# CVE-2026-109399: SWUpdate improper certificate validation in CMS signature verification

Source: https://junglewise.ai/threats/cve-2026-109399-swupdate-through-2026-05-1-contains-an-improper-certificate  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-12. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-109399 · Severity: high · CVSS 8.1 · Published 2026-10-12

SWUpdate is a firmware update system for embedded devices that verifies software authenticity using digital signatures. A flaw in the certificate validation logic allows an attacker with a signing key from a sibling certificate authority to forge updates that will be trusted and installed, enabling malware deployment to affected devices.

## Quick answers

**Is CVE-2026-109399 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-109399 being exploited in the wild.

**How severe is CVE-2026-109399?** High severity, with a CVSS score of 8.1, published 12 October 2026.

**Which products and versions are affected?** SWUpdate through 2026.05.1.

**Is there a fix for CVE-2026-109399?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-109399 attacked?** It is attacked over the network, in device firmware.

**Am I affected by CVE-2026-109399?** Check whether you run SWUpdate at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability is an improper certificate validation flaw in crypto/swupdate_cms_verify_openssl.c where the CMS signature verification process trusts untrusted X.509 certificates embedded in SignedData structures for chain building. An attacker possessing a private key from a sibling intermediate certificate under the same root CA can embed that intermediate in the malicious signature to bypass validation and install unauthorized firmware updates.

## Affected products

- SWUpdate through 2026.05.1

## Timeline

- 2026-10-12: disclosed

## References

- [https://github.com/sbabic/swupdate](https://github.com/sbabic/swupdate)
- [https://github.com/sbabic/swupdate/blob/2026.05.1/crypto/swupdate_cms_verify_openssl.c](https://github.com/sbabic/swupdate/blob/2026.05.1/crypto/swupdate_cms_verify_openssl.c)