Dangling Dns in Flask with Dynamodb

A dangling DNS risk in a Flask application that uses DynamoDB typically occurs when the application resolves a custom domain (for example, an internal or partner endpoint) and passes the resulting value into a DynamoDB operation without strict validation. Because middleBrick tests unauthenticated attack surfaces and includes SSRF and input validation checks among its 12 parallel security checks, it can surface cases where an attacker can control DNS resolution to indirectly influence DynamoDB-related network paths or metadata queries.

Consider a Flask route that looks up a record by a user-supplied hostname that is resolved via DNS before being used in a DynamoDB request:

import boto3
from flask import Flask, request

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

@app.route('/item')
def get_item():
    host = request.args.get('host')  # attacker-controlled
    ip = socket.gethostbyname(host)  # DNS resolution
    table = dynamodb.Table('Items')
    response = table.get_item(Key={'origin': ip})
    return response.get('Item', {})

If the hostname resolves to an internal or unexpected address (for example, an AWS metadata service IP such as 169.254.169.254), the DynamoDB request may be redirected or expose internal service interactions. An SSRF check within middleBrick would flag this behavior, while the input validation check would note that the hostname is not validated against an allowlist. In this configuration, a dangling DNS entry or a compromised DNS resolver can cause the application to interact with unintended endpoints, potentially leading to data exposure or SSRF when combined with DynamoDB metadata queries.

Additionally, if the application uses a custom endpoint for DynamoDB (for example, to route through a VPC endpoint or proxy), an attacker who can influence DNS may redirect traffic to a malicious proxy. middleByte’s SSRF and Unsafe Consumption checks are designed to detect unexpected network paths and over-permissive consumption patterns, highlighting cases where DNS-driven endpoint selection interacts with DynamoDB client configuration.

Another scenario involves DynamoDB streams or event sources that reference a DNS name (such as an SQS URL or a Lambda ARN that relies on DNS). If the DNS record is not properly controlled, updates to the DNS record can cause the application to consume events from unintended resources. The Inventory Management check in middleBrick examines configuration and runtime endpoints to surface mismatches between declared and observed targets.

To mitigate dangling DNS risks when using Flask with DynamoDB, enforce strict input validation, avoid DNS resolution of untrusted inputs, and explicitly configure endpoints and resource identifiers. Below are concrete, safe patterns and code examples aligned with the checks performed by middleBrick.

1. Validate hostnames against an allowlist and avoid runtime DNS resolution for user input

import boto3
from flask import Flask, request, abort

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

ALLOWED_ORIGINS = {'app.example.com', 'internal.example.com'}

@app.route('/item')
def get_item():
    host = request.args.get('host')
    if host not in ALLOWED_ORIGINS:
        abort(400, 'Invalid host')
    table = dynamodb.Table('Items')
    response = table.get_item(Key={'origin': host})
    return response.get('Item', {})

2. Use explicit resource identifiers instead of DNS-derived values for DynamoDB keys

@app.route('/item/v2')
def get_item_v2():
    item_id = request.args.get('id')
    # Validate item_id format to prevent injection or enumeration abuse
    if not item_id or not item_id.isalnum():
        abort(400, 'Invalid item identifier')
    table = dynamodb.Table('Items')
    response = table.get_item(Key={'id': item_id})
    return response.get('Item', {})

3. Avoid custom DynamoDB endpoints driven by DNS; if required, hardcode or securely configure them

import boto3

# Hardcode or load from a secure configuration, not from user input or dynamic DNS
dynamodb = boto3.client(
    'dynamodb',
    endpoint_url='https://dynamodb.us-east-1.amazonaws.com',
    region_name='us-east-1'
)

4. If integrating with event sources (e.g., DynamoDB Streams), validate stream ARNs and avoid DNS-based routing

# Example: verifying a stream ARN format before use
import re

def is_valid_dynamodb_stream_arn(arn: str) -> bool:
    pattern = r'^arn:aws:dynamodb:[a-z0-9\-]+:\d{12}:table/[^/]+/stream/[0-9]{4}\.[0-9]{2}\.[0-9]{2}T[0-9]{2}:[0-9]{2}:[0-9]{2}(\.[0-9]+)?Z$'
    return re.match(pattern, arn) is not None

# In application logic, ensure the stream ARN matches expected pattern before processing

By combining these practices with the continuous monitoring capabilities of the middleBrick Pro plan—where scans run on a configurable schedule and findings map to frameworks such as OWASP API Top 10—you can detect regressions in validation and configuration early. The CLI tool allows you to integrate checks into local development, and the GitHub Action can fail builds if a submitted endpoint configuration introduces unresolved DNS dependencies.