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CVE-2026-31421— net/sched: cls_fw: fix NULL pointer dereference on shared blocks

Quick assessment

Affected
Linux Linux
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在安全漏洞，该漏洞源于cls_fw模块在共享块上存在空指针取消引用，可能导致内核崩溃。

AI Predicted 5.5 Difficulty: Easy EPSS 0.17% · P6

Possible ATT&CK Techniques 1 AI

T1499 · Endpoint Denial of Service

Affected Version Matrix 18

VendorProduct Version RangeStatus
Linux Linux 1abf272022cf1d18469405f47b4ec49c6a3125db< d6d5bd62a09650856e1e2010eb09853eba0d64e1 affected
1abf272022cf1d18469405f47b4ec49c6a3125db< febf64ca79a2d6540ab6e5e197fa0f4f7e84473e affected
1abf272022cf1d18469405f47b4ec49c6a3125db< 3d41f9a314afa94b1c7c7c75405920123220e8cd affected
1abf272022cf1d18469405f47b4ec49c6a3125db< 18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28 affected
1abf272022cf1d18469405f47b4ec49c6a3125db< 5cf41031922c154aa5ccda8bcdb0f5e6226582ec affected
1abf272022cf1d18469405f47b4ec49c6a3125db< 3cb055df9e8625ce699a259d8178d67b37f2b160 affected
1abf272022cf1d18469405f47b4ec49c6a3125db< 96426c348def662b06bfdc65be3002905604927a affected
1abf272022cf1d18469405f47b4ec49c6a3125db< faeea8bbf6e958bf3c00cb08263109661975987c affected
… +10 more rows
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I. Basic Information for CVE-2026-31421

Vulnerability Information
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Vulnerability Title
net/sched: cls_fw: fix NULL pointer dereference on shared blocks
Source: CVE Program / CVE List V5
Vulnerability Description
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)
Source: CVE Program / CVE List V5
CVSS Information
N/A
Source: CVE Program / CVE List V5
Vulnerability Type
N/A
Source: CVE Program / CVE List V5
Vulnerability Title
Linux kernel 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在安全漏洞，该漏洞源于cls_fw模块在共享块上存在空指针取消引用，可能导致内核崩溃。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Linux Linux 1abf272022cf1d18469405f47b4ec49c6a3125db ~ d6d5bd62a09650856e1e2010eb09853eba0d64e1 -
Linux Linux 4.15 -

II. Public POCs for CVE-2026-31421

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III. Intelligence Information for CVE-2026-31421

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Same Patch Batch · Linux · 2026-04-13 · 15 CVEs total

CVE-2026-31414 9.8 CRITICAL netfilter: nf_conntrack_expect: use expect->helper
CVE-2026-31419 7.8 HIGH net: bonding: fix use-after-free in bond_xmit_broadcast()
CVE-2026-31417 7.5 HIGH net/x25: Fix overflow when accumulating packets
CVE-2026-31426 7.0 HIGH ACPI: EC: clean up handlers on probe failure in acpi_ec_setup()
CVE-2026-31428 netfilter: nfnetlink_log: fix uninitialized padding leak in NFULA_PAYLOAD
CVE-2026-31427 netfilter: nf_conntrack_sip: fix use of uninitialized rtp_addr in process_sdp
CVE-2026-31425 rds: ib: reject FRMR registration before IB connection is established
CVE-2026-31424 netfilter: x_tables: restrict xt_check_match/xt_check_target extensions for NFPROTO_ARP
CVE-2026-31423 net/sched: sch_hfsc: fix divide-by-zero in rtsc_min()
CVE-2026-31422 net/sched: cls_flow: fix NULL pointer dereference on shared blocks
CVE-2026-31420 bridge: mrp: reject zero test interval to avoid OOM panic
CVE-2026-31418 netfilter: ipset: drop logically empty buckets in mtype_del
CVE-2026-31416 netfilter: nfnetlink_log: account for netlink header size
CVE-2026-31415 ipv6: avoid overflows in ip6_datagram_send_ctl()

IV. Related Vulnerabilities

Same product: Linux
Same vendor: Linux

V. Comments for CVE-2026-31421

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