CVE-2021-21867 Unknown Vendor CVE debrief

Who should care

Industrial automation teams, OT/ICS administrators, and engineers who use Festo Automation Suite or install/manage CODESYS Development System components. Organizations that open or exchange project/profile files with these tools should treat this as a priority due to the command-execution impact.

Technical summary

The vulnerable behavior is an unsafe deserialization path in CODESYS Development System’s ObjectManager.plugin ObjectStream.ProfileByteArray functionality. The advisory states that a specially crafted file can cause arbitrary command execution. CISA’s CSAF advisory republishes Festo’s guidance and connects the CVE to Festo Automation Suite deployments, including versions where CODESYS was bundled as an external component or installed alongside the suite. The source corpus identifies the vulnerable CODESYS versions as 3.5.16 and 3.5.17.

Defensive priority

High. The combination of arbitrary command execution and file-triggered exposure makes this a serious risk in OT environments, especially where affected CODESYS components are installed or where users may open untrusted files.

Recommended defensive actions

• Update CODESYS Development System to the latest patched version provided by the vendor.
• Install Festo Automation Suite updates promptly; Festo’s guidance notes that starting with version 2.8.0.138, CODESYS is no longer bundled and must be obtained separately.
• Follow the official CODESYS installation and update instructions to ensure security fixes are applied correctly.
• Review where CODESYS components are present in your environment, including external components bundled with Festo Automation Suite.
• Limit exposure to untrusted or externally supplied files that could be opened by affected CODESYS tooling.
• Monitor vendor advisories from both Festo and CODESYS for follow-on guidance and patch notices.

Evidence notes

This debrief is grounded in the supplied CISA CSAF advisory (ICSA-26-076-01), which republishes Festo SE & Co. KG advisory FSA-202601, and in the advisory text stating that unsafe deserialization in CODESYS Development System 3.5.16 and 3.5.17 can lead to arbitrary command execution via a crafted file. Timing reflects the provided CVE published date of 2026-02-26 and modified date of 2026-03-17.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.