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Fencer Vulns
medium 4.4

CVE-2021-20177 json

A flaw was found in the Linux kernel's implementation of string matching within a packet.

published 2021-05-26 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-7634

cvss
4.4
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 20%
public exploits
0
none known

Summary

A flaw was found in the Linux kernel's implementation of string matching within a packet. A privileged user (with root or CAP_NET_ADMIN) when inserting iptables rules could insert a rule which can panic the system. Kernel before kernel 5.5-rc1 is affected.

Patch / fix

  • linux Debian 14, Debian 13, Debian 12 5.5.13-1

debian advisories DEBIAN-CVE-2021-20177

  • linux Ubuntu 20.04 LTS 5.4.0-66.74
  • linux-aws Ubuntu 20.04 LTS 5.4.0-1038.40
  • linux-gcp Ubuntu 20.04 LTS 5.4.0-1037.40
  • linux-gke Ubuntu 20.04 LTS 5.4.0-1036.38
  • linux-kvm Ubuntu 20.04 LTS 5.4.0-1033.34
  • linux-azure Ubuntu 20.04 LTS 5.4.0-1040.42
  • linux-gkeop Ubuntu 20.04 LTS 5.4.0-1010.11
  • linux-raspi Ubuntu 20.04 LTS 5.4.0-1029.32

ubuntu advisories UBUNTU-CVE-2021-20177

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • kernel-syms SLES for SAP 15 SP1, SLES 15 SP1-LTSS, SLES 15 SP1-BCL +6 more 4.12.14-197.83.1
  • kernel-source SLES for SAP 15 SP1, SLES 15 SP1-LTSS, SLES 15 SP1-BCL +6 more 4.12.14-197.83.1
  • kernel-default SLES for SAP 15 SP1, SLES 15 SP1-LTSS, SLES 15 SP1-BCL +8 more 4.12.14-197.83.1
  • kernel-zfcpdump SLES 15 SP1-LTSS, Manager Server 4.0 4.12.14-197.83.1
  • kernel-obs-build SLES for SAP 15 SP1, SLES 15 SP1-LTSS, SLES 15 SP1-BCL +6 more 4.12.14-197.83.1
  • kernel-rt SLE Real Time 12 SP5 4.12.14-10.31.1
  • kernel-syms-rt SLE Real Time 12 SP5 4.12.14-10.31.1
  • kernel-rt_debug SLE Real Time 12 SP5 4.12.14-10.31.1

suse advisories SUSE-SU-2021:0532-1SUSE-SU-2021:0433-1SUSE-SU-2021:0427-1SUSE-SU-2021:0354-1SUSE-SU-2021:0353-1SUSE-SU-2021:0348-1

  • kernel-syms openSUSE Leap 15.2 5.3.18-lp152.63.1
  • kernel-obs-qa openSUSE Leap 15.2 5.3.18-lp152.63.1
  • kernel-source openSUSE Leap 15.2 5.3.18-lp152.63.1
  • kernel-default openSUSE Leap 15.2 5.3.18-lp152.63.1
  • kernel-preempt openSUSE Leap 15.2 5.3.18-lp152.63.1
  • kernel-kvmsmall openSUSE Leap 15.2 5.3.18-lp152.63.1
  • kernel-obs-build openSUSE Leap 15.2 5.3.18-lp152.63.1
  • kernel-default-base openSUSE Leap 15.2 5.3.18-lp152.63.1.lp152.8.21.1

opensuse advisories openSUSE-SU-2021:0241-1

  • linux-lts Arch Linux 5.4.81-1

arch linux advisories AVG-1429

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-20177 updated cvss 4.4

kernel: iptables string match rule could result in kernel panic

  • Red Hat Enterprise Linux 5 kernel Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-alt, python-perf, kernel-kdump, +4 more Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more Red Hat Enterprise MRG 2 kernel-rt, mrg-rt-release, kernel-rt-trace, kernel-rt-vanilla, kernel-rt-firmware
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

0.28% chance of exploitation in the next 30 days, higher than 20% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 4.4 CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
NVD 2.0 2.1 AV:L/AC:L/Au:N/C:N/I:N/A:P

Affected products

  • n/a kernel

Classification

Weakness
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.