CVE-2021-20177

Kernel panic via stale packet-list pointer

Published May 26, 2021 · Updated Aug 3, 2024

Out-of-bounds read in Linux Kernel before 5.5-rc1 allows local users with CAP_NET_ADMIN to crash the system through an iptables rule. NF_HOOK_LIST removes packet buffers with list_del() but leaves skb->next linked, so string matching can follow stale packet-list state and panic the kernel. Root or CAP_NET_ADMIN access is required, and the documented consequence is a host-wide kernel panic rather than data access or modification.

CVSS severity4.4
Medium
EPSS probability0.28%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds read in Linux Kernel before 5.5-rc1 allows local users with CAP_NET_ADMIN to crash the system through an iptables rule. NF_HOOK_LIST removes packet buffers with list_del() but leaves skb->next linked, so string matching can follow stale packet-list state and panic the kernel. Root or CAP_NET_ADMIN access is required, and the documented consequence is a host-wide kernel panic rather than data access or modification.

The record

CVE
CVE-2021-20177
Published
May 26, 2021
Updated
Aug 3, 2024
Vendor
The Linux Kernel Organization
Product
Linux Kernel
Classifications
CWE-125, T1499.004
Attack vector
local
Privileges
admin

Timeline

How it unfolded

  1. May 26, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=kernel 5.5-rc1

What conditions does exploitation require?

Attack vectorlocal
Required privilegesadmin

What is affected?

The Linux Kernel Organization · Linux Kernelversion=kernel 5.5-rc1

Published CVSS scores

2.1NIST NVDAV:L/AC:L/Au:N/C:N/I:N/A:P
4.4Oracle LinuxCVSS:3.0/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
4.4Canonical UbuntuCVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
4.4NIST NVDCVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesadmin
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-125Out-of-bounds Read
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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