Technical Advisory – playSMS Pre-Authentication Remote Code Execution (CVE-2020-8644)

Vendor: playSMS
Vendor URL: https://playsms.org/
Versions affected: Before 1.4.3
Systems Affected: All
Author: Lucas Rosevear
Advisory URL / CVE Identifier: CVE-2020-8644
Risk: Critical

Summary:

PlaySMS is an open source SMS gateway, which has a web management portal written in PHP. PlaySMS supports a custom PHP templating system, called tpl (https://github.com/antonraharja/tpl).

PlaySMS double processes a server-side template, resulting in unauthenticated user control of input to the PlaySMS template engine. The template engine’s implementation then permits arbitrary code execution.

Location:

src/Playsms/Tpl.php:_compile()

Impact:

An attacker with network connectivity to an instance of PlaySMS can execute arbitrary code.

Details:

The vulnerability can be broken into two parts:

1. A server-side template is processed twice. After the first pass, user
   controlled input is present in the template when it is processed a second time.
2. The tpl template language is vulnerable to PHP code injection.

These are both detailed below.

Server-side Template Processed Twice

PlaySMS caches the last POST handled by each plugin/route in web/init.php with the following logic:

// save last $_POST in $_SESSION
if ($_POST['X-CSRF-Token']) {
  $_SESSION['tmp']['last_post'][md5(trim(_APP_ . _INC_ . _ROUTE_ . _INC_))] = $_POST;
}

The login flow in PlaySMS starts by dynamically loading a specified plugin in web/inc/app/main.php. For authentication, the web/plugin/core/auth/login.php file is included. When a user authenticates, a POST request is sent, populating the cache. The authentication plugin then populates a template, found in web/plugin/core/auth/templates/auth_login.html.

This template includes the following:

This template gets processed within the authentication plugin, executing the _lastpost method, which returns a value from the POST cache, populating the username field with the last submitted username.

At this point, if a user submitted a custom template string, such as {{1+1}}, this would not be processed, but would now be present in this HTML blob.

Subsequently, back in web/inc/app/main.php, the following logic is executed:

$content = ob_get_clean();

_p(themes_apply($content));

The $content variable contains the HTML with user-controlled input present from the authentication plugin. The themes_apply function ultimately ends up executing the following template:

  $tpl = array(
    'name' => $themes_layout,
    'vars' => array(
      'CONTENT' => $content,

    ) 
  );
  $content = tpl_apply($tpl, array(
    'core_config',
    'user_config' 
  ));

This logic maps the user-controlled $content to the CONTENT variable in the theme template, as seen in web/plugin/themes/common/templates/themes_layout.html

    

      {{ CONTENT }}
    

Template remote code execution

The vulnerable logic in the tpl templating system begins with variable substitution:

    // check static replaces
    if ($this->vars) {
      foreach ($this->vars as $key => $val) {
        $this->_setString($key, $val);
      }
      empty($this->vars);
    }

After this logic executes, the attacker controlled input is directly present in the template that is being processed. Subsequently, the following snippet is executed:

    $pattern = "{{(.*?)}}";
    preg_match_all("/" . $pattern . "/", $this->_result, $matches, PREG_SET_ORDER);

This looks for a pattern of form {{input}}. The input ultimately gets concatenated to a PHP string:

    foreach ($matches as $block) {
      $chunk = $block[0];
      $codes = '<?php ' . $this->config['echo'] . '(' . trim($block[1]) . ')' . '; ?>';
      $this->_result = str_replace($chunk, $codes, $this->_result);
    }

The resulting string is <?php echo(input); ?>

The code then executes this string in one of two ways:

1. tpl attempts to create a cache file with the malicious string as
   content. $cache_file = md5($this->_filename) . '.compiled'; $cache = $this->config['dir_cache'] . '/' . $cache_file; $fd = @fopen($cache, 'w+'); @fwrite($fd, $this->_result); @fclose($fd); If this succeeds, the code is executed by including the cache file: if (file_exists($cache)) { ob_start(); include $cache;
2. If caching fails, tpl directly evals the malicious string else { ob_start(); eval('?>' . $this->_result . '<?php ');
   
   

Reproduction:

Attempt to authenticate with the following username, and no password:

{{`ls -lah`}}

Notice the username field is replaced with the result of the command.

Recommendation:

Update to PlaySMS version 1.4.3.

Vendor Communication:

01/25/20: Initial contact with PlaySMS maintainer
01/30/20: Vulnerability details sent to PlaySMS
02/04/20: Vulnerability patched
02/04/20: Patch tested by NCC Group
02/05/20: PlaySMS 1.4.3 released

Thanks to:

Anton Raharja for their responsiveness and quickly patching the vulnerability.

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