Dangling Dns in Gin with Api Keys

A dangling DNS configuration in a Gin application that relies on API keys for routing or service selection can expose internal or external resolution behavior to unauthenticated attackers. When API keys are used as route parameters or headers to determine backend services, an attacker can supply a crafted key that maps to a DNS name under attacker control. If the application resolves this name at runtime, the DNS response may point to an internal or unexpected host, bypassing intended network segmentation. This becomes a dangling DNS issue when the resolved endpoint is not the intended service, for example when a staging or legacy hostname is returned. middleBrick detects this pattern during its DNS and input validation checks, flagging cases where DNS resolution is influenced by unauthenticated input such as API keys.

In Gin, routes often use path or header parameters to select tenants or services. Consider a handler that reads an X-API-Key header to look up a customer record containing a backend hostname, then forwards the request after resolving that hostname via DNS. If the API key is attacker-controlled and the stored hostname is a DNS alias that can be modified to point to an internal service, the attacker can cause the application to resolve to an internal IP such as 127.0.0.1 or a service in a private network. This is a dangling DNS scenario because the application trusts a DNS response that may change after resolution, potentially leading to SSRF or unauthorized internal access. middleBrick’s checks for SSRF and input validation highlight this by correlating API key usage with DNS resolution patterns, ensuring that unexpected hosts are identified.

Another angle involves OpenAPI specifications that define security schemes using API keys. If the spec describes key-to-service mappings but the runtime implementation performs DNS resolution based on those keys without strict validation, the discrepancy between spec and runtime can be exploited. An attacker may probe endpoints with arbitrary keys to observe resolution behavior, infer internal hostnames, or cause the application to route traffic to malicious infrastructure. middleBrick’s OpenAPI/Swagger analysis resolves $ref definitions and cross-references them with runtime findings, revealing mismatches where DNS-dependent routing is not tightly constrained. This is especially important for services that use API keys to route across environments such as development, staging, and production, where dangling DNS records are more common.

To remediate dangling DNS risks when using API keys in Gin, enforce strict validation of hostnames and avoid runtime DNS resolution based on unauthenticated or attacker-influenced input. Prefer static mappings defined in configuration or service discovery, and if DNS resolution is necessary, resolve only against a strict allowlist of known hosts. The following examples show secure patterns for handling API keys in Gin without introducing dangling DNS vulnerabilities.

First, define a fixed mapping between API keys and allowed backend hosts in configuration rather than storing dynamic hostnames that can be modified via DNS. Then validate the incoming key against this mapping and use a constant hostname for requests:

package main

import (
    "net/http"
    "strings"

    "github.com/gin-gonic/gin"
)

type KeyConfig map[string]string // maps API key to allowed backend host

var allowedBackends = KeyConfig{
    "tenantA-key-123": "api.service-a.internal",
    "tenantB-key-456": "api.service-b.internal",
}

func ProxyHandler(c *gin.Context) {
    apiKey := c.GetHeader("X-API-Key")
    if apiKey == "" {
        c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "missing API key"})
        return
    }
    backend, ok := allowedBackends[apiKey]
    if !ok {
        c.AbortWithStatusJSON(http.StatusForbidden, gin.H{"error": "invalid API key"})
        return
    }
    // Use a static, validated backend URL; do not resolve hostnames from external input
    req, _ := http.NewRequest(c.Request.Method, "https://"+backend+c.Request.URL.RequestURI(), nil)
    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        c.AbortWithStatusJSON(http.StatusBadGateway, gin.H{"error": "upstream error"})
        return
    }
    defer resp.Body.Close()
    for k, v := range resp.Header {
        c.Header(k, v[0])
    }
    c.Status(resp.StatusCode)
}

Second, if you must perform DNS resolution, resolve only against a predefined set of hosts and avoid using raw user input in the resolution process. Use net.LookupHost with strict hostname validation and reject responses that do not match expected patterns:

package main

import (
    "net"
    "net/http"
    "strings"

    "github.com/gin-gonic/gin"
)

var allowedSuffixes = map[string]string{
    "tenantA": "service-a.prod.example.com",
    "tenantB": "service-b.prod.example.com",
}

func SafeResolveHandler(c *gin.Context) {
    tenantID := c.Param("tenant")
    hostname, ok := allowedSuffixes[tenantID]
    if !ok {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "unknown tenant"})
        return
    }
    ips, err := net.LookupHost(hostname)
    if err != nil || len(ips) == 0 {
        c.AbortWithStatusJSON(http.StatusInternalServerError, gin.H{"error": "resolution failed"})
        return
    }
    // Use the first IP only if it matches expected subnet; otherwise reject
    if !strings.HasPrefix(ips[0], "10.0.") {
        c.AbortWithStatusJSON(http.StatusForbidden, gin.H{"error": "unexpected resolution"})
        return
    }
    c.JSON(http.StatusOK, gin.H{"resolved_ip": ips[0]})
}

These patterns ensure that API keys do not indirectly control DNS resolution paths, reducing the risk of dangling DNS and related SSRF or internal access issues. middleBrick’s authentication and input validation checks can verify that such mitigations are correctly implemented in your Gin routes.