Dangling Dns in Fastapi with Dynamodb

A dangling DNS record occurs when a hostname (CNAME or alias) points to a resource that no longer exists or has been reassigned. In a FastAPI application that uses Amazon DynamoDB, this can happen when an environment variable, configuration, or service discovery mechanism references a DNS name that was decommissioned or repurposed. If the FastAPI service resolves the dangling name to an unintended endpoint, requests may be routed to an unmanaged or attacker-controlled service instead of the intended DynamoDB endpoint.

When the FastAPI backend makes HTTP-based calls to a data service (for example, through a custom integration layer or a proxy) and uses a dangling DNS name, the application may inadvertently connect to a malicious host. An attacker who registers the orphaned DNS namespace can observe or manipulate traffic that the application believes is destined for DynamoDB. This can expose API keys, IAM credentials, or sensitive data if those are transmitted or handled in the request pipeline. Even when the application uses the AWS SDK directly, misconfigured endpoint URLs or custom AWS session endpoints based on dynamic configuration can similarly redirect traffic if a dangling DNS entry is used to construct the base URL.

In the context of middleBrick’s black-box scans, a dangling DNS issue may surface when runtime checks detect that a hostname used in endpoint construction does not resolve to the expected provider or service. For example, a misconfigured AWS SDK endpoint that points to a stale CNAME may still return successful TLS handshakes from an unexpected host, causing the scan to flag the inconsistency between the intended DynamoDB service and the observed endpoint. This becomes a security risk because the application’s data operations are no longer directed to the trusted AWS infrastructure, increasing the chance of data exposure or injection attacks.

Because FastAPI applications often rely on environment variables and dependency injection for configuration, a dangling DNS entry can be introduced through CI/CD changes or infrastructure updates without code modifications. The risk is compounded when the application also performs input validation based on hostname patterns or uses relaxed URL parsing that does not strictly enforce expected domain boundaries. middleBrick’s checks include DNS resolution and endpoint verification to highlight such inconsistencies before they can be exploited in production.

To prevent issues related to dangling DNS in a FastAPI application using DynamoDB, ensure that endpoint configuration is explicit, verified, and isolated from mutable or user-influenced sources. Use hardcoded AWS SDK endpoints only when necessary, and validate any dynamic endpoint values against an allowlist of known, expected domains.

Below are concrete code examples for a FastAPI service that safely interacts with DynamoDB using the AWS SDK for Python (boto3). The examples avoid dynamic endpoint construction from untrusted input and demonstrate explicit configuration.

from fastapi import FastAPI, HTTPException
import boto3
from botocore.exceptions import ClientError, EndpointConnectionError

app = FastAPI()

# Explicit endpoint configuration for DynamoDB (do not derive from user input)
AWS_REGION = "us-east-1"
DYNAMODB_TABLE = "example-table"

# Use a static session with a verified endpoint URL if required; avoid runtime concatenation
session = boto3.session.Session(region_name=AWS_REGION)
dynamodb = session.resource(
    "dynamodb",
    endpoint_url=None,  # Explicitly None ensures AWS SDK uses the default, trusted endpoint
)

table = dynamodb.Table(DYNAMODB_TABLE)

@app.get("/items/{item_id}")
async def read_item(item_id: str):
    if not item_id or not item_id.isalnum():
        raise HTTPException(status_code=400, detail="Invalid item_id")
    try:
        response = table.get_item(Key={"id": item_id})
        item = response.get("Item")
        if item is None:
            raise HTTPException(status_code=404, detail="Item not found")
        return item
    except ClientError as e:
        raise HTTPException(status_code=500, detail=f"AWS error: {e.response['Error']['Code']}")
    except EndpointConnectionError:
        raise HTTPException(status_code=503, detail="Unable to connect to data store")

If you must use a custom endpoint (for example, when working with a proxy or a local mock), validate it strictly:

import re
from urllib.parse import urlparse

ALLOWED_ENDPOINT_DOMAINS = {"dynamodb.us-east-1.amazonaws.com"}

def validate_endpoint(url: str) -> bool:
    parsed = urlparse(url)
    return parsed.hostname in ALLOWED_ENDPOINT_DOMAINS

custom_endpoint = "https://dynamodb.us-east-1.amazonaws.com"
if not validate_endpoint(custom_endpoint):
    raise ValueError("Disallowed endpoint")

# Safe construction when necessary
session = boto3.session.Session(region_name=AWS_REGION)
dynamodb = session.resource("dynamodb", endpoint_url=custom_endpoint if validate_endpoint(custom_endpoint) else None)

These patterns reduce the risk of a dangling DNS entry redirecting traffic by removing implicit or mutable resolution steps and by enforcing domain checks. When combined with continuous scanning using a tool like middleBrick Pro, which supports configurable schedules and CI/CD integration, you can detect configuration drift and DNS inconsistencies before they reach production.