# CVE-2019-18420: Denial of service in n/a

> Malicious x86 PV guests can crash the Xen hypervisor via a specific hypercall, causing a denial of service.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/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:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 31 Oct 2019
- **Updated:** 5 Aug 2024
- **Also known as:** ALPINE-CVE-2019-18420, DEBIAN-CVE-2019-18420, GHSA-54p2-qq73-f565, UBUNTU-CVE-2019-18420

## What is CVE-2019-18420?

CVE-2019-18420 is a medium-severity denial of service vulnerability in Debian Debian Linux, published on 31 Oct 2019. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

An issue was discovered in Xen through 4.12.x allowing x86 PV guest OS users to cause a denial of service via a VCPUOP_initialise hypercall. hypercall_create_continuation() is a variadic function which uses a printf-like format string to interpret its parameters. Error handling for a bad format character was done using BUG(), which crashes Xen. One path, via the VCPUOP_initialise hypercall, has a bad format character. The BUG() can be hit if VCPUOP_initialise executes for a sufficiently long period of time for a continuation to be created. Malicious guests may cause a hypervisor crash, resulting in a Denial of Service (DoS). Xen versions 4.6 and newer are vulnerable. Xen versions 4.5 and earlier are not vulnerable. Only x86 PV guests can exploit the vulnerability. HVM and PVH guests, and guests on ARM systems, cannot exploit the vulnerability.

Weakness: CWE-134 (Use of Externally-Controlled Format String)

## Which versions are affected by CVE-2019-18420?

It affects 3 products.


Products: Debian Debian Linux, Fedoraproject Fedora, Xen Xen.

## How do I fix CVE-2019-18420?

Apply the vendor's patch.

## Is CVE-2019-18420 being exploited?

There is no known exploitation of CVE-2019-18420 in the wild and no public exploit code.

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- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2019-18420 and fix it if it is affected.

What it is: CVE-2019-18420 is a medium-severity denial of service vulnerability in Debian Debian Linux, published on 31 Oct 2019. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2019-18420 in the wild and no public exploit code.

Affected product: Debian Debian 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-2019-18420/ (Markdown: https://insights.sbomify.com/CVE-2019-18420/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2019-18420
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2019-18420
- OSV: https://osv.dev/vulnerability/CVE-2019-18420

---
From sbomify Insights: https://insights.sbomify.com/CVE-2019-18420/
Summaries are generated and may contain mistakes; the official description above is the record.