Dns Cache Poisoning in Chi with Dynamodb

DNS cache poisoning can occur in a Chi application that relies on Amazon DynamoDB for service discovery or configuration when DNS responses are manipulated and cached by a resolver used by the client or server. If Chi resolves service endpoints (such as a DynamoDB endpoint or a custom domain-backed table endpoint) using a poisoned cache, requests may be directed to an attacker-controlled host, leading to data exposure or injection. This is an application-layer risk amplified by how Chi performs hostname resolution and how DynamoDB clients construct requests based on resolved addresses.

In practice, if a Chi service performs hostname-to-IP resolution at startup or per request without validating DNS responses, and an attacker can inject a fraudulent response for the target hostname (e.g., a DynamoDB-compatible endpoint or a custom domain fronting DynamoDB), the poisoned entry may be used for subsequent requests. Because DynamoDB requires precise endpoint routing and signed requests, forwarding requests to an unintended host may cause failures or, in a multi-tenant setup, potential data leakage if the attacker can simulate a compatible service. The risk is not in DynamoDB itself but in how Chi uses DNS to locate services and in how requests are formed based on those lookups.

When using DynamoDB with Chi, consider the following: if Chi resolves a hostname such as dynamodb.example.com and the DNS cache is poisoned to return an attacker IP, any signed requests sent to that endpoint may be intercepted or altered. This is particularly relevant when using custom endpoints or when integrating with services that rely on DNS-based failover. The DynamoDB client in Chi must ensure that endpoint resolution is performed securely and that cached DNS entries do not persist beyond safe time-to-live (TTL) values.

Remediation focuses on ensuring DNS resolution is performed securely and that endpoint usage in Chi does not trust cached DNS indefinitely. Prefer IP literals or tightly controlled service discovery mechanisms where feasible, and enforce short DNS caches. Below are concrete examples for Chi using DynamoDB with a custom endpoint, demonstrating how to reduce risk.

Example 1: Using a static IP with DynamoDB client in Chi

By specifying an IP address or a controlled hostname with strict validation, you reduce reliance on external DNS caches.

import chi
import boto3
from botocore.config import Config

# Use a controlled endpoint with DNS validation disabled for static IPs
endpoint_url = "https://192.0.2.1:8443"  # static, pre-validated IP
client = boto3.client(
    "dynamodb",
    endpoint_url=endpoint_url,
    config=Config(signature_version="v4"),
    aws_access_key_id="YOUR_ACCESS_KEY",
    aws_secret_access_key="YOUR_SECRET_KEY"
)

response = client.get_item(
    TableName="secure-table",
    Key={"id": {"S": "item-123"}}
)
print(response.get("Item"))

Example 2: Short-lived DNS resolution with forced refresh in Chi

Force re-resolution on each request and avoid long-lived caches. This example shows how to configure a custom HTTP client in Chi to minimize cached DNS misuse.

import chi
import net
import boto3
from botocore.session import Session

# Create a custom resolver that re-resolves for each request
class ShortCacheResolver:
    def __init__(self):
        self.cache = {}

    def resolve(self, host):
        if host not in self.cache:
            # Perform fresh resolution; in real usage integrate with system resolver
            addrs = net.lookup_host(host)
            self.cache[host] = addrs[0] if addrs else None
        return self.cache[host]

resolver = ShortCacheResolver()
session = Session()
endpoint = resolver.resolve("dynamodb.example.com")
endpoint_url = f"https://{endpoint}:443"

client = session.create_client(
    "dynamodb",
    endpoint_url=endpoint_url,
    region_name="us-east-1"
)

# Use client as normal; ensure TLS verification is enabled
response = client.scan(TableName="my-table")
for item in response.get("Items", []):
    print(item)

Example 3: Validating hostname against a pinned list in Chi

Pin allowed hostnames or IPs and reject any resolution that does not match. This prevents poisoned entries from being used even if DNS cache is compromised.

import chi
import boto3

allowed_hosts = {"dynamodb.us-east-1.amazonaws.com", "198.51.100.2"}

hostname = "dynamodb.us-east-1.amazonaws.com"
if hostname not in allowed_hosts:
    raise ValueError("Unauthorized endpoint hostname")

client = boto3.client(
    "dynamodb",
    endpoint_url="https://" + hostname,
    region_name="us-east-1"
)

# Proceed with secure request
resp = client.get_item(TableName="approved-table", Key={"pk": {"S": "key"}})
print(resp.get("Item"))