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PatchSiren cyber security CVE debrief

CVE-2021-21707 Festo Didactic SE CVE debrief

CVE-2021-21707 describes a PHP filename-handling flaw in certain XML parsing functions, including simplexml_load_file(). When a filename is URL-decoded and contains a URL-encoded NUL byte, PHP may treat the NUL as the end of the filename and open a different file than the caller intended. In the Festo Didactic SE MES PC advisory context, the vendor directs operators to a replacement Factory Control Panel release for affected MES PCs.

Vendor
Festo Didactic SE
Product
MES PC
CVSS
MEDIUM 5.3
CISA KEV
Not listed in stored evidence
Original CVE published
2024-02-27
Original CVE updated
2026-01-27
Advisory published
2024-02-27
Advisory updated
2026-01-27

Who should care

Festo Didactic MES PC operators, OT/ICS defenders, and administrators running PHP 7.3.x below 7.3.33, 7.4.x below 7.4.26, or 8.0.x below 8.0.13 in any workflow that passes filenames into XML parsing functions.

Technical summary

The supplied advisory data says certain PHP XML parsing functions URL-decode the filename parameter before use. If an attacker can supply a URL-encoded NUL character, the effective path may be truncated at the NUL and a different file may be read. The advisory maps the issue to CWE-159 and gives a CVSS 3.1 vector of AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N.

Defensive priority

Medium. Prioritize patching or replacement where the affected PHP runtime is present on exposed or user-influenced systems, especially MES PC deployments that process filenames through XML parsers.

Recommended defensive actions

  • Obtain and deploy Festo's current Factory Control Panel replacement for MES PCs, as directed in the advisory, through Festo technical support.
  • Ensure any affected PHP deployment is updated to at least 7.3.33, 7.4.26, or 8.0.13, depending on the installed branch.
  • Audit code and integrations that call XML parsing functions such as simplexml_load_file() so untrusted input cannot influence filenames.
  • Reject URL-encoded NUL bytes and other unexpected encodings before any filename is passed to the parser.
  • Reduce exposure of any service that processes external input into file paths until remediation is complete.

Evidence notes

The CISA CSAF source item for ICSA-26-027-02 and its references state the affected PHP versions, describe the URL-decoded NUL filename issue, and provide the vendor remediation path for Festo MES PC. The advisory metadata also supplies the CVSS vector and score. The supplied data shows no KEV listing.

Sources and references

Verified primary and authoritative sources

  • CVE-2021-21707 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2021-21707

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2021-21707 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2021-21707

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

  • Source item URL

    Unverified legacy reference

    URL: https://raw.githubusercontent.com/cisagov/CSAF/develop/csaf_files/OT/white/2026/icsa-26-027-02.json

    cisa_csaf

  • Source reference

    Unverified legacy reference

    URL: https://festo.csaf-tp.certvde.com/.well-known/csaf/white/2024/fsa-202402.json

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://certvde.com/en/advisories/vendor/festo/

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://festo.com/psirt

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://certvde.com/en/advisories/VDE-2023-065

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/news-events/ics-advisories/icsa-26-027-02

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/news-events/ics-alerts/ics-alert-10-301-01

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/resources-tools/resources/ics-recommended-practices

    Reference

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.