Xpath Injection in Flask with Bearer Tokens

Xpath Injection occurs when an attacker can influence an XPath expression constructed from user-controlled input, leading to authentication bypass or data exfiltration. In Flask applications that rely on XML-based authentication or configuration, combining XPath queries with Bearer Tokens can unintentionally expose sensitive logic or data if token handling and query construction are not carefully separated.

Consider a Flask route that extracts a Bearer Token from the Authorization header and uses it to query an XML store (for example, an XML database or SAML assertion) to resolve user roles:

from flask import request, jsonify
import defusedxml.ElementTree as ET

@app.route("/profile")
def profile():
    auth = request.headers.get("Authorization", "")
    token = auth.replace("Bearer ", "", 1) if auth.startswith("Bearer ") else ""
    # Unsafe: token value used to build an XPath expression
    expr = f"//user[token=\"{token}\"]/role"
    # Assume xml_data is loaded from an XML source
    result = xml_data.find(expr)
    if result is not None:
        return jsonify({"role": result.text})
    return jsonify({"error": "unauthorized"}), 401

If the token contains characters that affect XPath syntax (such as single quotes, double quotes, or function-like constructs), an attacker can manipulate the expression. For example, a token like ' or 1=1 or 'a'='a would alter the predicate, potentially returning roles for unintended users or bypassing authorization checks entirely.

Even when the token is expected to be opaque, treating it as raw data in an XPath query is risky. Bearer Tokens are often long, random strings, but they should never be directly interpolated into XPath expressions because:

• XPath has its own syntax and escaping rules; a token containing ' or " can break query structure.
• XML parsers may normalize or interpret certain characters, leading to unexpected matches.
• An XML store might include metadata or configuration nodes that could be enumerated or traversed via injection, exposing internal logic or relationships.

In a black-box scan, middleBrick would flag this as a BOLA/IDOR and Input Validation issue, noting that the application combines authentication material (Bearer Token) with data queries in an unsafe way. The scanner would also highlight that the XPath construction does not validate or sanitize the token, violating secure coding practices for input handling.

To understand the impact, compare this to a safer pattern where the token is used only for transport-layer authentication (e.g., validating the token via an introspection endpoint or JWT verification) and a separate, parameterized lookup identifies the user in the XML store:

from flask import request, jsonify
import jwt  # example JWT library
import defusedxml.ElementTree as ET

def get_user_role_from_xml(user_id):
    # Parameterized lookup: user_id is not part of the XPath string
    expr = "//user[@id=\"{}\"]/role".format(escape_for_xml(user_id))
    return xml_data.find(expr)

@app.route("/profile")
def profile():
    auth = request.headers.get("Authorization", "")
    token = auth.replace("Bearer ", "", 1) if auth.startswith("Bearer ") else ""
    try:
        payload = jwt.decode(token, options={"verify_signature": False})
        user_id = payload.get("sub")
        role = get_user_role_from_xml(user_id)
        if role is not None:
            return jsonify({"role": role.text})
    except Exception:
        pass
    return jsonify({"error": "unauthorized"}), 401

In this improved version, the Bearer Token is used strictly for authentication (e.g., JWT validation), while user identification is handled through a safe, parameterized approach that avoids XPath injection. middleBrick’s checks for Authentication and Input Validation would highlight the insecure pattern and guide developers toward separating concerns and using parameterized queries.

Remediation focuses on ensuring Bearer Tokens are never directly interpolated into XPath expressions and that token validation is separated from data access logic. Below are concrete, secure patterns for Flask applications.

1. Use token validation, not token-based XPath construction

Validate the Bearer Token using a library appropriate for your token format (e.g., PyJWT for JWTs). Do not use the raw token in queries:

from flask import request, jsonify, abort
import jwt
SECRET_KEY = "your-secret"

def verify_token(token):
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=["HS256"])
        return payload
    except jwt.InvalidTokenError:
        return None

@app.route("/profile")
def profile():
    auth = request.headers.get("Authorization", "")
    if not auth.startswith("Bearer "):
        abort(401, "Missing Bearer token")
    token = auth.split(" ", 1)[1]
    payload = verify_token(token)
    if not payload:
        abort(401, "Invalid token")
    user_id = payload.get("sub")
    role = safe_get_role_by_user_id(user_id)
    if role:
        return jsonify({"role": role})
    abort(403, "Insufficient permissions")

2. Parameterized XPath queries

If you must query XML, ensure user-derived identifiers are parameterized and escaped, not string-interpolated:

import defusedxml.ElementTree as ET
from markupsafe import escape

def safe_get_role_by_user_id(user_id):
    safe_id = escape(str(user_id))
    expr = f'//user[@id="{safe_id}"]/role'
    return xml_data.find(expr)

# Example usage within a route remains the same as above

3. Avoid XPath where possible; use structured APIs

For new features, prefer JSON-based APIs or database queries with ORM/parameterized statements instead of XML and XPath. If XML is required, use XSLT or DOM methods that accept parameters rather than string-based expressions.

4. MiddleBrick tooling integration

Using the CLI, you can validate these patterns by running:

middlebrick scan https://your-api.example.com

The dashboard and GitHub Action integrations can enforce that routes handling authentication do not exhibit unsafe XPath construction. The MCP Server allows you to scan API definitions directly from your IDE and catch issues before deployment.