# CVE-2026-63990: bonding: refuse to enslave CAN devices

> A null pointer dereference in the Linux kernel bonding driver allows local attackers to cause a system crash by enslaving virtual CAN devices.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **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:** Yes
- **Published:** 19 Jul 2026
- **Updated:** 19 Jul 2026
- **Also known as:** BELL-CVE-2026-63990, DEBIAN-CVE-2026-63990, GHSA-vcmq-h7g2-2m6j, ROOT-OS-UBUNTU-2204-CVE-2026-63990, UBUNTU-CVE-2026-63990

## What is CVE-2026-63990?

CVE-2026-63990 is a medium-severity vulnerability in Linux kernel, published on 19 Jul 2026. 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:

bonding: refuse to enslave CAN devices

syzbot reported a kernel paging request crash in
can_rx_unregister() inside net/can/af_can.c. The crash occurs
because a virtual CAN device (vxcan) is being enslaved to a
bonding master.

During the enslavement process, the bonding driver mutates
and modifies the network device states to fit an Ethernet-like
aggregation model. However, CAN devices operate on a completely
different Layer 2 architecture, relying on the CAN mid-layer
private data structure (can_ml_priv) instead of standard
Ethernet structures. Since bonding does not initialize or
maintain these CAN structures, subsequent operations on the
half-enslaved interface (such as closing associated sockets
via isotp_release) lead to a null-pointer dereference when
accessing the CAN receiver lists.

Bonding CAN interfaces is architecturally invalid as CAN lacks
MAC addresses, ARP capabilities, and standard Ethernet
link-layer mechanisms. While generic loopback devices are
blocked globally in net/core/dev.c, virtual CAN devices
bypass this check because they do not carry the IFF_LOOPBACK
flag, despite acting as local software-loopbacks.

Fix this by explicitly blocking network devices of type
ARPHRD_CAN from being enslaved at the very beginning of
bond_enslave(). This prevents illegal state mutations,
eliminates the resulting KASAN crashes, and avoids potential
memory leaks from incomplete socket cleanups.

As the CAN support has been added a long time after bonding
the Fixes-tag points to the introduction of ARPHRD_CAN that
would have needed a specific handling in bonding_main.c.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2026-63990?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-63990?

Apply the vendor's patch.

## Is CVE-2026-63990 being exploited?

There is no known exploitation of CVE-2026-63990 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63990 and fix it if it is affected.

What it is: CVE-2026-63990 is a medium-severity vulnerability in Linux kernel, published on 19 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-63990 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63990/ (Markdown: https://insights.sbomify.com/CVE-2026-63990/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63990
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63990
- OSV: https://osv.dev/vulnerability/CVE-2026-63990

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63990/
Summaries are generated and may contain mistakes; the official description above is the record.