CVE-2026-81891: elFinder: ZIP extraction bypasses uploadDeny MIME filter allowing PHP file upload (RCE)

Summary

elFinder provides uploadDeny and uploadAllow options in its connector configuration to restrict which MIME types may be uploaded. When uploadDeny includes text/x-php, direct upload of .php, .phtml, and .phar files is correctly blocked. However, the extract command (ZIP decompression) internally calls checkExtractItems(), which invokes mimetypeInternalDetect() directly without passing the result through mimeTypeNormalize(). Because phtml, phar, and similar PHP-executable extensions are absent from mime.types, they are not resolved to text/x-php at the detection stage, causing the MIME filter to be silently bypassed. An attacker who is permitted to upload ZIP archives can therefore extract PHP-executable files into the web-accessible files/ directory. If the server is configured to execute the affected extension (e.g., .phtml, .phar) as PHP — which is the case in common Apache and Nginx deployments — this results in Remote Code Execution.

---

Details

elFinder's MIME validation pipeline for direct uploads (upload command) is:

mimetype() └─ mimetypeInternalDetect() // stage 1: extension → MIME via mime.types └─ mimeTypeNormalize() // stage 2: apply staticMimeMap phtml: → text/x-php phar: → text/x-php php5: → text/x-php └─ allowPutMime() // blocked: text/x-php ∈ uploadDeny

The extract command (checkExtractItems() in elFinderVolumeDriver.class.php, line 7110) uses a shortened pipeline:

php // line 7110 — stage 2 (mimeTypeNormalize) is never called if ($chkMime && ($mimeByName = elFinderVolumeDriver::mimetypeInternalDetect($name)) && !$this->allowPutMime($mimeByName)) {

Because phtml and phar are not present in mime.types, mimetypeInternalDetect() returns a generic type (e.g., application/octet-stream) for these extensions. Without mimeTypeNormalize(), the staticMimeMap entries that would map phtml: → text/x-php are never applied, so allowPutMime() sees a non-blocked MIME and permits extraction.

Affected extensions confirmed: .phtml, .phar, .php5, .php3 Not bypassed: .php (present in mime.types, detected as text/x-php in stage 1)

---

PoC

Requirements: - elFinder 2.1.69 deployed under Apache/Nginx (PHP-FPM or modphp) - Connector configured with uploadDeny = ['text/x-php'] and uploadAllow including application/zip - files/ directory served under a public web path

Step 1 — Confirm direct upload is blocked Open elFinder in a browser and click the Upload button. Select hello.phtml (content: <?php phpinfo(); ?>). → Upload is rejected with: "Upload file hello.phtml: File type not allowed (text/x-php)" <img width="1061" height="408" alt="1" src="https://github.com/user-attachments/assets/22f833d9-7d2a-4197-85c2-3b0eb19ecfbc" />

Step 2 — Upload a ZIP containing the payload Create bypass.zip containing hello.phtml. Upload bypass.zip via the Upload button. → ZIP is accepted (MIME: application/zip ∈ uploadAllow). <img width="886" height="320" alt="2" src="https://github.com/user-attachments/assets/30c3498f-91f4-45cf-8c2b-cc806716e3a4" />

Step 3 — Extract the ZIP Right-click bypass.zip in the file list → Extract files. → hello.phtml appears in the file list without any error. → File is now present at {filesdir}/hello.phtml on the server. <img width="1123" height="792" alt="3" src="https://github.com/user-attachments/assets/49e7db8e-566c-4653-a57f-e80fb31ea61d" />

Step 4 — Execute the extracted PHP file

Navigate to: http://<target>/elFinder/files/hello.phtml → Apache processes the file as PHP and renders the full phpinfo() output, confirming Remote Code Execution.

--- <img width="1059" height="739" alt="4" src="https://github.com/user-attachments/assets/ad85fdf7-bdc7-4514-bdec-e45cfb2e2bd3" />

Impact

Any user with ZIP upload permission can bypass the uploadDeny MIME restriction, place PHP-executable files in a web-accessible directory, and achieve Remote Code Execution on the server.

Concrete impact: - Arbitrary PHP code execution on the web server - Full server environment disclosure via phpinfo() (paths, PHP version, loaded modules, environment variables) - Potential access to server filesystem, database credentials, and internal network services - Complete compromise of the web application if an attacker substitutes phpinfo() with a web shell (e.g., <?php system($GET['cmd']); ?>)

Extensions confirmed executable on Apache (default config): phtml, phar, php5, php3

Recommended fix:

Apply mimeTypeNormalize() inside checkExtractItems() so that the full MIME pipeline is used consistently:

php // elFinderVolumeDriver.class.php, line 7110 // Before (vulnerable): $mimeByName = elFinderVolumeDriver::mimetypeInternalDetect($name)

// After (fixed): $mimeByName = $this->mimeTypeNormalize( elFinderVolumeDriver::mimetypeInternalDetect($name), $name, pathinfo($name, PATHINFOEXTENSION) )

---

---