# CVE-2026-6899: Systerel S2OPC improper certificate revocation check in CycloneCrypto

Source: https://junglewise.ai/threats/cve-2026-6899-systerel-s2opc-improper-certificate-revocation-check-in  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-09. Updated: 2026-06-09  
License: CC BY 4.0 with a link back

CVE-2026-6899 · Severity: medium · CVSS 5.6 · Published 2026-06-09

A security flaw exists in the S2OPC library, which is used for secure industrial communications (OPC UA). The software fails to properly check all lists of revoked security certificates, potentially allowing a device or user with a cancelled certificate to maintain a connection to the server. This could lead to unauthorized access or the continued operation of compromised devices that should have been blocked from the network.

Technologies: [Systerel S2OPC](https://junglewise.ai/threats/technologies/s2opc). Vendors: [Systerel](https://junglewise.ai/threats/vendors/systerel).

## Quick answers

**Is CVE-2026-6899 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-6899 being exploited in the wild.

**How severe is CVE-2026-6899?** Medium severity, with a CVSS score of 5.6, published 9 June 2026.

**Which products and versions are affected?** Systerel S2OPC a442f47200aaf3e58194b209e0738a4f9056393a.

**Is there a fix for CVE-2026-6899?** 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-2026-6899 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-6899?** Check whether you run Systerel S2OPC 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 classified as an improper check for certificate revocation (CWE-299) within the `crt_verifycrl_and_check_revocation` function of the S2OPC CycloneCrypto wrapper. The implementation incorrectly terminates its search after finding the first matching CRL for a Certificate Authority, ignoring subsequent valid CRLs that may contain the certificate's revocation status. An attacker using a revoked certificate could successfully establish or maintain an OPC UA connection if the revocation entry exists in a CRL not processed by the library. The issue was identified in commit a442f472 and requires a fix to iterate through all associated CRLs to ensure comprehensive revocation verification.

## Affected products

- Systerel S2OPC a442f47200aaf3e58194b209e0738a4f9056393a

## Timeline

- 2026-06-09: disclosed
- 2026-06-09: advisory

## References

- [https://gitlab.com/systerel/S2OPC/-/work_items/1739](https://gitlab.com/systerel/S2OPC/-/work_items/1739)

## Related threats

- [S2OPC null pointer dereference in notification message allocation](https://junglewise.ai/threats/cve-2026-90782-s2opc-null-pointer-dereference-in-notification-message-allocation)
- [Systerel S2OPC use-after-free in monitored item event filter](https://junglewise.ai/threats/cve-2026-82619-systerel-s2opc-use-after-free-in-monitored-item-event-filter)
- [Systerel S2OPC out-of-bounds read in string array range writing](https://junglewise.ai/threats/cve-2026-82618-systerel-s2opc-out-of-bounds-read-in-string-array-range-writing)
- [Systerel S2OPC out-of-bounds read in AddNodes Service](https://junglewise.ai/threats/cve-2026-78049-systerel-s2opc-out-of-bounds-read-in-addnodes-service)
- [Systerel S2OPC heap-based buffer overflow in EventFilter handling](https://junglewise.ai/threats/cve-2026-67868-systerel-s2opc-heap-based-buffer-overflow-in-eventfilter-handling)