Request Smuggling in Fiber with Dynamodb

Request smuggling occurs when an HTTP request is interpreted differently by a frontend proxy and a backend server. In a Fiber application that uses DynamoDB as a persistence layer, this often arises from inconsistent handling of HTTP message framing—typically between a reverse proxy or load balancer and the Fiber server. If the proxy and Fiber parse Content-Length and Transfer-Encoding headers differently, an attacker can craft a request that is processed as two separate requests by the two layers. The first request might target the proxy’s routing logic, while the second is interpreted by Fiber as a distinct request that reaches application code and DynamoDB operations.

Because DynamoDB operations in Fiber are typically invoked within handler functions, a smuggled request can cause unintended data access or mutations. For example, a request that smuggles an extra path or method may cause the application to issue a GetItem or PutItem call with an unexpected identifier, exposing or modifying data belonging to another user. This becomes especially risky when authorization checks are performed inconsistently across layers; the proxy may allow access based on route patterns while Fiber relies on application-level middleware. The vulnerability is not in DynamoDB itself, but in how requests reach DynamoDB calls within Fiber when message parsing diverges between infrastructure and application logic.

Consider a scenario where a proxy accepts a request with both Content-Length: 0 and Transfer-Encoding: chunked. Fiber may process only the chunked body, while the proxy interprets the request as two separate requests. An attacker can smuggle a second request such as POST /users/other-id/profile, which Fiber processes with a valid session but an attacker-controlled ID. If the handler directly uses the ID to construct a DynamoDB GetItem input without strict validation, the attacker can read or overwrite data they should not access.

To detect this with middleBrick, you can submit your Fiber endpoint’s base URL for a black-box scan. The scanner runs checks including Input Validation and BOLA/IDOR in parallel, and because it tests the unauthenticated attack surface, it can surface inconsistencies in how requests are interpreted before they reach DynamoDB. Findings include severity ratings and remediation guidance mapped to frameworks such as OWASP API Top 10, helping you prioritize fixes specific to your Fiber + DynamoDB stack.

Remediation focuses on normalizing HTTP message parsing in Fiber and ensuring DynamoDB inputs are strictly validated and independent of potentially smuggled headers. Below are concrete, idiomatic Go examples for a Fiber handler that safely interacts with DynamoDB using the AWS SDK for Go v2.

1. Enforce a single message parsing strategy

Configure your reverse proxy to reject requests that contain both Content-Length and Transfer-Encoding. On the Fiber side, avoid relying on c.Request().Header for routing or ID extraction when headers may be ambiguous. Instead, normalize inputs explicitly.

2. Validate and sanitize all user-supplied identifiers before DynamoDB use

Never trust path parameters or headers that may have been duplicated or altered by smuggling. Use strict schema validation and canonicalization.

// main.go
package main

import (
	"context"
	"fmt"
	"net/http"
	"regexp"
	"strings"

	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/fiber/v2/middleware/limiter"
	"github.com/aws/aws-sdk-go-v2/service/dynamodb"
	"github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
)

// sanitizeUserID ensures the ID contains only safe characters and is non-empty.
func sanitizeUserID(input string) (string, bool) {
	// Allow alphanumeric, underscore, hyphen; reject path traversal or control chars.
	re := regexp.MustCompile(`^[A-Za-z0-9_-]{1,64}$`)
	if !re.MatchString(input) {
		return "", false
	}
	return strings.TrimSpace(input), true
}

func main() {
	app := fiber.New()

	// Apply global rate limiting to reduce abuse surface.
	app.Use(limiter.New(limiter.Config{
		Max:     100,
		Expiry:  60,
	}))

	dynamoClient := dynamodb.NewFromConfig(awsConfig)

	app.Get("/users/:id/profile", func(c *fiber.Ctx) error {
		userID := c.Params("id")
		safeID, ok := sanitizeUserID(userID)
		if !ok {
			return c.Status(http.StatusBadRequest).SendString("invalid user id")
		}

		// Use the sanitized ID exclusively for DynamoDB input construction.
		out, err := dynamoClient.GetItem(c.Context(), &dynamodb.GetItemInput{
			TableName: aws.String("Users"),
			Key: map[string]types.AttributeValue{
				"user_id": &types.AttributeValueMemberS{Value: safeID},
			},
		})
		if err != nil {
			return c.Status(http.StatusInternalServerError).SendString("failed to fetch profile")
		}
		if out.Item == nil {
			return c.Status(http.StatusNotFound).SendString("not found")
		}

		// Process and return safe data.
		return c.JSON(fiber.Map{
			"user_id": safeID,
			"data":    out.Item,
		})
	})

	// Start server on a controlled port.
	app.Listen(":3000")
}

In this example, the user ID is extracted from the route parameter, not from potentially duplicated headers introduced by smuggling. The sanitize function enforces a strict allowlist, preventing path traversal, oversized inputs, or injection of extra routing segments. The DynamoDB GetItem call uses only this validated ID, ensuring that even if a request is smuggled, the second request reaching Fiber will fail validation or target a different, attacker-controlled ID that the application does not use for the original session context.

3. Use middleware to normalize headers and reject ambiguous requests

Add Fiber middleware that removes or rejects requests containing both Content-Length and Transfer-Encoding before they reach route handlers. This reduces the chance that the proxy and Fiber interpret the same request differently.

// middleware/header_normalize.go
package middleware

import (
	"github.com/gofiber/fiber/v2"
)

func NormalizeHeaders() fiber.Handler {
	return func(c *fiber.Ctx) error {
		cl := c.Request().Header.Peek("Content-Length")
		te := c.Request().Header.Peek("Transfer-Encoding")
		if len(cl) > 0 && len(te) > 0 {
			// Reject ambiguous messages to prevent smuggling.
			return c.Status(fiber.StatusBadRequest).SendString("conflicting transfer encodings")
		}
		return c.Next()
	}
}

Then use it early in your Fiber setup:

app.Use(middleware.NormalizeHeaders())

By combining strict header normalization, robust input validation, and safe DynamoDB usage patterns, you reduce the risk that request smuggling can affect data access or integrity in your Fiber application.

middleBrick can scan your Fiber endpoint to surface related findings under Input Validation and BOLA/IDOR. Its unauthenticated checks complement runtime testing, and its CLI and GitHub Action integrations help you enforce secure configurations before deployment.