# CVE-2026-63891: thunderbolt: property: Cap recursion depth in __tb_property_parse_dir()

> A crafted Thunderbolt peer can cause a kernel stack overflow via infinite recursion in property parsing, leading to denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **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-2026-63891, DEBIAN-CVE-2026-63891, GHSA-67r4-fvhf-7vqf, ROOT-OS-UBUNTU-2204-CVE-2026-63891, UBUNTU-CVE-2026-63891

## What is CVE-2026-63891?

CVE-2026-63891 is a denial of service vulnerability in Linux, 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:

thunderbolt: property: Cap recursion depth in __tb_property_parse_dir()

A DIRECTORY entry's value field is used as the dir_offset for a
recursive call into __tb_property_parse_dir() with no depth counter.
A crafted peer that chains DIRECTORY entries into a back-reference
loop drives the parser until the kernel stack is exhausted and the
guard page fires.  Any untrusted XDomain peer (cable, dock, in-line
inspector, adjacent host) that reaches the PROPERTIES_REQUEST
control-plane exchange can trigger this without authentication.

Thread a depth counter through tb_property_parse() and
__tb_property_parse_dir(), and reject blocks that exceed
TB_PROPERTY_MAX_DEPTH = 8.  That is comfortably larger than any
observed legitimate XDomain layout.

Operators who do not need XDomain host-to-host discovery can disable
the path entirely with thunderbolt.xdomain=0 on the kernel command
line.

## Which versions are affected by CVE-2026-63891?


## How do I fix CVE-2026-63891?

Apply the vendor's patch.

## Is CVE-2026-63891 being exploited?

There is no known exploitation of CVE-2026-63891 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63891 and fix it if it is affected.

What it is: CVE-2026-63891 is a denial of service vulnerability in Linux, 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-2026-63891 in the wild and no public exploit code.

Affected product: Linux. 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-2026-63891/ (Markdown: https://insights.sbomify.com/CVE-2026-63891/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63891
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63891
- OSV: https://osv.dev/vulnerability/CVE-2026-63891

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63891/
Summaries are generated and may contain mistakes; the official description above is the record.