Dns Cache Poisoning in Buffalo

DNS cache poisoning in Buffalo applications typically arises when user-controlled input is used to construct DNS queries without proper validation, enabling attackers to inject malicious DNS responses into resolver caches. Buffalo's github.com/gobuffalo/buffalo framework encourages rapid development, which can lead to oversight in input sanitization for network operations. A common pattern involves using query parameters or form data to dynamically set DNS server addresses or hostnames in internal tooling, such as custom admin panels for network diagnostics.

For example, a Buffalo handler might accept a hostname parameter to perform a DNS lookup for debugging purposes:

func DebugDNSHandler(c buffalo.Context) error {
    hostname := c.Param("hostname")
    if hostname == "" {
        return c.Error(400, errors.New("hostname required"))
    }
    // Vulnerable: no validation of hostname before DNS lookup
    addrs, err := net.LookupHost(hostname)
    if err != nil {
        return c.Error(500, err)
    }
    return c.Render(200, r.JSON(map[string]interface{}{
        "host": hostname,
        "ips": addrs,
    }))
}

An attacker could supply a value like attacker-controlled-domain.com%0AIN%20A%2010.0.0.1 (URL-encoded newline and DNS record) to attempt cache poisoning if the application uses a vulnerable DNS resolver or forwards the input to an internal DNS proxy. While Buffalo itself does not perform DNS resolution, the net.LookupHost call uses the system resolver, which may be susceptible to poisoning if the resolver cache is shared and the attacker can predict transaction IDs or exploit source port randomization flaws (e.g., CVE-2008-1447). In microservices architectures, such endpoints could be chained with SSRF to poison internal service discovery DNS entries.

middleBrick detects this vector during its Input Validation and SSRF checks by identifying unsanitized user input passed to network-facing functions like net.LookupHost, net.ResolveTCPAddr, or HTTP clients that may trigger DNS lookups. It flags endpoints where user input influences DNS queries without validation against an allowlist of safe characters or domains.

Identifying DNS cache poisoning risks in Buffalo applications requires scanning for patterns where user input flows into DNS resolution functions without adequate sanitization. middleBrick’s black-box scan analyzes endpoint behavior and OpenAPI specifications (if available) to detect such flows. For instance, if a Buffalo API defines a GET /debug/dns/{hostname} endpoint in its Swagger spec, middleBrick will test it with payloads designed to provoke DNS-based anomalies.

During a scan, middleBrick sends active probes such as:

• Hostnames containing DNS meta-characters (e.g., %0A for newline, % for hex encoding)
• Subdomains designed to trigger predictable resolver behavior (e.g., random12345.example.com)
• Payloads mimicking known attack patterns from CVE-2008-1447, such as repeated queries for non-existent subdomains to increase poisoning window

If the application reflects user input in DNS responses or makes external HTTP calls based on resolved IPs (indicating successful resolution of attacker-influenced names), middleBrick logs this as a potential Input Validation or SSRF finding with medium to high severity. The scan correlates runtime behavior with spec definitions: if the OpenAPI document marks the hostname parameter as string without pattern or enum constraints, and the endpoint is unauthenticated, the risk score increases.

For example, middleBrick’s CLI might output:

$ middlebrick scan https://api.example.com
...
[INPUT VALIDATION] Medium risk: User input in 'hostname' parameter passed to net.LookupHost without sanitization
[SSRF] Low risk: Potential for DNS rebinding via uncontrolled hostname resolution
...

This detection works without agents or configuration—simply providing the API URL triggers the check. Teams can integrate this into CI via the GitHub Action to block PRs if the risk score for Input Validation exceeds a threshold (e.g., score > 30 = fail build).

Remediating DNS cache poisoning risks in Buffalo applications centers on strict input validation and avoiding direct use of user input in DNS queries. Buffalo’s ecosystem supports robust validation through the github.com/gobuffalo/validate package and custom middleware. The fix involves validating the hostname parameter against a strict allowlist before any network operation.

Updated handler using Buffalo’s validation:

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/validate"
    "net"
)

func DebugDNSHandler(c buffalo.Context) error {
    hostname := c.Param("hostname")
    v := validate.New()
    // Only allow alphanumeric, hyphen, dot; max length 253 (RFC 1035)
    v.StringMatch(hostname, "^[a-zA-Z0-9][a-zA-Z0-9.-]{1,251}[a-zA-Z0-9]$", "hostname", "must be a valid hostname")
    if v.HasAny() {
        return c.Error(400, v)
    }
    // Additional safety: ensure it's not an IP address (if hostnames only are allowed)
    if net.ParseIP(hostname) != nil {
        return c.Error(400, errors.New("IP addresses not allowed"))
    }
    addrs, err := net.LookupHost(hostname)
    if err != nil {
        return c.Error(500, err)
    }
    return c.Render(200, r.JSON(map[string]interface{}{
        "host": hostname,
        "ips": addrs,
    }))
}

For applications requiring broader input (e.g., user-provided domains for legitimate purposes), implement an allowlist of approved domains or use a dedicated DNS library with built-in validation like github.com/miekg/dns, which constructs messages safely and avoids injection risks. Never concatenate user input into raw DNS queries.

Architecturally, consider:

• Using Buffalo middleware to centralize validation for all incoming parameters
• Restricting debug endpoints to internal networks or requiring authentication
• Logging and monitoring DNS query patterns for anomalies (e.g., sudden spikes in NXDOMAIN responses)

middleBrick’s Pro plan includes continuous monitoring that can re-scan APIs on a schedule to ensure validation logic remains effective as code evolves. After deploying fixes, rescan via the Dashboard or CLI to confirm the Input Validation risk score improves. Remember: middleBrick detects and reports—it does not apply fixes. Remediation guidance is provided, but implementation rests with the development team using Buffalo’s native tools.