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Fencer Vulns
high 7.1

CVE-2025-21920 json

vlan: enforce underlying device type

published 2025-04-01 19 mo ago modified 2026-06-17 4 mo ago also EUVD-2025-9401

cvss
7.1
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 14%
public exploits
0
none known

Summary

In the Linux kernel, the following vulnerability has been resolved:

vlan: enforce underlying device type

Currently, VLAN devices can be created on top of non-ethernet devices.

Besides the fact that it doesn't make much sense, this also causes a bug which leaks the address of a kernel function to usermode.

When creating a VLAN device, we initialize GARP (garp_init_applicant) and MRP (mrp_init_applicant) for the underlying device.

As part of the initialization process, we add the multicast address of each applicant to the underlying device, by calling dev_mc_add.

__dev_mc_add uses dev->addr_len to determine the length of the new multicast address.

This causes an out-of-bounds read if dev->addr_len is greater than 6, since the multicast addresses provided by GARP and MRP are only 6 bytes long.

This behaviour can be reproduced using the following commands:

ip tunnel add gretest mode ip6gre local ::1 remote ::2 dev lo ip l set up dev gretest ip link add link gretest name vlantest type vlan id 100

Then, the following command will display the address of garp_pdu_rcv:

ip maddr show | grep 01:80:c2:00:00:21

Fix the bug by enforcing the type of the underlying device during VLAN device initialization.

Patch / fix

  • Linux Linux 5.4.291, 5.10.235, 5.15.179, 6.1.131, 6.6.83, 6.12.19, 6.13.7, 6.14
  • linux Debian 12 6.1.133-1
  • linux Debian 14, Debian 13 6.12.19-1

debian advisories DEBIAN-CVE-2025-21920

  • linux Ubuntu 20.04 LTS 5.4.0-216.236
  • linux Ubuntu 22.04 LTS 5.15.0-140.150
  • linux Ubuntu 24.04 LTS 6.8.0-84.84
  • linux-aws Ubuntu 20.04 LTS 5.4.0-1146.156
  • linux-aws Ubuntu 22.04 LTS 5.15.0-1084.91
  • linux-aws Ubuntu 24.04 LTS 6.8.0-1039.41
  • linux-gcp Ubuntu 20.04 LTS 5.4.0-1149.158
  • linux-gcp Ubuntu 22.04 LTS 5.15.0-1083.92

ubuntu advisories UBUNTU-CVE-2025-21920

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 RHEL 9 5.14.0-570.21.1.el9_6
  • rv RHEL 9 5.14.0-570.21.1.el9_6
  • perf RHEL 9 5.14.0-570.21.1.el9_6
  • rtla RHEL 9 5.14.0-570.21.1.el9_6
  • libperf RHEL 9 5.14.0-570.21.1.el9_6
  • kernel-rt RHEL 9 5.14.0-570.21.1.el9_6
  • kernel-64k RHEL 9 5.14.0-570.21.1.el9_6
  • kernel-core RHEL 9 5.14.0-570.21.1.el9_6

red hat advisories RHSA-2025:8643

  • kernel-coco SLE Module for Confidential Computing Technical Preview 15 SP6 6.4.0-15061.28.coco15sp6.1
  • kernel-syms-coco SLE Module for Confidential Computing Technical Preview 15 SP6 6.4.0-15061.28.coco15sp6.1
  • kernel-coco_debug SLE Module for Confidential Computing Technical Preview 15 SP6 6.4.0-15061.28.coco15sp6.1
  • kernel-source-coco SLE Module for Confidential Computing Technical Preview 15 SP6 6.4.0-15061.28.coco15sp6.1
  • dtb-aarch64 openSUSE Leap 15.6 6.4.0-150600.23.60.3
  • kernel-64kb SLE Module for Basesystem 15 SP6, openSUSE Leap 15.6 6.4.0-150600.23.60.4
  • kernel-syms SLE Module for Development Tools 15 SP6, openSUSE Leap 15.6 6.4.0-150600.23.60.4
  • kernel-obs-qa openSUSE Leap 15.6 6.4.0-150600.23.60.2

suse advisories SUSE-SU-2025:02923-1SUSE-SU-2025:02538-1SUSE-SU-2025:20498-1SUSE-SU-2025:20483-1SUSE-SU-2025:02335-1SUSE-SU-2025:02334-1

