Deep Dive: CVE-2023-50917 - Unmasking an Unauthenticated Remote Code Execution Flaw in MajorDoMo's Thumb Module

Introduction

MajorDoMo, a beacon in Russian home automation, particularly favored by Raspberry Pi aficionados, has been a trusted name for over a decade. With over 380 stars on its official GitHub repository at the time of writing, its popularity is evident. However, lurking within its thumb.php module is a severe unauthenticated Remote Code Execution (RCE) vulnerability. This article intricately explores this critical flaw, detailing its roots, distinct exploitation methods, and possible ramifications.

Disclosure Timeline:

• October 28, 2023: Initial discovery of the vulnerability (CVE-2023-50917).
• October 29, 2023: Contacted MajorDoMo team detailing the vulnerability.
• November 6, 2023: After no response from MajorDoMo's team for over a week, submitted a CVE request to the appropriate CNA.
• November 14, 2023: New attempt to contact the MajorDoMo team. Received a response from the team within a few hours. The patch has been applied.
• December 15, 2023: Public disclosure of CVE-2023-50917.

Technical Background: The Vulnerable Code

The script /modules/thumb/thumb.php is primarily designed for thumbnail generation in MajorDoMo. It serves to facilitate the creation of thumbnails from various media sources. But within this benign purpose lies a significant vulnerability:

Key Code Snippets and Analysis:

1. URL Decoding:

   $url = base64_decode($url);

   The script takes a base64 encoded url parameter and decodes it. This decoding process is pivotal, as it allows attackers to obfuscate their payloads, skirting around simple checks.

2. Pattern Checks:

   if (preg_match('/^rtsp:/is', $url) || preg_match('/\/dev/', $url)) {
   ...
   }

   The script then checks if the decoded url adheres to specific patterns (rtsp: or /dev). This is a rudimentary check to decide whether to process the URL. With the help of base64 encoding, it becomes trivial for attackers to bypass this verification.

3. Direct Command Construction:

   if ($_GET['transport']) {
       $stream_options = '-rtsp_transport ' . $_GET['transport'] . ' ' . $stream_options;
   }

   Here lies the crux of the vulnerability. The transport parameter is taken directly and embedded within a system command without adequate sanitization. This glaring oversight allows for arbitrary command injections. By crafting the transport parameter, an attacker can introduce and execute arbitrary commands. The subsequent command is executed via the exec function, which poses a significant security risk.

The Core Vulnerability

The vulnerability's essence is the unchecked and unsanitized user input (from the transport parameter) that gets directly incorporated into a system command. This allows attackers to run arbitrary commands on the server, potentially taking full control of the MajorDoMo instance.

Exploitation Avenues:

1. Bypassing URL Validation: The script's initial validation checks for patterns such as rtsp: or /dev. By using base64 encoded strings like cnRzcDovL2EK (decoding to rtsp://a), these checks can be easily bypassed.

2. Command Injection via the transport Parameter: The transport parameter is used directly within a system command. With no sanitization in place, this can be exploited for command injections, leading to RCE. For instance, the command ||echo; echo $(command_here) can be used to break out of the intended command and execute any arbitrary command.

Potential Impact

The severity of this RCE vulnerability is high. Given MajorDoMo's integral role in home automation, successful exploitation can result in an attacker compromising physical security systems, gaining access to surveillance cameras, or even taking control of other connected IoT devices.

Recommendations for Mitigation

• Thorough Input Validation: It is essential to rigorously validate all inputs. This can prevent malicious payloads from being processed.
• Sanitize Before Execution: Inputs should be sanitized before being incorporated into any system commands.
• Limit Direct Command Execution: Prefer using built-in PHP functions or secure APIs over direct system command execution.

Conclusion

This vulnerability underscores the importance of thorough code reviews and robust input validation. Even established software projects like MajorDoMo are not immune to critical vulnerabilities. The discovery serves as a reminder of the ever-present need for diligence and a proactive approach to security in all software development stages.