CVE-2026-81925 Concrete CMS CVE debrief

Who should care

Defenders managing Concrete CMS installations, particularly those using versions before 9.5.3, should assess their exposure and prioritize updates or mitigations. This includes reviewing security configurations, verifying system versions, and implementing necessary patches or workarounds to prevent exploitation.

Why it matters

This low-severity vulnerability in Concrete CMS could lead to reflected XSS attacks if not properly addressed. Defenders should prioritize updates and review security configurations to prevent exploitation.

• Defenders need to verify and update Concrete CMS to prevent exploitation.
• Security teams should review and enhance endpoint security configurations.
• CSRF token implementation is crucial for preventing similar attacks.

Technical summary

The vulnerability exists in Concrete CMS before version 9.5.3. User-supplied custom date formats are not properly neutralized when rendering conversation messages, allowing for reflected cross-site scripting (XSS). The attack involves a crafted POST request to the conversation view endpoint, which lacks a CSRF token, enabling cross-origin POST requests without authentication. This issue was reported by Yonatan Drori from Tenzai and given a CVSS v4.0 score of 2.1. Defenders should prioritize verifying and updating Concrete CMS to version 9.5.3 or later, and review conversation view endpoint security configurations.

Defensive priority

Defenders should prioritize verifying and updating Concrete CMS to version 9.5.3 or later, and review conversation view endpoint security configurations.

Recommended defensive actions

• Verify Concrete CMS version and update to 9.5.3 or later
• Review conversation view endpoint security configurations
• Implement CSRF tokens for endpoints handling user input
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record and NVD entry provide details on the vulnerability, including its CVSS v4.0 score of 2.1. The Concrete CMS security team assessed the vulnerability, and Yonatan Drori from Tenzai reported it.

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.