# CVE-2026-40279: BACnet Stack: Undefined-behavior signed left shift in `decode_signed32()`

> Undefined behavior from signed left shifts in BACnet Stack can cause crashes or unpredictable behavior when processing specific BACnet data.

- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 21 Apr 2026
- **Updated:** 21 Apr 2026
- **Also known as:** GHSA-326g-j95f-gmxv

## What is CVE-2026-40279?

CVE-2026-40279 is a low-severity vulnerability in bacnet-stack, published on 21 Apr 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 1.4.3, decode_signed32() in src/bacnet/bacint.c reconstructs a 32-bit signed integer from four APDU bytes using signed left shifts. When any of the four bytes has bit 7 set (value ≥ 0x80), the left-shift operation overflows a signed int32_t, which is undefined behavior per the C standard. This is flagged thousands of times per minute by UndefinedBehaviorSanitizer on any BACnet input containing signed-integer property values with high-bit-set bytes. This vulnerability is fixed in 1.4.3.

Weakness: CWE-758 (Reliance on Undefined, Unspecified, or Implementation-Defined Behavior)

## Which versions are affected by CVE-2026-40279?

It affects 1 product.


Products: Bacnetstack Bacnet Stack.

## How do I fix CVE-2026-40279?

No fix has been published for CVE-2026-40279 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-40279 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-40279 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-2026-40279 and fix it if it is affected.

What it is: CVE-2026-40279 is a low-severity vulnerability in bacnet-stack, published on 21 Apr 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-2026-40279 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-2026-40279/ (Markdown: https://insights.sbomify.com/CVE-2026-40279/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40279
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40279
- OSV: https://osv.dev/vulnerability/CVE-2026-40279

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40279/
Summaries are generated and may contain mistakes; the official description above is the record.