Analysis of Unauthenticated RCE in Motorola MR2600 Router

The author of this research is currently pursuing a personal challenge: discovering at least one Remote Code Execution (RCE) vulnerability for every single router manufacturer. This report details the discovery of a critical flaw in the Motorola MR2600.

🎯 Target Overview

Most Motorola routers keep their firmware locked away, distributing updates exclusively via over-the-air (OTA) channels. However, the MR2600 (a Wi-Fi 5 device) proved to be an exception. The most recent firmware, v1.0.22, was released in mid-2024 and was available for download.

Resource: Motorola MR2600 Software Updates

After extracting the filesystem, the researcher analyzed the SOAP handlers and CGI scripts to reverse-engineer the manual update mechanism. The process consists of two distinct phases:

Image Upload: Sending the firmware file to the device.
Validation & Flashing: Triggering the routine that writes the image to memory.

🛠️ Phase 1: Uploading Malicious Firmware

The router exposes a specific endpoint for firmware uploads: POST /WEBCGI1/prog.fcgi?method=/cgi-bin/fwupload.cgi

The UpdateFirmware_Moto.html page utilizes a form with the following attributes:

<form id="fwupload" action="/WEBCGI1/prog.fcgi?method=/cgi-bin/fwupload.cgi" method="post" enctype="multipart/form-data">

The Validation Flaw

To prevent arbitrary file uploads, the router checks if the file is a valid SEAMA image by verifying the first four bytes (the "magic numbers"). These bytes must be: $\text{Magic Bytes} = \{0x27, 0x05, 0x19, 0x56\}$

However, a critical implementation error exists. The developers attempted to validate the magic numbers against the raw multipart request body rather than the extracted file content.

Incorrect Logic Flow: ~~Extract file $\rightarrow$ Check magic bytes~~ $\rightarrow$ Check raw HTTP body $\rightarrow$ Extract file

Because a standard multipart request starts with a boundary string (e.g., ------WebKitFormBoundary...), the first four bytes will always be ----, causing legitimate requests to fail.

The Bypass: An attacker can simply ignore the multipart format and send the raw binary firmware image directly. The system will see the magic bytes at the very start of the request and accept the file.

The Authentication Gap

While the developers attempted to restrict this endpoint to authenticated users, the check is misplaced. The router first writes the uploaded data to /tmp/firmware.img and then checks for authentication. If the user is unauthenticated, the request is rejected, but the file is ~~not~~ deleted from the /tmp directory.

⚡ Phase 2: Flashing the Firmware

To move the uploaded file from temporary storage to the actual system kernel, a SOAP request is required:

POST /WEBCGI1/ HTTP/1.1
Content-Type: text/xml; charset=utf-8
SOAPAction: "http://purenetworks.com/WEBCGI1/LoadFirmwareValidation"

<?xml version="1.0"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <LoadFirmwareValidation xmlns="http://purenetworks.com/WEBCGI1/" />
  </soap:Body>
</soap:Envelope>

This triggers the LoadFirmwareValidation function, which performs a CRC32 checksum to ensure file integrity. Since there is no cryptographic signing, anyone can craft a valid image. Once validated, the router executes:

mtd_write -r -w write /tmp/firmware.img Kernel

(Note: The -r flag forces a reboot after flashing).

Bypassing Authentication via Logic Flaw

The authentication mechanism uses inconsistent comparison operators to determine if a URI is "allowed."

Check Type	Logic Used	Result
Allowlist	Substring Match	If URI contains `Login.html` $\rightarrow$ Allow
Denylist	Exact Match	If URI is exactly `/WEBCGI1/` $\rightarrow$ Block

By appending an allowlisted string as a parameter, an attacker can trick the router. For example, using /WEBCGI1/?Login.html:

Does it contain Login.html? Yes $\rightarrow$ Pass Allowlist.
Is it exactly /WEBCGI1/? No $\rightarrow$ Bypass Denylist.

The Final Payload:

POST /WEBCGI1/?Login.html HTTP/1.1
Content-Type: text/xml; charset=utf-8
SOAPAction: "http://purenetworks.com/WEBCGI1/LoadFirmwareValidation"

[SOAP Envelope as shown above]

🚀 Exploit Summary

The complete attack chain is visualized below:

Exploit Checklist

Craft malicious SEAMA image with correct magic bytes.
Upload raw binary to /WEBCGI1/prog.fcgi?method=/cgi-bin/fwupload.cgi.
Trigger flash via /WEBCGI1/?Login.html using the LoadFirmwareValidation SOAP action.
Wait for reboot to gain full control.

Exploitability: This vulnerability can be triggered by any unauthenticated actor with network access to the router's web interface.