# CVE-2026-21878: BACnet Stack Improperly Limits Pathnames to a Restricted Directory

> Attackers can write files to arbitrary directories due to lack of path validation in the BACnet Stack library, potentially compromising embedded systems.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 13 Feb 2026
- **Updated:** 13 Feb 2026
- **Also known as:** GHSA-p8rx-c26w-545j

## What is CVE-2026-21878?

CVE-2026-21878 is a high-severity path traversal vulnerability in bacnet-stack, published on 13 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

BACnet Stack is a BACnet open source protocol stack C library for embedded systems. Prior to 1.5.0.rc3, a vulnerability has been discovered in BACnet Stack's file writing functionality where there is no validation of user-provided file paths, allowing attackers to write files to arbitrary directories. This affects apps/readfile/main.c and ports/posix/bacfile-posix.c. This vulnerability is fixed in 1.5.0.rc3.

Weakness: CWE-22 (Improper Limitation of a Pathname to a Restricted Directory)

## Which versions are affected by CVE-2026-21878?

It affects 1 product.


Products: Bacnetstack Bacnet Stack.

## How do I fix CVE-2026-21878?

No fix has been published for CVE-2026-21878 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21878 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21878 exists.

## Other vulnerabilities in bacnet-stack

- [CVE-2023-51773](https://insights.sbomify.com/CVE-2023-51773/): Memory corruption in n/a (critical 9.1)
- [CVE-2026-41475](https://insights.sbomify.com/CVE-2026-41475/): BACnet Stack: Out-of-Bounds Read in WritePropertyMultiple Decoder via Deprecated Tag Parser (high 8.7)
- [CVE-2026-41502](https://insights.sbomify.com/CVE-2026-41502/): BACnet Stack: Off-by-One Out-of-Bounds Read in ReadPropertyMultiple Object ID Decoder (high 8.7)
- [CVE-2026-41503](https://insights.sbomify.com/CVE-2026-41503/): BACnet Stack: Out-of-Bounds Read in ReadPropertyMultiple Property Decoder via Deprecated Tag Parser (high 8.7)
- [CVE-2026-26264](https://insights.sbomify.com/CVE-2026-26264/): BACnet Stack WriteProperty decoding length underflow leads to OOB read and crash (high 7.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21878 and fix it if it is affected.

What it is: CVE-2026-21878 is a high-severity path traversal vulnerability in bacnet-stack, published on 13 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21878 exists.

Affected product: bacnet-stack. 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-2026-21878/ (Markdown: https://insights.sbomify.com/CVE-2026-21878/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21878
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21878
- OSV: https://osv.dev/vulnerability/CVE-2026-21878

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-21878/
Summaries are generated and may contain mistakes; the official description above is the record.