Xml External Entities in Flask with Dynamodb

XML External Entity (XXE) injection occurs when an application processes XML input that references external entities. In Flask applications that accept XML payloads and forward data to AWS DynamoDB, the risk is twofold: the web layer can be abused to read local files or interact with internal systems, and the data sent to DynamoDB may reflect maliciously crafted content if not properly validated.

Flask does not parse XML by default, but if a developer adds an XML parser such as xml.etree.ElementTree or lxml and uses user-supplied XML to build requests, the parser may resolve external entities. A typical pattern is accepting an XML upload, extracting fields, and writing them to a DynamoDB table via the AWS SDK. If the XML includes entities like &file SYSTEM 'file:///etc/passwd', the parser may read sensitive files and include that content in the item stored in DynamoDB or in responses.

Because DynamoDB stores the data as provided, malicious content can persist and be retrieved later via read operations or exported backups. If the application later returns stored XML fragments or uses the data in HTML or JSON contexts without escaping, it may lead to reflected or stored injection. In an API security scan, findings related to Input Validation and Data Exposure would flag the absence of entity disabling and insufficient sanitization before persistence.

Consider a Flask route that accepts XML and writes it to DynamoDB:

from flask import Flask, request
import boto3
import xml.etree.ElementTree as ET

app = Flask(__name__)
dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
table = dynamodb.Table('Items')

@app.route('/submit', methods=['POST'])
def submit():
    xml_data = request.data
    root = ET.fromstring(xml_data)  # Potentially vulnerable
    item_id = root.find('id').text
    payload = root.find('data').text
    table.put_item(Item={'id': item_id, 'data': payload})
    return 'ok', 200

If the XML contains an external entity, ET.fromstring with a non-secure configuration may resolve it, depending on the parser backend and system settings. An attacker could craft a request that reads /etc/passwd and stores it in data, leading to Data Exposure. The DynamoDB table itself does not introduce XXE, but it becomes a storage sink for attacker-controlled content, increasing impact.

An API security scan would highlight missing controls around XML parsing, lack of schema validation, and potential Data Exposure when sensitive data is stored without sanitization. Without controls such as entity expansion limits and strict input validation, the attack surface includes both the web layer and the persistence layer.

Remediation focuses on preventing entity resolution at the parser level and validating data before it reaches DynamoDB. Use a parser configuration that disables external entities and DOCTYPE declarations. For xml.etree.ElementTree in Python 3.7+, you can use XMLParser with resolve_entities=False. For stricter safety, prefer defusedxml libraries which are designed to block XXE by default.

Below is a secure Flask example using defusedxml.ElementTree and defensive data handling before writing to DynamoDB:

from flask import Flask, request, jsonify
import boto3
from defusedxml.ElementTree import fromstring as safe_fromstring

app = Flask(__name__)
dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
table = dynamodb.Table('Items')

@app.route('/submit', methods=['POST'])
def submit():
    xml_data = request.data
    try:
        root = safe_fromstring(xml_data)  # Blocks external entities
    except Exception as e:
        return jsonify({'error': 'Invalid XML'}), 400

    item_id_elem = root.find('id')
    data_elem = root.find('data')
    if item_id_elem is None or data_elem is None:
        return jsonify({'error': 'Missing fields'}), 400

    item_id = item_id_elem.text
    payload = data_elem.text

    # Basic validation before persistence
    if not isinstance(item_id, str) or not isinstance(payload, str):
        return jsonify({'error': 'Invalid types'}), 400
    if len(item_id) > 256 or len(payload) > 65535:
        return jsonify({'error': 'Field length limits exceeded'}), 400

    # Store sanitized data to DynamoDB
    table.put_item(Item={
        'id': item_id,
        'data': payload
    })
    return jsonify({'status': 'ok'}), 200

Key remediation steps:

• Disable external entities and DOCTYPE processing in the XML parser.
• Validate and sanitize all extracted fields before sending to DynamoDB (type, length, allowed characters).
• Apply least privilege IAM roles to the DynamoDB table so that the Flask service can only write expected item structures.
• Log and monitor rejected inputs to detect probing attempts without exposing internal details to the client.

Even with secure parsing, treat data from external sources as untrusted. DynamoDB will store what you allow through your parser and validation logic. Combine secure coding practices with API security scanning to detect missing controls early, using tools that check Input Validation and Data Exposure checks alongside OWASP API Top 10 mappings.