CVE-2026-31421: net/sched: cls_fw: fix NULL pointer dereference on shared blocks
Vulnerability summary
CVE-2026-31421 is a medium (CVSS 5.5) NULL Pointer Dereference vulnerability in linux. EPSS estimates a 0.12% chance of exploitation in the next 30 days, higher than 1.8% of scored vulnerabilities.
Not severe (CVSS 5.5, local access); no exploitation reported by the 5 checked sources
Source freshness unavailable
How is this decided?Advisory lineage
Upstream 0 · Downstream 32 · Related 30Timeline
- 13-04-2026PublishedVulnerability first disclosed.
- 15-10-2026Last modifiedVulnerability information updated.
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)
CVSS metrics
3 scores · 3 sources · 1 version- OSV · UbuntuEcosystem advisory · calculated5.5MediumCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
- OSV · DebianEcosystem advisory · calculated5.5MediumCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
EPSS trend
Probability
0.12%
Chance of exploitation in the next 30 days
Percentile
2%
Versus all other CVEs
30-day change
Decrease -0.05 pp
From 0.17% on 04-10-2026
Known exploited
Compare KEV catalogs
CISA KEVNot in the CISA KEV catalog.
ENISA CNWNot in the ENISA CNW catalog.
CIRCLNot in the CIRCL catalog.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeAffected systems
Known conditions: local access, low-privilege account, no user interaction (CVSS vector). Other conditions: not stated in public data.
Ubuntu156 products
- linuxAll versions (No fix recorded) | < 5.15.0-186.196 | < 6.8.0-136.136
- linux-allwinner-5.19All versions (No fix recorded)
- linux-awsAll versions (No fix recorded) | < 5.15.0-1112.119 | < 6.8.0-1061.64
- linux-aws-5.0All versions (No fix recorded)
- linux-aws-5.11All versions (No fix recorded)
Linux2 products
- linux≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < d6d5bd62a09650856e1e2010eb09853eba0d64e1 | ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < febf64ca79a2d6540ab6e5e197fa0f4f7e84473e | ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < 3d41f9a314afa94b1c7c7c75405920123220e8cd | ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < 18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28 | ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < 5cf41031922c154aa5ccda8bcdb0f5e6226582ec | ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < 3cb055df9e8625ce699a259d8178d67b37f2b160 | ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < 96426c348def662b06bfdc65be3002905604927a | ≥ 1abf272022cf1d18469405f47b4ec49c6a3125db, < faeea8bbf6e958bf3c00cb08263109661975987c | 4.15
- linux_kernel≥ 4.15, < 5.10.253 | ≥ 5.11, < 5.15.203 | ≥ 5.16, < 6.1.168 | ≥ 6.2, < 6.6.134 | ≥ 6.7, < 6.12.81 | ≥ 6.13, < 6.18.22 | ≥ 6.19, < 6.19.12 | 7.0:rc1 | 7.0:rc2 | 7.0:rc3 | 7.0:rc4 | 7.0:rc5 | 7.0:rc6
Debian1 product
- linux< 6.1.170-1 | < 6.12.85-1 | < 6.19.12-1
3 vendors · 159 products
