CVE-2025-21920 vlan: enforce underlying device type
A Linux kernel vulnerability allows local attackers to leak kernel function addresses by creating VLAN devices on non-ethernet interfaces, aiding privilege escalation exploits.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-21920?
CVE-2025-21920 is a high-severity vulnerability in Linux kernel, published on 1 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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.
Weakness: CWE-125
Which versions are affected by CVE-2025-21920?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21920?
Apply the vendor's patch.
Is CVE-2025-21920 being exploited?
There is no known exploitation of CVE-2025-21920 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-59943, AZL-59976, BELL-CVE-2025-21920, DEBIAN-CVE-2025-21920, GHSA-p79h-jcrm-6qrc, ROOT-OS-UBUNTU-2404-CVE-2025-21920 and 1 more.