Server Side Template Injection in Flask with Basic Auth

Server Side Template Injection (SSTI) in Flask becomes particularly concerning when Basic Authentication is used for access control but is not complemented by robust input validation and context-aware output encoding. The vulnerability arises when user-influenced data is incorporated into a Jinja2 template without proper escaping, allowing an attacker to break out of the intended rendering context and execute arbitrary template logic. In Flask, this often occurs when values from request headers, query parameters, or form fields are passed into macros or directly into template strings that are rendered with render_template_string.

Basic Auth introduces a specific vector: the Authorization header itself can become an untrusted input if the application extracts and uses the username or password fields in template rendering. For example, if a developer logs or personalizes content using request.authorization.username and passes it into a Jinja2 template without sanitization, an attacker can supply a crafted Basic Auth credential containing template syntax. Because Basic Auth credentials are base64-encoded but not encrypted, the injected payload is delivered verbatim to the server in the header, making it straightforward to test and iterate against the unauthenticated attack surface that middleBrick scans.

During a scan, middleBrick’s 12 security checks run in parallel and test the unauthenticated attack surface. For SSTI in this context, the system checks for template injection indicators in inputs that may originate from headers, including the Authorization header when credentials are present. A typical payload might be {{7*7}} or {% set ns = namespace() %} to confirm template code execution. If the application uses Flask with Jinja2 and reflects injected values in the response, middleBrick’s Input Validation and Data Exposure checks can surface the finding with severity guidance and remediation steps, even when no authentication is required to trigger the endpoint.

Flask’s default configuration does not automatically escape variables in render_template_string, so developers must explicitly apply escaping or avoid dynamic template strings altogether. The presence of Basic Auth does not mitigate SSTI; it can inadvertently supply new injection channels. An attacker may leverage predictable patterns such as log injection or error messages that include the username to refine their payload. This highlights the importance of validating and encoding all external inputs, including authentication artifacts, and favoring safer APIs like render_template with static templates.

Remediation focuses on preventing untrusted data from reaching Jinja2 templates and ensuring that any use of authentication-derived values is either omitted from rendering or rigorously controlled. The safest approach is to avoid using request.authorization in templates entirely. If you must display user context, pass only validated, non-sensitive data through controlled variables and use autoescaping mechanisms.

Secure Flask example with Basic Auth and safe rendering

from flask import Flask, request, render_template, jsonify
from werkzeug.security import check_password_hash

app = Flask(__name__)

# Hardcoded user store for demonstration; use a secure backend in production
USERS = {
    "alice": "pbkdf2:sha256:260000$...$...",  # pre-hashed password
}

@app.route('/profile')
def profile():
    auth = request.authorization
    if auth and auth.username in USERS and check_password_hash(USERS[auth.username], auth.password):
        # Safe: only pass explicitly validated, non-sensitive data to the template
        return render_template('profile.html', username=auth.username)
    return jsonify({"error": "Unauthorized"}), 401

Avoiding render_template_string with dynamic input

Never use render_template_string with user-controlled values, including those derived from Basic Auth. If dynamic templates are unavoidable, rigorously validate and escape input:

from flask import Flask, request, render_template_string
import re

app = Flask(__name__)

@app.route('/search')
def search():
    auth = request.authorization
    query = request.args.get('q', '')
    # Strict allowlist validation for any variable used in a dynamic template
    if not re.match(r'^[\w\-\s]+$', query):
        return "Invalid input", 400
    # Even with validation, prefer render_template for complex outputs
    template = "Search results for {{ query | e }}"
    return render_template_string(template, query=query)

Additional remediation steps include enforcing HTTPS to protect credentials in transit, setting the WWW-Authenticate header correctly, and using secure flags on cookies. Regularly test your endpoints with tools that include LLM security probes, as middleBrick does, to uncover indirect channels such as error messages or logging endpoints that might expose sensitive context when credentials are manipulated.