# Jinja2 Server-Side Template Injection via render_template_string
Language: Python
Severity: Critical
CWE: CWE-1336
## Source
9
## Flow
9-10-11
## Sink
11
## Vulnerable Code
```python
from flask import Flask, request, render_template_string
import boto3
app = Flask(__name__)
s3_client = boto3.client('s3')
@app.route('/cloud/bucket-report')
def generate_bucket_report():
bucket_name = request.args.get('bucket', 'default-bucket')
report_title = request.args.get('title', 'S3 Bucket Analysis')
template_markup = f"{report_title}Report for: {bucket_name}
"
return render_template_string(template_markup)
```
## Explanation
The application accepts user-controlled input through the 'title' parameter (line 9) and directly embeds it into a template string using an f-string (line 10), which is then passed to render_template_string() (line 11). This allows attackers to inject Jinja2 template expressions that will be executed server-side, potentially leading to remote code execution.
## Remediation
The fix replaces the dangerous f-string interpolation of user input directly into the template string with Jinja2's native variable placeholders ({{ title }} and {{ bucket }}). By passing user inputs as context variables to render_template_string(), Jinja2 automatically escapes them, preventing any template expressions in user input from being interpreted as code.
## Secure Code
```python
from flask import Flask, request, render_template_string
import boto3
from markupsafe import escape
app = Flask(__name__)
s3_client = boto3.client('s3')
@app.route('/cloud/bucket-report')
def generate_bucket_report():
bucket_name = request.args.get('bucket', 'default-bucket')
report_title = request.args.get('title', 'S3 Bucket Analysis')
template_markup = "{{ title }}Report for: {{ bucket }}
"
return render_template_string(template_markup, title=report_title, bucket=bucket_name)
```