# CVE-2020-28388: Weak cryptography in APOGEE PXC Compact (BACnet)

> Predictable TCP Initial Sequence Numbers allow attackers to hijack sessions or spoof connections in Siemens building automation systems.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L/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:** 9 Feb 2021
- **Updated:** 2 Jun 2026
- **Also known as:** GHSA-j6gp-g45q-2wm2

## What is CVE-2020-28388?

CVE-2020-28388 is a medium-severity vulnerability in Siemens APOGEE PXC Compact (BACnet), published on 9 Feb 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 < V5.2), Nucleus ReadyStart V3 (All versions < V2012.12), Nucleus Source Code (All versions), PLUSCONTROL 1st Gen (All versions), TALON TC Compact (BACnet) (All versions < V3.5.5), TALON TC Modular (BACnet) (All versions < V3.5.5). Initial Sequence Numbers (ISNs) for TCP connections are derived from an insufficiently random source. As a result, the ISN of current and future TCP connections could be predictable. An attacker could hijack existing sessions or spoof future ones.

Weakness: CWE-342 (Predictable Exact Value from Previous Values)

## Which versions are affected by CVE-2020-28388?

It affects 5 products.


Products: Siemens Capital Vstar, Siemens Nucleus Net, Siemens Nucleus Readystart, Siemens Nucleus Source Code, Siemens Pluscontrol 1st Gen.

## How do I fix CVE-2020-28388?

No fix has been published for CVE-2020-28388 yet. Limit exposure of the affected software until one is available.

## Is CVE-2020-28388 being exploited?

There is no known exploitation of CVE-2020-28388 in the wild and no public exploit code.

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

- [CVE-2021-25663](https://insights.sbomify.com/CVE-2021-25663/): Denial of service in Capital Embedded AR Classic 431-422 (high 8.7)
- [CVE-2021-25664](https://insights.sbomify.com/CVE-2021-25664/): Denial of service in Capital Embedded AR Classic 431-422 (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-28388 and fix it if it is affected.

What it is: CVE-2020-28388 is a medium-severity vulnerability in Siemens APOGEE PXC Compact (BACnet), published on 9 Feb 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-2020-28388 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-2020-28388/ (Markdown: https://insights.sbomify.com/CVE-2020-28388/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-28388
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-28388
- OSV: https://osv.dev/vulnerability/CVE-2020-28388

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From sbomify Insights: https://insights.sbomify.com/CVE-2020-28388/
Summaries are generated and may contain mistakes; the official description above is the record.