VulnerabilityPublished 13 Apr 2026Updated 5 Nov 2026

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

Affects Linux kernelSource CVE Database V5

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Not known to be exploited · fixed in febf64ca79a2d6540ab6e5e197fa0f4f7e84473e : update Linux kernel when convenient.

Data as of 5 Nov 2026, 23:24 UTC

Severity

Unrated

Rated by Threat Radar · No CVSS score from NVD or the CNA

Thin evidence

One scorer or none

EPSS

0.1% chance of exploitation in 30 days – higher than 1% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

Exploitation: No evidence of active exploitation and no public proof of concept
Automatable: Attackers cannot reliably automate steps 1–4 of the kill chain
Technical impact: Limited control over the software or limited information exposure

CISA SSVC assessment, 2026-11-05.

Patch

Available – fixed in febf64ca79a2d6540ab6e5e197fa0f4f7e84473e, faeea8bbf6e958bf3c00cb08263109661975987c, d6d5bd62a09650856e1e2010eb09853eba0d64e1 +5

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Apr 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 5 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade to a fixed release listed under Affected software.

Affected software

Product	Affected	Fixed in
LinuxLinux	Affected ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < febf64ca79a2d6540ab6e5e197fa0f4f7e84473e	Fixed in febf64ca79a2d6540ab6e5e197fa0f4f7e84473e
LinuxLinux	Affected ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < faeea8bbf6e958bf3c00cb08263109661975987c	Fixed in faeea8bbf6e958bf3c00cb08263109661975987c
LinuxLinux	Affected ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < d6d5bd62a09650856e1e2010eb09853eba0d64e1	Fixed in d6d5bd62a09650856e1e2010eb09853eba0d64e1
LinuxLinux	Affected ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < 96426c348def662b06bfdc65be3002905604927a	Fixed in 96426c348def662b06bfdc65be3002905604927a
LinuxLinux	Affected ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < 18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28	Fixed in 18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28
LinuxLinux	Affected ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < 5cf41031922c154aa5ccda8bcdb0f5e6226582ec	Fixed in 5cf41031922c154aa5ccda8bcdb0f5e6226582ec

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)

References

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