Dns Cache Poisoning in Gorilla Mux with Dynamodb

DNS cache poisoning can arise in a Gorilla Mux routing setup backed by DynamoDB when an API service relies on host-based routing or redirection logic that is influenced by data stored in DynamoDB. If an attacker can influence the data in DynamoDB—such as a hostname, redirect target, or CNAME stored as part of routing configuration—and that data is later read by the Gorilla Mux router to construct or select a downstream endpoint, poisoned values can redirect traffic to malicious hosts.

Gorilla Mux is a URL router and reverse proxy often used to route requests to backend services. When DynamoDB is used as a configuration or service registry, a compromised or malicious entry can change where requests are forwarded. For example, if route hostnames or target URLs are stored in DynamoDB and retrieved at runtime to build redirect URLs or select upstream services, an attacker who can modify those records can effectively poison the decision logic within Gorilla Mux without directly touching the router config.

This becomes a practical concern when DynamoDB data is used to populate HTTP redirect targets or to select backend hosts based on hostname matching. If the application does not validate or sanitize data from DynamoDB before using it to construct URLs or set redirect headers, an attacker can inject a malicious hostname or path. Because Gorilla Mux operates at the HTTP routing layer, the poisoned hostname may cause requests to be forwarded to an attacker-controlled server, enabling interception of credentials or session tokens intended for legitimate services.

The threat chain typically involves: (1) compromise of a DynamoDB entry that holds routing configuration or redirect mappings; (2) retrieval of that entry by application code used by Gorilla Mux; (3) unsafe use of the retrieved values to construct a redirect or route selection; and (4) exploitation by a victim whose request is redirected to a malicious host. This pattern is especially risky when combined with trust placed in DNS names stored in DynamoDB, because DNS cache poisoning on the client or resolver side can compound the issue by causing repeated redirection to a malicious IP even after the DynamoDB entry is corrected.

To detect such issues, scanning with tools that perform black-box testing combined with spec analysis can surface routes where host-based routing or redirect logic depends on external data sources like DynamoDB. middleBrick, for example, runs checks that correlate OpenAPI routing definitions with runtime behavior and can flag endpoints where redirects or host selections reference dynamic values without strict validation, helping highlight configurations that could be abused via poisoned DynamoDB entries.

Remediation focuses on ensuring that any data read from DynamoDB before being used by Gorilla Mux for routing or redirect decisions is validated, normalized, and never directly reflected into URLs or headers without strict allowlisting.

First, validate and restrict hostnames or redirect targets against a strict allowlist before using them in Gorilla Mux routes. Do not allow arbitrary values from DynamoDB to set the Host header or redirect location.

import (
    "fmt"
    "net/http"
    "github.com/gorilla/mux"
    "github.com/aws/aws-sdk-go/service/dynamodb"
)

var allowedHosts = map[string]bool{
    "api.example.com": true,
    "app.example.com": true,
}

func safeRedirectHandler(dbSvc DynamoDBLookup) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Assume GetTargetHost queries DynamoDB and returns a hostname
        target, err := dbSvc.GetTargetHost(r.Context(), &dynamodb.GetItemInput{
            TableName: aws.String("RouterConfig"),
            Key: map[string]*dynamodb.AttributeValue{
                "RouteID": {S: aws.String("v1-api")}, // example key
            },
        })
        if err != nil {
            http.Error(w, "service unavailable", http.StatusServiceUnavailable)
            return
        }
        if !allowedHosts[target] {
            http.Error(w, "invalid route configuration", http.StatusBadRequest)
            return
        }
        http.Redirect(w, r, "https://"+target+r.URL.RequestURI(), http.StatusTemporaryRedirect)
    }
}

type DynamoDBLookup interface {
    GetTargetHost(context.Context, *dynamodb.GetItemInput) (string, error)
}

Second, avoid using raw DynamoDB values in Location headers or in constructing absolute URLs that Gorilla Mux might serve as redirects. If you must store redirect mappings in DynamoDB, store only path segments or internal identifiers, and map them to fixed, validated base URLs in code.

func buildSafeRedirect(pathID string) (string, error) {
    // Fetch a path segment or key from DynamoDB, not a full URL
    destPath, err := getPathFromDynamo(pathID)
    if err != nil {
        return "", err
    }
    base := "https://api.example.com"
    // Validate destPath to prevent open redirects
    if !strings.HasPrefix(destPath, "/safe/") {
        return "", fmt.Errorf("invalid path")
    }
    return base + destPath, nil
}

Third, enforce least privilege for the DynamoDB IAM role used by the application. The role should only have permission to read the specific items needed for routing configuration, and not broader write or administrative rights that an attacker could abuse to modify routing entries.

Finally, monitor and log redirect outcomes and hostname resolution events. Correlating logs with DynamoDB access patterns can reveal anomalous changes to routing configuration that may indicate an attempted poisoning or compromise. middleBrick scans can help identify routes where redirect or host selection logic lacks strict validation, providing findings mapped to relevant standards such as OWASP API Top 10 and CWE categories to support remediation planning.