Cross Site Request Forgery in Gorilla Mux with Dynamodb

Cross Site Request Forgery (CSRF) in a Gorilla Mux service that uses DynamoDB can occur when state-changing HTTP methods (POST, PUT, DELETE) rely on cookie-based session identifiers without additional anti-CSRF controls. In this stack, the web application typically uses cookie-based session management (for example a session cookie set after login), and Gorilla Mux routes requests to backend handlers that directly invoke AWS SDK operations against DynamoDB. Because DynamoDB itself is an authenticated backend service (calls use AWS credentials on the server), the CSRF risk centers on the web layer: a malicious site can trick a user’s browser into issuing authenticated requests to the Gorilla Mux endpoints, and those requests result in legitimate DynamoDB writes or deletions on behalf of the user.

Consider a handler created with Gorilla Mux that deletes a DynamoDB item using the AWS SDK for Go. If the handler trusts only the session cookie and does not validate a same-site cookie attribute or an anti-CSRF token, an attacker can craft a form on another site that submits a DELETE or POST to that endpoint. The browser automatically includes the session cookie, the handler parses the Gorilla Mux route variables (e.g., an item ID from the URL path), and issues a DynamoDB DeleteItem or UpdateItem call. From the attacker’s perspective, the malicious page initiates a request that the vulnerable Gorilla Mux route accepts and executes against DynamoDB, leading to unauthorized data modification or deletion.

Specific factors in this combination increase exposure. Gorilla Mux encourages explicit route definitions that can tightly bind path parameters to backend actions (for example, /items/{id}), making it straightforward for an attacker to guess or enumerate resource identifiers. If the application does not implement per-user authorization checks that ensure the authenticated user owns or is permitted to modify the targeted DynamoDB item, a forged request may succeed purely based on a valid session cookie and a known item ID pattern. Moreover, if CORS is misconfigured and preflight responses are overly permissive, browser-based scripts from untrusted origins can more easily issue authenticated requests to the Gorilla Mux handlers, compounding the CSRF surface that ultimately results in unintended DynamoDB operations.

Remediation focuses on ensuring every request that results in a DynamoDB mutation is bound to the authenticated user and validated within the application logic, rather than trusting path parameters or cookies alone. In Gorilla Mux, use middleware to verify session integrity and enforce anti-CSRF tokens for state-changing methods, and ensure DynamoDB operations include user identity checks in the condition expressions or key condition expressions.

Example secure handler with middleware and DynamoDB conditional write

The following example shows a Gorilla Mux handler that requires a valid session, validates an anti-CSRF token for mutating requests, and performs a DynamoDB UpdateItem with a condition that ties the operation to the authenticated user’s identity, preventing tampering on the item ID path parameter.

//go
package handlers

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

// sessionClaims is a simplified representation of validated session data.
type sessionClaims struct {
	UserID string
}

// getSession extracts and validates session claims from the request.
func getSession(r *http.Request) (*sessionClaims, bool) {
	// Validate cookie/session token; return nil, false if invalid.
	// This is app-specific and omitted for brevity.
	return &sessionClaims{UserID: "user-123"}, true
}

// requireCSRFToken validates a per-request CSRF token for unsafe methods.
func requireCSRFToken(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.Method == http.MethodPost || r.Method == http.MethodPut || r.Method == http.MethodDelete {
			reqToken := r.Header.Get("X-CSRF-Token")
			sessionToken := r.Header.Get("X-Session-Token") // example: stored in session
			if reqToken == "" || reqToken != sessionToken {
				http.Error(w, "invalid csrf token", http.StatusForbidden)
				return
			}
		}
		next.ServeHTTP(w, r)
	})
}

// deleteItemHandler safely deletes a DynamoDB item after CSRF and ownership checks.
func deleteItemClient(dynamoClient *dynamodb.Client) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		session, ok := getSession(r)
		if !ok {
			http.Error(w, "unauthorized", http.StatusUnauthorized)
			return
		}

		vars := mux.Vars(r)
		itemID := vars["id"]
		if itemID == "" {
			http.Error(w, "missing item id", http.StatusBadRequest)
			return
		}

		// Ensure the item belongs to the authenticated user.
		// This check can be performed via a pre-condition or a prior read.
		_, err := dynamoClient.GetItem(r.Context(), &dynamodb.GetItemInput{
			TableName: aws.String("Items"),
			Key: map[string]types.AttributeValue{
				"id": &types.AttributeValueMemberS{Value: itemID},
			},
		})
		if err != nil {
			http.Error(w, "unable to validate item", http.StatusInternalServerError)
			return
		}

		// Perform a conditional update/delete that includes the user ID.
		_, err = dynamoClient.DeleteItem(r.Context(), &dynamodb.DeleteItemInput{
			TableName: aws.String("Items"),
			Key: map[string]types.AttributeValue{
				"id": &types.AttributeValueMemberS{Value: itemID},
			},
			ConditionExpression: aws.String("user_id = :uid"),
			ExpressionAttributeValues: map[string]types.AttributeValue{
				":uid": &types.AttributeValueMemberS{Value: session.UserID},
			},
		})
		if err != nil {
			// Handle ConditionalCheckFailedException and other errors explicitly.
			http.Error(w, "deletion condition not met", http.StatusForbidden)
			return
		}

		w.WriteHeader(http.StatusNoContent)
	}
}

In this example, the Gorilla Mux route should be registered with the CSRF middleware for mutating methods, and the handler uses a ConditionExpression on the DynamoDB DeleteItem call to ensure the item’s user_id matches the authenticated user’s ID. This prevents an attacker from successfully deleting or modifying another user’s items even if they can guess or enumerate item IDs. Additionally, ensure cookies are sent with SameSite=Strict or Lax and that CORS policies restrict origins and methods to those required by your frontend.

Comparing insecure vs secure patterns

Additional recommendations: use SameSite cookies, implement strict CORS rules, and prefer POST or DELETE with a request body (where feasible) rather than relying solely on URL path parameters. For sensitive operations, consider requiring re-authentication (e.g., password or MFA confirmation) before executing destructive DynamoDB actions.