# CVE-2021-25664: Denial of service in Capital Embedded AR Classic 431-422

> Attackers can cause a denial of service in Siemens Nucleus RTOS products by sending crafted IPv6 packets that trigger an infinite loop in the Hop-by-Hop header processing.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 22 Apr 2021
- **Updated:** 11 Mar 2025
- **Also known as:** GHSA-q7wx-433j-27hv

## What is CVE-2021-25664?

CVE-2021-25664 is a high-severity denial of service vulnerability in Siemens Capital Embedded AR Classic 431-422, published on 22 Apr 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A vulnerability has been identified in Capital Embedded AR Classic 431-422 (All versions), Capital Embedded AR Classic R20-11 (All versions < V2303), Nucleus NET (All versions), Nucleus ReadyStart V3 (All versions < V2017.02.4), Nucleus ReadyStart V4 (All versions < V4.1.0), Nucleus Source Code (All versions including affected IPv6 stack). The function that processes the Hop-by-Hop extension header in IPv6 packets and its options lacks any checks against the length field of the header, allowing attackers to put the function into an infinite loop by supplying arbitrary length values.

Weakness: CWE-835 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2021-25664?

It affects 5 products.


Products: Siemens Capital Vstar, Siemens Nucleus Net, Siemens Nucleus Readystart V3, Siemens Nucleus Readystart V4, Siemens Nucleus Source Code.

## How do I fix CVE-2021-25664?

No fix has been published for CVE-2021-25664 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-25664 being exploited?

There is no known exploitation of CVE-2021-25664 in the wild and no public exploit code.

## Other vulnerabilities in Siemens Capital Embedded AR Classic 431-422

- [CVE-2021-25663](https://insights.sbomify.com/CVE-2021-25663/): Denial of service in Capital Embedded AR Classic 431-422 (high 8.7)
- [CVE-2020-28388](https://insights.sbomify.com/CVE-2020-28388/): Weak cryptography in APOGEE PXC Compact (BACnet) (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-25664 and fix it if it is affected.

What it is: CVE-2021-25664 is a high-severity denial of service vulnerability in Siemens Capital Embedded AR Classic 431-422, published on 22 Apr 2021. 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-2021-25664 in the wild and no public exploit code.

Affected product: Siemens Capital Embedded AR Classic 431-422. 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-2021-25664/ (Markdown: https://insights.sbomify.com/CVE-2021-25664/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-25664
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-25664
- OSV: https://osv.dev/vulnerability/CVE-2021-25664

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-25664/
Summaries are generated and may contain mistakes; the official description above is the record.