CVE-2026-40279

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.

CVSS v3.1 Score

3.7

LOW

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS — Exploit Prediction

0.0004

Probability of exploitation

0.13%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-758 CWE-758

Affected Products

Vendor	Product	Versions
bacnetstack	bacnet_stack	< 1.4.3

References

Advisories & Patches

https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-326g-j95f-gmxv https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-326g-j95f-gmxv

Exploits

https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-326g-j95f-gmxv https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-326g-j95f-gmxv

Frequently Asked Questions

What is CVE-2026-40279? +

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. It has a CVSS v3.1 base score of 3.7 (LOW).

How severe is CVE-2026-40279? +

CVE-2026-40279 has a CVSS v3.1 score of 3.7 out of 10, rated LOW. This is a low-severity vulnerability with limited impact. The EPSS score is 0.0004, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-40279? +

CVE-2026-40279 affects products from bacnetstack, specifically: bacnet_stack. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-40279? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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