# CVE-2026-40279

> Vulnerability · severity: **LOW** (CVSS 3.7).

## 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.

## Key facts

- **CVE ID:** CVE-2026-40279
- **Published:** 2026-04-21
- **CVSS severity:** LOW
- **CVSS base score:** 3.7
- **CWE codes:** CWE-758

## Affected products

- `bacnetstack:bacnet_stack`

## Primary sources

- NIST NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40279

## Citation

> AI Analytics. CVE-2026-40279. Retrieved 2026-07-26 from https://api.ai-analytics.org/cve/CVE-2026-40279. Derived from NIST NVD. Licensed CC0.

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