Broken Access Control in Gorilla Mux with Dynamodb

Broken Access Control occurs when authorization checks are missing or bypassed, allowing one user to access or modify another user's resources. Using Gorilla Mux—a popular HTTP router for Go—and Amazon DynamoDB together can unintentionally expose this vulnerability when route parameters (such as {id}) are directly used as DynamoDB keys without validating that the authenticated user is allowed to access the corresponding item.

In a typical setup, a route like /users/{id} maps to a handler that reads from DynamoDB using the path parameter as the primary key value. If the handler skips ownership verification—such as confirming that the authenticated user matches the requested user ID—an attacker can change the ID in the request and enumerate or modify other users' data. This is a classic BOLA/IDOR pattern, and DynamoDB does not enforce application-level ownership; it only enforces permissions attached to the credentials used to call the API. Therefore, if the service identity has broad read/write access, a missing per-request authorization check in Gorilla Mux leads to direct data exposure.

Moreover, DynamoDB's flexible schema can inadvertently amplify the risk. For example, if a table uses a composite key (partition key + sort key) and the handler only validates the partition key while allowing arbitrary sort key values via user input, an attacker may traverse unrelated data partitions. The combination of Gorilla Mux routing and DynamoDB key construction means that improper parameter-to-key mapping directly translates to an access control gap. Attack patterns like IDOR are commonly observed against APIs that expose sequential or guessable IDs without contextual authorization checks, and DynamoDB responses will return the requested item if the caller’s IAM policy permits it, regardless of business-level ownership.

OpenAPI/Swagger analysis can surface these issues by correlating path parameters with security schemes and required scopes, but the implementation must enforce checks in each handler. Without explicit authorization logic in the Gorilla Mux handler chain—such as extracting the authenticated subject and comparing it with the resource’s owning user ID—an attacker can leverage crafted requests to assume other identities. Instrumenting routes with middleware that validates ownership before constructing DynamoDB key conditions is essential to prevent broken access control in this stack.

To remediate Broken Access Control, enforce user-level authorization before issuing any DynamoDB operation in your Gorilla Mux handlers. Always derive the partition key from the authenticated subject rather than from user-supplied path parameters, or if you must accept a parameter, verify that it matches the authenticated identity.

// Example: secure handler with Gorilla Mux and DynamoDB
package main

import (
	"context"
	"net/http"
	"github.com/gorilla/mux"
	"github.com/aws/aws-sdk-go-v2/service/dynamodb"
	"github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
)

type identityResolver func(r *http.Request) (string, error) // returns userID

type handler struct {
	db       *dynamodb.Client
	resolver identityResolver
}

func (h *handler) getUserProfile(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	// 1) Authenticated subject (e.g., from JWT)
	subject, err := h.resolver(r)
	if err != nil {
		http.Error(w, "unauthorized", http.StatusUnauthorized)
		return
	}

	// 2) Route parameter (potentially attacker-controlled)
	vars := mux.Vars(r)
	requestedID := vars["id"]

	// 3) Authorization check: ensure subject matches requestedID
	if subject != requestedID {
		http.Error(w, "forbidden: cannot access other user's data", http.StatusForbidden)
		return
	}

	// 4) Build key from subject, not from vars, to avoid injection or mismatch
	key := map[string]types.AttributeValue{
		"user_id": &types.AttributeValueMemberS{Value: subject},
	}
	out, err := h.db.GetItem(ctx, &dynamodb.GetItemInput{
		TableName:                 aws.String("Users"),
		Key:                       key,
		ConsistentRead:            aws.Bool(true),
	})
	if err != nil {
		http.Error(w, "internal error", http.StatusInternalServerError)
		return
	}
	if out.Item == nil {
		http.Error(w, "not found", http.StatusNotFound)
		return
	}
	// serialize out.Item as needed
	w.WriteHeader(http.StatusOK)
}

In this pattern, the Gorilla Mux route can stay generic (e.g., /users/{id}), but the handler resolves the authenticated identity separately and compares it to the path parameter before using any user input to form the DynamoDB key. This ensures that even if an attacker changes {id}, the authorization check blocks access to other users' items.

For queries that require filtering by attributes beyond the primary key, use the authenticated subject as a filter expression condition rather than relying on sort key ranges supplied by the client. For example, if you must scan or query a table with a global secondary index, enforce the subject condition on the client side after retrieval, but prefer designing the partition key to include the user ID so that GetItem or Query operations are naturally scoped to a single user. This DynamoDB-specific remediation, combined with Gorilla Mux middleware that centralizes ownership validation, closes the broken access control vector.