Dns Cache Poisoning on Azure

DNS cache poisoning in Azure environments typically exploits misconfigurations in Azure DNS services and custom DNS resolvers deployed on Azure Virtual Machines or Azure Kubernetes Service (AKS). The attack pattern involves injecting malicious DNS records into Azure DNS caches, causing clients to resolve legitimate domain names to attacker-controlled IP addresses.

A common Azure-specific manifestation occurs when developers configure custom DNS resolvers using Azure Functions or Azure Container Instances without proper validation. Consider this vulnerable Azure Function DNS resolver:

const dns = require('dns');
const fetch = require('node-fetch');

module.exports = async function (context, req) {
    const domain = req.query.domain;
    
    // Vulnerable: No validation of DNS response
    const records = await dns.promises.resolve(domain, 'A');
    
    context.res = {
        body: { records }
    };
};

This function is susceptible because it blindly trusts DNS responses and caches them without validation. An attacker can poison the cache by responding to DNS queries with forged records before legitimate responses arrive.

Another Azure-specific scenario involves Azure Load Balancer configurations that use DNS-based health probes. If the health probe endpoints are compromised or use untrusted DNS resolvers, attackers can redirect traffic by poisoning the DNS cache that the health probe relies on.

Azure App Service applications face similar risks when using custom DNS configurations. Without proper DNSSEC validation or response validation, malicious actors can intercept traffic by returning fake DNS records that Azure services cache.

Detecting DNS cache poisoning in Azure requires both runtime monitoring and proactive scanning. middleBrick's Azure-specific scanning identifies cache poisoning vulnerabilities by testing how Azure services handle DNS responses.

For Azure Functions, middleBrick tests for:

• Missing DNS response validation in custom resolvers
• Insecure caching mechanisms that don't validate TTL values
• Lack of DNSSEC validation when querying external DNS servers
• Unprotected DNS endpoints that accept queries from any source

Here's how middleBrick detects DNS cache poisoning in Azure environments:

// middleBrick Azure DNS scan simulation
const scanAzureDns = async (azureFunctionUrl) => {
    const testCases = [
        { domain: 'example.com', spoofedResponse: '192.168.1.1' },
        { domain: 'azure.com', spoofedResponse: '10.0.0.1' },
        { domain: 'microsoft.com', spoofedResponse: '172.16.0.1' }
    ];
    
    const results = [];
    
    for (const test of testCases) {
        const response = await fetch(`${azureFunctionUrl}?domain=${test.domain}`, {
            headers: { 'X-Spoof-Attack': 'true' }
        });
        
        const data = await response.json();
        
        if (data.records.includes(test.spoofedResponse)) {
            results.push({
                domain: test.domain,
                vulnerability: 'DNS cache poisoning',
                severity: 'high',
                recommendation: 'Implement DNS response validation'
            });
        }
    }
    
    return results;
};

For Azure Kubernetes Service, middleBrick scans for:

• Insecure CoreDNS configurations that allow cache poisoning
• Missing DNS validation in service discovery mechanisms
• Unprotected DNS endpoints in cluster configurations

Azure Monitor can also be configured to detect unusual DNS patterns that might indicate cache poisoning attempts, such as:

• Sudden spikes in DNS query failures
• Unexpected changes in DNS response times
• Anomalous DNS traffic patterns from specific IP ranges

Remediating DNS cache poisoning in Azure requires implementing multiple layers of defense using Azure's native capabilities. The primary approach involves validating DNS responses and implementing secure caching mechanisms.

For Azure Functions DNS resolvers, implement response validation:

const dns = require('dns');
const crypto = require('crypto');

async function validateDnsResponse(domain, records) {
    // Validate record count and types
    if (!Array.isArray(records) || records.length === 0) {
        throw new Error('Invalid DNS response: empty records');
    }
    
    // Validate IP addresses
    for (const record of records) {
        if (!/^(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\./.test(record)) {
            throw new Error('Invalid DNS record format');
        }
    }
    
    // Implement DNSSEC validation if available
    try {
        const dnssec = await dns.promises.resolve(domain, 'DS');
        if (!dnssec || dnssec.length === 0) {
            console.warn('DNSSEC not available for domain');
        }
    } catch (error) {
        console.warn('DNSSEC validation failed:', error.message);
    }
    
    return true;
}

module.exports = async function (context, req) {
    const domain = req.query.domain;
    
    try {
        const records = await dns.promises.resolve(domain, 'A');
        
        // Validate response before caching
        await validateDnsResponse(domain, records);
        
        context.res = {
            body: { records, timestamp: Date.now() }
        };
    } catch (error) {
        context.res = {
            status: 500,
            body: { error: error.message }
        };
    }
};

For Azure App Service, configure secure DNS settings in the application configuration:

# Azure CLI commands for secure DNS configuration
az webapp config set --resource-group myResourceGroup \
    --name myAppService \
    --generic-configurations @{
        "DNS_RESOLVER": "8.8.8.8,1.1.1.1",
        "DNS_VALIDATION_ENABLED": "true",
        "DNS_CACHE_TTL_MINUTES": "5"
    }

# Configure Azure DNS with DNSSEC validation
az network dns record-set update --resource-group myResourceGroup \
    --zone-name mydomain.com \
    --name www \
    --type A \
    --target-resource /subscriptions/.../resourceGroups/.../providers/Microsoft.Network/dnszones/mydomain.com/A/www \
    --dnssec-enabled true

For Azure Kubernetes Service, secure CoreDNS configuration:

# Azure CLI for DNS traffic restrictions
az network nsg rule create --resource-group myResourceGroup \
    --nsg-name myNetworkSecurityGroup \
    --name DNSRestriction \
    --priority 200 \
    --direction Inbound \
    --access Allow \
    --source-address-prefixes 192.168.0.0/16 \
    --destination-address-prefixes 10.0.0.0/8 \
    --destination-port-ranges 53 \
    --protocol UDP

az network nsg rule create --resource-group myResourceGroup \
    --nsg-name myNetworkSecurityGroup \
    --name TrustedDNS \
    --priority 201 \
    --direction Outbound \
    --access Allow \
    --destination-address-prefixes 8.8.8.8/32 1.1.1.1/32 \
    --destination-port-ranges 53 \
    --protocol UDP