CVE-2026-53599 redaxo CVE debrief

Who should care

Administrators of REDAXO CMS installations, especially those with untrusted authenticated users, should prioritize patching this vulnerability to prevent potential code execution. Security teams should review the vulnerability details and assess the exposure of their REDAXO deployments. IT operations teams should plan and apply the vendor-supported updates or mitigations through normal change control where exposure is confirmed. Monitoring and incident response teams should be prepared to detect and respond to potential exploitation attempts. Asset inventory and vulnerability management teams should ensure that affected systems are identified and prioritized for remediation.

Technical summary

The rex_mediapool::isAllowedExtension function in REDAXO CMS versions 5.18.2 to 5.21.1 allows authenticated backend users with media upload permissions to upload malicious files, such as a JPEG/PHP polyglot named shell.php.any.jpg, which can be executed as the web-server user on systems with multi-extension PHP handlers. This issue can lead to arbitrary code execution. The vulnerability is fixed in version 5.21.1, which corrects the file upload validation logic.

Defensive priority

Authenticated users with media upload permissions can execute arbitrary code; prioritize patching and restrict upload privileges.

Recommended defensive actions

• Apply the patch in version 5.21.1
• Restrict media upload permissions to necessary users
• Monitor for suspicious file uploads and system changes
• 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 indicates a high-severity vulnerability in REDAXO content management system versions from 5.18.2 to 5.21.1. An authenticated backend user with media upload permission can upload a malicious file that can be executed as the web-server user. The issue is fixed in version 5.21.1. Evidence is limited to the CVE record and NVD detail page. Defenders should verify affected REDAXO installations, review media upload permissions, and monitor for suspicious file uploads and system changes.

Official resources