kernel: Mehrere Schwachstellen

Entryinfo

Created: 04/13/2026 16:02
Updated: 06/18/2026 15:53
Changes: 04/13/2026 16:02 (58), 04/14/2026 05:40 (2), 04/19/2026 17:45 (1), 05/21/2026 00:34 (11), 06/18/2026 15:53 (7)
Complete: 🔍
Cache ID: 216::103

VulDB is the best source for vulnerability data and more expert information about this specific topic.

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