Server Side Template Injection

Template injection allows an attacker to include template code into an existing (or not) template. A template engine makes designing HTML pages easier by using static template files which at runtime replaces variables/placeholders with actual values in the HTML pages

Summary

Tools
Methodology
References

Tools

Hackmanit/TInjA - An effiecient SSTI + CSTI scanner which utilizes novel polyglots

tinja url -u "http://example.com/?name=Kirlia" -H "Authentication: Bearer ey..."
tinja url -u "http://example.com/" -d "username=Kirlia"  -c "PHPSESSID=ABC123..."

epinna/tplmap - Server-Side Template Injection and Code Injection Detection and Exploitation Tool

python2.7 ./tplmap.py -u 'http://www.target.com/page?name=John*' --os-shell
python2.7 ./tplmap.py -u "http://192.168.56.101:3000/ti?user=*&comment=supercomment&link"
python2.7 ./tplmap.py -u "http://192.168.56.101:3000/ti?user=InjectHere*&comment=A&link" --level 5 -e jade

vladko312/SSTImap - Automatic SSTI detection tool with interactive interface based on epinna/tplmap

python3 ./sstimap.py -u 'https://example.com/page?name=John' -s
python3 ./sstimap.py -u 'https://example.com/page?name=Vulnerable*&message=My_message' -l 5 -e jade
python3 ./sstimap.py -i -A -m POST -l 5 -H 'Authorization: Basic bG9naW46c2VjcmV0X3Bhc3N3b3Jk'

Methodology

Identify the Vulnerable Input Field

The attacker first locates an input field, URL parameter, or any user-controllable part of the application that is passed into a server-side template without proper sanitization or escaping.

For example, the attacker might identify a web form, search bar, or template preview functionality that seems to return results based on dynamic user input.

TIP: Generated PDF files, invoices and emails usually use a template.

Inject Template Syntax

The attacker tests the identified input field by injecting template syntax specific to the template engine in use. Different web frameworks use different template engines (e.g., Jinja2 for Python, Twig for PHP, or FreeMarker for Java).

Common template expressions:

{{7*7}} for Jinja2 (Python).
#{7*7} for Thymeleaf (Java).

Find more template expressions in the page dedicated to the technology (PHP, Python, etc).

In most cases, this polyglot payload will trigger an error in presence of a SSTI vulnerability:

${{<%[%'"}}%\.

The Hackmanit/Template Injection Table is an interactive table containing the most efficient template injection polyglots along with the expected responses of the 44 most important template engines.

Enumerate the Template Engine

Based on the successful response, the attacker determines which template engine is being used. This step is critical because different template engines have different syntax, features, and potential for exploitation. The attacker may try different payloads to see which one executes, thereby identifying the engine.

Python: Django, Jinja2, Mako, ...
Java: Freemarker, Jinjava, Velocity, ...
Ruby: ERB, Slim, ...

The post "template-engines-injection-101" from @0xAwali summarize the syntax and detection method for most of the template engines for JavaScript, Python, Ruby, Java and PHP and how to differentiate between engines that use the same syntax.

Escalate to Code Execution

Once the template engine is identified, the attacker injects more complex expressions, aiming to execute server-side commands or arbitrary code.

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