SSTI

Server-side template injection (SSTI) occurs when an attacker exploits native template syntax to inject and execute a malicious payload on the server.

Detect

Plaintext context

Most template languages support a 'text' context where you can directly input HTML

http://vulnerable-website.com/?username=user

The server:

render('Hello ' + username)
Hello user

You might think to test for XSS, but it can also be vulnerable to template injection. If you find XSS, test also for SSTI. So, you should try:

http://vulnerable-website.com/?username=${7*7}

If the output is 49, it means the mathematical operation is being processed server-side.

Code context

This context is easily missed during assessment because it doesn't result in obvious XSS.

  1. First establish that the parameter doesn't contain a direct XSS vulnerability by injecting HTML

http://vulnerable-website.com/?greeting=data.username<tag>

Without XSS, this usually results in a blank output, encoded tags, or an error message.

  1. Next, break out of the statement with common templating syntax and inject arbitrary HTML

http://vulnerable-website.com/?greeting=data.username}}<tag>

If this results in an error or blank output, you may have used the wrong templating syntax, or SSTI isn't possible. If the output renders correctly with the arbitrary HTML, it indicates a SSTI vulnerability.

Hello user<tag>

  1. Exploit

http://vulnerable-website.com/?greeting=data.username}}<PAYLOAD>

Identification

  • Smarty (PHP)

    • ${7*7}

    • a{*comment*}b

  • Mako (Python)

    • ${7*7}

    • ${"z".join("ab")}

  • Jinja2 (Python)

    • {{7*7}} returns error

    • {{7*'7'}} returns 7777777

  • Twig (PHP)

    • {{7*7}} returns 49

    • {{7*'7'}} returns 49

  • ERB (Ruby)

    • <%= 7*7 %> returns 49

  • Unknown

    • ${7*7}

    • {{7*'7'}}

  • Check if not vulnerable

    • ${7*7} -> {{7*'7'}}

    • In this case these payloads don't have to work

Note: there are many other template languages.

Exploitation

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