# CVE-2020-29487: Denial of service in n/a

> A malicious guest can cause host denial of service by monopolizing memory in dom0 through inefficient xenstore key watching logic in the XAPI toolstack.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 15 Dec 2020
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-q329-j6wm-vf6q

## What is CVE-2020-29487?

CVE-2020-29487 is a high-severity denial of service vulnerability in n/a, published on 15 Dec 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An issue was discovered in Xen XAPI before 2020-12-15. Certain xenstore keys provide feedback from the guest, and are therefore watched by toolstack. Specifically, keys are watched by xenopsd, and data are forwarded via RPC through message-switch to xapi. The watching logic in xenopsd sends one RPC update containing all data, any time any single xenstore key is updated, and therefore has O(N^2) time complexity. Furthermore, message-switch retains recent (currently 128) RPC messages for diagnostic purposes, yielding O(M*N) space complexity. The quantity of memory a single guest can monopolise is bounded by xenstored quota, but the quota is fairly large. It is believed to be in excess of 1G per malicious guest. In practice, this manifests as a host denial of service, either through message-switch thrashing against swap, or OOMing entirely, depending on dom0's configuration. (There are no quotas in xenopsd to limit the quantity of keys that result in RPC traffic.) A buggy or malicious guest can cause unreasonable memory usage in dom0, resulting in a host denial of service. All versions of XAPI are vulnerable. Systems that are not using the XAPI toolstack are not vulnerable.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2020-29487?

It affects 1 product.


Products: Xen Xapi.

## How do I fix CVE-2020-29487?

No fix has been published for CVE-2020-29487 yet. Limit exposure of the affected software until one is available.

## Is CVE-2020-29487 being exploited?

There is no known exploitation of CVE-2020-29487 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2022-33749](https://insights.sbomify.com/CVE-2022-33749/): Denial of service in Xapi (medium 5.3)
- [CVE-2024-31144](https://insights.sbomify.com/CVE-2024-31144/): Xapi: Metadata injection attack against backup/restore functionality (low 3.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-29487 and fix it if it is affected.

What it is: CVE-2020-29487 is a high-severity denial of service vulnerability in n/a, published on 15 Dec 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2020-29487 in the wild and no public exploit code.

Affected product: n/a. 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-2020-29487/ (Markdown: https://insights.sbomify.com/CVE-2020-29487/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-29487
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-29487
- OSV: https://osv.dev/vulnerability/CVE-2020-29487

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From sbomify Insights: https://insights.sbomify.com/CVE-2020-29487/
Summaries are generated and may contain mistakes; the official description above is the record.