# CVE-2020-27736: Memory corruption in APOGEE PXC Compact (BACnet)

> An attacker can cause denial of service or leak memory by sending malformed DNS responses to affected Siemens building automation and network devices.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H/E:P/RL:O/RC:C`)
- **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:** 4 Aug 2024
- **Also known as:** GHSA-6xw6-4hxh-3v2w

## What is CVE-2020-27736?

CVE-2020-27736 is a medium-severity memory corruption vulnerability in Siemens APOGEE PXC Compact (BACnet), 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 APOGEE PXC Compact (BACnet) (All versions < V3.5.5), APOGEE PXC Compact (P2 Ethernet) (All versions < V2.8.20), APOGEE PXC Modular (BACnet) (All versions < V3.5.5), APOGEE PXC Modular (P2 Ethernet) (All versions < V2.8.20), Nucleus NET (All versions), Nucleus ReadyStart V3 (All versions < V2017.02.3), Nucleus ReadyStart V4 (All versions < V4.1.0), Nucleus Source Code (Versions including affected DNS modules), SIMOTICS CONNECT 400 (All versions < V0.5.0.0), TALON TC Compact (BACnet) (All versions < V3.5.5), TALON TC Modular (BACnet) (All versions < V3.5.5). The DNS domain name label parsing functionality does not properly validate the null-terminated name in DNS-responses. The parsing of malformed responses could result in a read past the end of an allocated structure. An attacker with a privileged position in the network could leverage this vulnerability to cause a denial-of-service condition or leak the read memory.

Weakness: CWE-125 (Out-of-bounds Read), CWE-170 (Improper Null Termination)

## Which versions are affected by CVE-2020-27736?

It affects 5 products.


Products: Siemens Nucleus Net, Siemens Nucleus Readystart V3, Siemens Nucleus Readystart V4, Siemens Nucleus Source Code, Siemens Simotics Connect 400 Firmware.

## How do I fix CVE-2020-27736?

No fix has been published for CVE-2020-27736 yet. Limit exposure of the affected software until one is available.

## Is CVE-2020-27736 being exploited?

There is no known exploitation of CVE-2020-27736 in the wild and no public exploit code.

## Other vulnerabilities in Siemens APOGEE PXC Compact (BACnet)

- [CVE-2020-15795](https://insights.sbomify.com/CVE-2020-15795/): Memory corruption in APOGEE PXC Compact (BACnet) (high 8.1)
- [CVE-2020-27009](https://insights.sbomify.com/CVE-2020-27009/): Memory corruption in APOGEE PXC Compact (BACnet) (high 8.1)
- [CVE-2020-27737](https://insights.sbomify.com/CVE-2020-27737/): Memory corruption in APOGEE PXC Compact (BACnet) (medium 6.5)
- [CVE-2020-27738](https://insights.sbomify.com/CVE-2020-27738/): Memory corruption in APOGEE PXC Compact (BACnet) (medium 6.5)
- [CVE-2021-25677](https://insights.sbomify.com/CVE-2021-25677/): Other in APOGEE PXC Compact (BACnet) (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-27736 and fix it if it is affected.

What it is: CVE-2020-27736 is a medium-severity memory corruption vulnerability in Siemens APOGEE PXC Compact (BACnet), 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-2020-27736 in the wild and no public exploit code.

Affected product: Siemens APOGEE PXC Compact (BACnet). 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-2020-27736/ (Markdown: https://insights.sbomify.com/CVE-2020-27736/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-27736
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-27736
- OSV: https://osv.dev/vulnerability/CVE-2020-27736

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From sbomify Insights: https://insights.sbomify.com/CVE-2020-27736/
Summaries are generated and may contain mistakes; the official description above is the record.