Open Redirect in Gorilla Mux with Dynamodb

An open redirect in a Go service using Gorilla Mux and DynamoDB typically occurs when an endpoint accepts a user-supplied URL or host value and then performs an HTTP redirect without strict validation. If the redirect destination is derived from DynamoDB-stored configuration or user input without canonicalization and allowlisting, an attacker can supply a malicious location that causes the application to redirect victims to arbitrary external sites.

Consider a profile endpoint that reads a user’s preferred landing page from a DynamoDB table and redirects the browser:

• The application retrieves a redirect_url attribute from a DynamoDB item (e.g., via GetItem or Query).
• It then passes this value directly to http.Redirect or a Gorilla Mux handler that calls http.Redirect with http.StatusFound.

If the stored or supplied URL lacks validation, an attacker who can control the DynamoDB attribute (through compromised credentials or a secondary injection) or can influence the selection logic (through IDOR or BOLA) can cause the application to redirect any user to a phishing site. Because Gorilla Mux routes requests based on path patterns, an open redirect may be chained with IDOR to target specific users: an authenticated user’s stored redirect URL in DynamoDB can be abused to create a trusted-looking outbound redirect from your domain.

Common patterns that increase risk include:

• Using a query parameter such as next or returnTo that is validated only by prefix checks (e.g., starts with /) but not against a strict allowlist of trusted hosts.
• DynamoDB attributes that store hostnames or full URLs without schema enforcement, enabling mismatched schemes (e.g., javascript:...) or host confusion (e.g., example.attacker.com that passes a domain check due to substring matching).
• Inconsistent validation between inbound request validation and stored configuration values, allowing a malicious item to persist in DynamoDB and be used later in redirects.

Remediation centers on strict validation of any redirect target, whether sourced from DynamoDB or from request parameters. Use an allowlist of permitted hosts, normalize and parse URLs, and avoid using user-controlled values as the sole redirect target. Below are concrete, safe patterns for a Gorilla Mux handler that reads from DynamoDB.

1. Validate against an allowlist and use a canonical URL

Define a small set of trusted hosts and ensure the parsed host matches exactly (or via a controlled mapping). Reject unknown hosts and disallow dangerous schemes.

import (
	"net/url"
	"net/http"
	"github.com/gorilla/mux"
)

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

func safeRedirectFromDynamoDB(w http.ResponseWriter, r *http.Request) {
	vars := mux.Vars(r)
	userID := vars["userID"]

	// Fetch item from DynamoDB (pseudo-code; use AWS SDK in practice)
	item, err := getUserRedirectConfig(userID)
	if err != nil {
		http.Error(w, "internal error", http.StatusInternalServerError)
		return
	}

	target := item.RedirectURL
	if target == "" {
		target = "/dashboard"
	}

	if !isValidRedirect(target) {
		http.Error(w, "invalid redirect", http.StatusBadRequest)
		return
	}

	http.Redirect(w, r, target, http.StatusFound)
}

func isValidRedirect(raw string) bool {
	parsed, err := url.Parse(raw)
	if err != nil || parsed.Scheme != "" && parsed.Scheme != "https" {
		return false
	}
	if parsed.Host == "" {
		// path-only redirect; ensure it starts with /
		return parsed.Path != "" && parsed.Path[0] == '/'
	}
	return allowedRedirectHosts[parsed.Host]
}

2. DynamoDB fetch example with strict attribute checks

When reading a redirect URL from DynamoDB, treat the attribute as untrusted. Validate and canonicalize before any use. The example below uses the AWS SDK for Go (v2) and demonstrates safe retrieval and validation.

import (
	"context"
	"fmt"
	"net/url"
	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/service/dynamodb"
	"github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
)

type RedirectConfig struct {
	UserID string
	RedirectURL string
}

func getUserRedirectConfig(userID string) (*RedirectConfig, error) {
	cfg, err := getDynamoDBConfig()
	if err != nil {
		return nil, err
	}
	client := dynamodb.NewFromConfig(cfg)

	out, err := client.GetItem(context.TODO(), &dynamodb.GetItemInput{
		TableName: aws.String("UserPreferences"),
		Key: map[string]types.AttributeValue{
			"user_id": &types.AttributeValueMemberS{Value: userID},
		},
	})
	if err != nil {
		return nil, err
	}
	if out.Item == nil {
		return nil, fmt.Errorf("no item")
	}

	redirectURL := ""
	if v, ok := out.Item["redirect_url"]; ok {
		if s, ok := v.(*types.AttributeValueMemberS); ok {
			redirectURL = s.Value
		}
	}

	parsed, err := url.Parse(redirectURL)
	if err != nil || (parsed.Host != "" && parsed.Scheme != "https") {
		return nil, fmt.Errorf("invalid redirect_url in DynamoDB")
	}
	if parsed.Host != "" && !allowedRedirectHosts[parsed.Host] {
		return nil, fmt.Errorf("host not allowed")
	}

	return &RedirectConfig{
		UserID:      userID,
		RedirectURL: redirectURL,
	}, nil
}

3. Use Gorilla Mux route composition to constrain flows

Design routes so that redirect decisions are server-controlled where possible. If you must accept a target, ensure it is a relative path or a validated absolute URL that matches your allowlist. Combine this with middleware that logs suspicious redirect attempts for further analysis.

Mapping to compliance and scanning

Findings related to open redirects in this stack map to the OWASP API Top 10 (e.g., A05:2023 Security Misconfiguration) and can be surfaced by a scan via the middleBrick Web Dashboard or the CLI tool middlebrick scan <url>. For continuous assurance, the Pro plan supports scheduled scans and GitHub Action integration to fail builds if a redirect-related issue is detected, while the MCP Server lets you run checks directly from AI coding assistants in your IDE.