CVE-2026-81913 Concrete CMS CVE debrief

Who should care

Defenders and administrators of Concrete CMS versions 9.5.0 through 9.5.2 should assess exposure and prioritize mitigation, as open redirect vulnerabilities can be used for phishing and credential theft.

Why it matters

CVE-2026-81913 is a medium-severity vulnerability in Concrete CMS versions 9.5.0 through 9.5.2, allowing open redirects via the rcURL parameter, which can facilitate phishing and credential theft. Defenders should prioritize verifying and mitigating open redirect vulnerabilities, especially in authentication and registration flows.

• Phishing and credential theft are possible due to open redirects in authentication and registration flows.
• Defenders must verify and mitigate open redirect vulnerabilities to prevent potential security incidents.
• Additional security measures are required for authentication and registration flows to prevent exploitation.

Technical summary

Concrete CMS versions 9.5.0 through 9.5.2 are vulnerable to open redirects via the rcURL parameter, allowing attackers to craft links that send users to arbitrary external sites after authentication, facilitating phishing and credential theft. The vulnerability has a CVSS v4.0 score of 5.3. The same handling is present in the registration flow, giving a second entry point on sites with registration enabled. Concrete CMS versions prior to 9.5.0 do not include the rcURL parameter or this allowlist and are not affected.

Defensive priority

Defenders should prioritize verifying and mitigating open redirect vulnerabilities, especially in authentication and registration flows.

Recommended defensive actions

• Verify and mitigate open redirect vulnerabilities in Concrete CMS versions 9.5.0 through 9.5.2
• Implement additional security measures for authentication and registration flows
• Monitor for potential phishing and credential theft attempts
• 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 vulnerability was reported by Michal M. and assigned a CVSS v4.0 score of 5.3 by the Concrete CMS security team. The CVE record was published on 2026-09-11T20:19:14.140Z and has not been modified since then. Concrete CMS versions prior to 9.5.0 do not include the rcURL parameter or this allowlist and are not affected. The same handling is present in the registration flow, giving a second entry point on sites with registration enabled.

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.