CVE-2026-8204 Concrete CMS CVE debrief

Who should care

Concrete CMS administrators, developers, and security teams that use calendar blocks or expose calendar content to public users should review this CVE. Sites that separate public and private calendars are especially relevant because the reported bypass can bridge that boundary.

Technical summary

The NVD record describes an authorization bypass in the Calendar Event Frontend Dialog. The key security impact is cross-calendar data disclosure: a public calendar block may be used as a pivot point to access private calendar data. NVD maps the weakness to CWE-639 and records the CVSS v4.0 vector as CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N, indicating network-reachable exploitation with low attack complexity and no required privileges or user interaction, but with a present authorization prerequisite in the vector.

Defensive priority

Medium. The flaw is publicly disclosed and can expose private calendar data, so organizations using affected Concrete CMS versions should treat it as a timely access-control issue even though the scored severity is not critical.

Recommended defensive actions

• Confirm whether your Concrete CMS deployment is running version 9.5.0 or earlier and treat it as potentially affected.
• Review the Concrete CMS release notes and apply the vendor fix or upgrade path for a non-vulnerable version newer than 9.5.0.
• Audit public calendar blocks and related frontend dialog permissions to ensure private calendar data cannot be reached through public entry points.
• Verify authorization checks on calendar event retrieval and dialog rendering paths, especially any code that distinguishes public versus private calendars.
• Review logs and access patterns for unusual or unauthorized access to calendar content.
• If calendar data separation matters to your environment, temporarily reduce exposure of public calendar features until remediation is completed.

Evidence notes

All statements above are grounded in the supplied CVE/NVD metadata and the referenced Concrete CMS release-notes URL. The CVE description explicitly states Concrete CMS 9.5.0 and below are vulnerable to an authorization bypass in the Calendar Event Frontend Dialog that can allow cross-calendar data disclosure. NVD records the weakness as CWE-639 and provides the CVSS v4.0 vector. No exploit details, proof-of-concept steps, or unverified fix version were used.

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.