  • kernel AlmaLinux 9 5.14.0-570.21.1.el9_6
  • rv AlmaLinux 9 5.14.0-570.21.1.el9_6
  • perf AlmaLinux 9 5.14.0-570.21.1.el9_6
  • rtla AlmaLinux 9 5.14.0-570.21.1.el9_6
  • libperf AlmaLinux 9 5.14.0-570.21.1.el9_6
  • kernel-rt AlmaLinux 9 5.14.0-570.21.1.el9_6
  • kernel-64k AlmaLinux 9 5.14.0-570.21.1.el9_6
  • kernel-core AlmaLinux 9 5.14.0-570.21.1.el9_6

almalinux advisories ALSA-2025:8643

  • kernel Rocky Linux 9 5.14.0-570.21.1.el9_6

rocky linux advisories RLSA-2025:8643

  • kernel Azure Linux 3.0 6.6.85.1-2
  • kernel CBL-Mariner 2.0 5.15.180.1-1

azure linux advisories AZL-59943AZL-59976

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

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-2025-21920 updated cvss 7.1

kernel: vlan: enforce underlying device type

  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 10 kernel
    affected: no fix available “Fix deferred”
  • Red Hat Enterprise Linux AppStream (v. 9) rv 5.14.0-570.21.1.el9_6, perf 5.14.0-570.21.1.el9_6, rtla 5.14.0-570.21.1.el9_6, python3-perf 5.14.0-570.21.1.el9_6, kernel-devel-matched 5.14.0-570.21.1.el9_6, kernel-64k-devel-matched 5.14.0-570.21.1.el9_6, +7 more Red Hat Enterprise Linux BaseOS (v. 9) kernel 5.14.0-570.21.1.el9_6, kernel-64k 5.14.0-570.21.1.el9_6, kernel-core 5.14.0-570.21.1.el9_6, kernel-tools 5.14.0-570.21.1.el9_6, kernel-modules 5.14.0-570.21.1.el9_6, kernel-64k-core 5.14.0-570.21.1.el9_6, +27 more Red Hat Enterprise Linux CodeReady Linux Builder (v. 9) libperf 5.14.0-570.21.1.el9_6, kernel-debuginfo-common-s390x 5.14.0-570.21.1.el9_6, kernel-debuginfo-common-x86_64 5.14.0-570.21.1.el9_6, kernel-debuginfo-common-aarch64 5.14.0-570.21.1.el9_6, kernel-debuginfo-common-ppc64le 5.14.0-570.21.1.el9_6 Red Hat Enterprise Linux Real Time (v. 9) kernel-rt 5.14.0-570.21.1.el9_6, kernel-rt-64k 5.14.0-570.21.1.el9_6, kernel-rt-core 5.14.0-570.21.1.el9_6, kernel-rt-modules 5.14.0-570.21.1.el9_6, kernel-rt-64k-core 5.14.0-570.21.1.el9_6, kernel-rt-debug-core 5.14.0-570.21.1.el9_6, +14 more Red Hat Enterprise Linux Real Time for NFV (v. 9) kernel-rt 5.14.0-570.21.1.el9_6, kernel-rt-kvm 5.14.0-570.21.1.el9_6, kernel-rt-core 5.14.0-570.21.1.el9_6, kernel-rt-modules 5.14.0-570.21.1.el9_6, kernel-rt-debug-kvm 5.14.0-570.21.1.el9_6, kernel-rt-debug-core 5.14.0-570.21.1.el9_6, +6 more
    fixed: vendor fix RHSA-2025:8643

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

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
13 mo ago

Exploit prediction

0.24% chance of exploitation in the next 30 days, higher than 14% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CISA ADP 3.1 7.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
NVD 3.1 7.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
CNA 3.1 7.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H

Affected products

  • Linux Linux

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.