Dns Cache Poisoning on Aws

In AWS environments, DNS cache poisoning typically appears when an application relies on the AmazonProvidedDNS resolver (the default VPC DNS) to resolve user‑supplied hostnames before making outbound calls. If an attacker can inject a malicious DNS response into the resolver’s cache, subsequent lookups for a benign domain will return an attacker‑controlled IP address. This can turn harmless functionality — such as a webhook URL parameter, an image‑fetch API, or a custom endpoint override — into a vector for SSRF, data exfiltration, or access to internal services.

A common AWS‑specific code path is found in Lambda functions or EC2‑based services that accept a url or host query parameter, then pass it straight to the AWS SDK or to http.request without validation. Example (Node.js):

const https = require('https');
exports.handler = async (event) => {
  const target = event.queryStringParameters.url; // user‑supplied
  const req = https.get(target, (res) => { /* … */ });
  req.end();
};

If the VPC’s DNS resolver has been poisoned to resolve example.com to 169.254.169.254 (the EC2 instance metadata service), the Lambda will inadvertently make a request to the metadata endpoint, leaking IAM role credentials. Similar patterns appear in API Gateway integrations that forward the host header to an upstream service, or in Step Functions tasks that invoke a Lambda with a user‑provided endpoint ARN.

The attack does not require credentials; it is purely unauthenticated and exploits the trust placed in the VPC DNS resolver.

Detecting DNS cache poisoning risk in AWS starts with finding APIs that accept uncontrolled domain or URL inputs and then use those values in outbound network calls. middleBrick’s unauthenticated black‑box scan checks for exactly this class of flaws:

• Input Validation – middleBrick sends fuzzed hostnames (e.g., example.com%0Ainternal) and observes whether the API reflects or acts on the tainted value.
• SSRF – the scanner attempts to cause the API to make requests to internal AWS endpoints (e.g., 169.254.169.254, fd00:ec2::254) by supplying hostnames that would resolve to those addresses if the DNS cache were poisoned.
• Property Authorization / Data Exposure – any leakage of credentials or metadata in the response is flagged as a finding.

When middleBrick detects that a user‑supplied hostname influences an outbound request without proper validation, it reports a finding with severity, remediation guidance, and a link to the OWASP API‑Top‑10 category A1:2023 – Broken Object Level Authorization (often manifested as SSRF via DNS manipulation).

Beyond middleBrick, AWS native telemetry can help confirm exposure:

• VPC Flow Logs – look for outbound traffic from Lambda/EC2 to the metadata service IP (169.254.169.254) that originates from a function that should only call public endpoints.
• Amazon GuardDuty – its DNS‑based threat detection flags queries to known malicious domains or unusual patterns (e.g., sudden bursts of queries for a domain that resolves to an internal IP).
• CloudWatch Logs Insights – query Lambda START and END logs for process.env.AWS_ENDPOINT_URL overrides or dns.lookup calls with user input.

Combining middleBrick’s API‑level findings with these AWS observability sources gives a clear picture of whether the API is susceptible to DNS cache poisoning.

The most effective mitigation is to eliminate the reliance on user‑supplied hostnames for DNS resolution altogether. When that is not possible, enforce strict validation and use AWS‑provided safeguards.

1. **Allowlist validation** – before making any outbound call, verify that the hostname matches a pre‑approved list. Example in Python (boto3‑based Lambda):

import re
ALLOWED_PATTERN = re.compile(r'^[a-z0-9.-]+\.(example\.com|trustedpartner\.net)$', re.IGNORECASE)

def validate_host(host):
    return bool(ALLOWED_PATTERN.match(host))

# In the handler
if not validate_host(user_host):
    raise ValueError('Host not allowed')
# Proceed with AWS SDK call using the validated host

2. **Disable endpoint overrides** – the AWS SDK permits custom endpoints via environment variables (AWS_ENDPOINT_URL) or the endpoint parameter. Ensure that code never reads these from user input. If you must support configurable endpoints, store them in AWS Systems Manager Parameter Store or Secrets Manager and retrieve them at init time, not per request.

3. **Use AWS PrivateLink or VPC Endpoints** – for calls to AWS services, provision interface VPC Endpoints so traffic never leaves the VPC and does not depend on the public DNS resolver. This removes the need to resolve public service names via AmazonProvidedDNS.

4. **Enable Route 53 Resolver DNS Firewall** – create a DNS Firewall rule group that blocks queries to known malicious domains and prevents resolution of IP ranges reserved for AWS internal services (e.g., 169.254.0.0/16). Attach the rule group to the VPC via the AWS Console, CLI, or CloudFormation:

aws route53resolver create-firewall-rule-group \
    --name block-internal-ips \
    --query 'FirewallRuleGroup.Id'

aws route53resolver create-firewall-rule \
    --firewall-rule-group-id $GRP_ID \
    --action BLOCK \
    --block-override DNS_REDIRECT \
    --block-override-domain '.' \
    --max-ttl 0 \
    --name block-metadata \
    --priority 100 \
    --block-response NODATA \
    --query 'FirewallRule.Id'

# Associate with VPC
aws route53resolver associate-firewall-rule-group \
    --firewall-rule-group-id $GRP_ID \
    --vpc-id $VPC_ID \
    --priority 100

5. **Leverage Amazon GuardDuty and VPC Flow Logs for runtime detection** – set up CloudWatch Alarms on GuardDuty findings of type Backdoor:EC2/C&CActivity.B!DNS or on Flow Logs entries showing traffic to 169.254.169.254 from unexpected sources. This provides an additional detection layer should a poisoned cache slip through.

By combining strict input validation, removal of user‑controlled endpoint overrides, VPC‑native traffic routing, and DNS Firewall controls, you eliminate the conditions that allow DNS cache poisoning to affect AWS‑hosted APIs.