CVE-2020-13671: Drupal core Un-restricted Upload of File

CVE-2020-13671 is an unrestricted file upload weakness in Drupal core stemming from improper sanitization of file name extensions. Attackers who can reach an upload path may be able to place files that the application later treats as executable or otherwise dangerous content. For any organization running Drupal, this matters because a successful upload can lead to code execution, site takeover, or further lateral movement inside the environment. Confirm exact impact and fixed releases against the vendor advisory.

How it works

The flaw belongs to CWE-434: unrestricted upload of a file with a dangerous type. Drupal core failed to sanitize extension file names properly, so an uploaded file could retain or be given an extension that the web server or PHP interpreter would later execute or process in an unintended way. An attacker with the ability to submit a file through a legitimate upload feature (or any other path that accepts user-controlled filenames) could craft a name that bypasses the incomplete checks. Once the file is stored in a web-accessible location, a subsequent request could cause the server to interpret it as code or as a configuration file. No public exploit mechanics beyond this class behavior are supplied here; defenders should treat any unauthenticated or low-privilege upload surface as potentially reachable and verify details in the official Drupal security advisory.

Am I affected? How to find it in your systems

Drupal core is commonly deployed as the foundation of public websites, intranets, and content-management platforms, often behind reverse proxies or load balancers and frequently paired with PHP-FPM or Apache/Nginx. Inventory every host and container that runs PHP and serves Drupal:

• Search package inventories, configuration-management databases, and container images for Drupal core installations.
• Examine composer.lock, core/lib/Drupal.php, or the administrative status report to identify the installed core version.
• Review any custom or contributed modules that expose file-upload forms; the core sanitization flaw can affect those paths as well.
• Check web-server and application logs for unexpected file creations under sites/default/files or other public upload directories, especially files whose extensions do not match their content type or that appear shortly before anomalous PHP execution or 500 errors.

Because precise vulnerable version ranges are not listed in the supplied facts, compare every discovered instance against the vendor advisory before declaring a system safe.

How to remediate

Apply the updates supplied by Drupal for this CVE as the primary remediation; the CISA-required action is simply to follow the vendor instructions. After patching:

• Verify that the core update is present on every instance, including development, staging, and any cloned environments.
• Re-test upload functionality to confirm that dangerous extensions are now rejected or renamed.
• Harden the upload pipeline for this class of weakness: store uploaded files outside the web root when possible, serve them through a clean controller that forces safe Content-Type and Content-Disposition headers, and maintain a strict allow-list of extensions rather than a block-list.
• Ensure the PHP interpreter and web server are configured so that files in upload directories cannot be executed (for example, by removing script handlers from those locations).

If you can't patch immediately

Until the vendor update can be deployed, reduce exposure with compensating controls:

• Segment Drupal hosts so that only necessary management and content-authoring networks can reach upload endpoints.
• Place a web-application firewall or reverse-proxy rule that blocks requests containing suspicious double extensions, null bytes, or known dangerous suffixes in multipart form data.
• Temporarily disable or restrict any non-essential file-upload features, including those provided by contributed modules.
• Increase monitoring on the upload directories and on PHP error logs; alert on creation of files with executable extensions or on sudden spikes in POST requests to upload handlers.
• If feasible, move the entire upload storage volume to a location that is not directly web-accessible and serve files only through authenticated, sanitized download handlers.

If your data may have been exposed

Actively exploited upload vulnerabilities frequently serve as the initial foothold for broader compromise, data theft, or ransomware deployment (though ransomware use is not documented for this specific CVE). If you have reason to believe an upload path was abused, treat the host as potentially compromised: isolate it, preserve forensic images, rotate credentials, and review downstream systems for lateral movement. As a quick additional check, you can run a free exposure scan of your email addresses against known breach data sets to see whether associated accounts have already appeared in public dumps.