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

Severity: low (CVSS 3.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-04-21 · updated 2026-06-17
Weakness: CWE-758

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.

Affected products

bacnet-stack: before 1.4.3 — fixed in 1.4.3

How to fix

Upgrade to a fixed version: bacnet-stack 1.4.3. Check the vendor advisory for mitigations.

References

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