Spring4shell in Fiber with Hmac Signatures

The combination of Spring4Shell (CVE-2022-22965), a web framework like Fiber for Go, and Hmac Signatures introduces a nuanced risk profile that middleBrick identifies as part of its authentication and input validation checks. Spring4shell exploits a flaw in Spring’s Data Binding mechanism where arbitrary object properties can be modified via crafted payloads, often leveraging deeply nested or unexpected parameter names. When a Fiber service uses Hmac Signatures to validate request integrity, the presence of untrusted parameters in the request body or query string can bypass developer assumptions that signature validation alone prevents injection.

Specifically, if a Fiber application parses JSON or form data into a Go struct and then recomputes an Hmac Signature over the raw bytes for verification, an attacker can inject additional fields or modify nested objects before signature verification occurs. This can lead to Insecure Deserialization-like behaviors and BOLA/IDOR patterns when the application uses the deserialized data to locate resources. middleBrick’s authentication and property authorization checks are designed to detect scenarios where untrusted input influences authorization decisions, highlighting how Hmac Signatures may not protect against post-deserialization data tampering.

Furthermore, if the Hmac verification is performed after data binding rather than on a canonical, pre-parsed representation, subtle discrepancies between expected and actual payload structure may be overlooked. For example, an attacker could include extra parameters that are ignored by the application logic but still affect internal routing or object mapping. middleBrick’s input validation and SSRF checks help identify whether such unexpected parameters result in side effects, such as SSRF or unauthorized access to internal endpoints, especially when combined with unchecked deserialization paths.

In practice, the risk is not in the cryptographic strength of Hmac Signatures but in how and when the signature is applied relative to data parsing and authorization. If the signature covers only a subset of headers or a limited portion of the payload, an attacker can manipulate other areas of the request. middleBrick’s OpenAPI/Swagger spec analysis, with full $ref resolution, cross-references declared schemas with runtime behavior to uncover mismatches between documented and actual input handling, exposing potential gaps in integrity checks.

Finally, logging or error handling that reflects deserialized objects may inadvertently disclose sensitive context in the event of malformed requests, contributing to Data Exposure findings. By scanning unauthenticated attack surfaces, middleBrick surfaces these subtle interactions between framework behavior, signature placement, and input handling, enabling teams to align their security posture with the actual runtime flow rather than assumed protections.

To mitigate risks when using Hmac Signatures in a Fiber application, apply signature verification before data binding and on a canonical representation of the request. This ensures that any modifications to the payload are detected before the data influences business logic or authorization decisions. Below are concrete, realistic code examples that demonstrate secure handling.

Example 1: Hmac verification on raw body in Fiber

Read the raw body once, compute the Hmac, and validate it before unmarshaling JSON or form data. This prevents attackers from injecting additional fields that could affect deserialization.

// main.go
package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"io"
	"net/http"

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

func main() {
	app := fiber.New()
	secret := []byte("your-256-bit-secret")

	app.Post("/webhook", func(c *fiber.Ctx) error {
		// Read raw body
		raw, err := io.ReadAll(c.Request().Body())
		if err != nil {
			return c.Status(http.StatusBadRequest).SendString("failed to read body")
		}

		// Extract signature from header
		sig := c.Get("X-Hub-Signature-256")
		if sig == "" {
			return c.Status(http.StatusUnauthorized).SendString("missing signature")
		}

		// Verify Hmac-SHA256
		mac := hmac.New(sha256.New, secret)
		mac.Write(raw)
		expected := mac.Sum(nil)

		if !hmac.Equal(expected, parseSignature(sig)) {
			return c.Status(http.StatusUnauthorized).SendString("invalid signature")
		}

		// Now safely unmarshal
		var payload map[string]interface{}
		if err := json.Unmarshal(raw, &payload); err != nil {
			return c.Status(http.StatusBadRequest).SendString("invalid json")
		}

		// Proceed with business logic using payload
		// Ensure strict schema validation and avoid using raw user input for resource lookup
		return c.SendString("ok")
	})

	app.Listen(":3000")
}

func parseSignature(hmacHeader string) []byte {
	// Example: "sha256=abcd1234"
	const prefix = "sha256="
	if len(hmacHeader) > len(prefix) && hmacHeader[:len(prefix)] == prefix {
		// In production, use hex.DecodeString and handle errors
		return []byte(hmacHeader[len(prefix):])
	}
	return nil
}

Example 2: Schema-driven validation after Hmac verification

Combine Hmac checks with strict JSON Schema validation to enforce allowed fields and nesting, reducing the impact of unexpected inputs that could interact with framework internals.

// validate.go
package main

import (
	"encoding/json"
	"github.com/xeipuuv/gojsonschema"
)

var payloadSchema = `{
	"type": "object",
	"properties": {
		"action": {"type": "string", "enum": ["create", "update"]},
		"resource_id": {"type": "string", "pattern": "^[a-zA-Z0-9-]+$"}
	},
	"required": ["action", "resource_id"]
}`

func validatePayload(data []byte) error {
	loader := gojsonschema.NewStringLoader(payloadSchema)
	documentLoader := gojsonschema.NewBytesLoader(data)

	result, err := gojsonschema.Validate(loader, documentLoader)
	if err != nil {
		return err
	}
	if !result.Valid() {
		return fmt.Errorf("validation failed: %s", result.Errors())
	}
	return nil
}

Operational practices

• Treat the raw body as the source of truth for Hmac computation and avoid secondary parsing before verification.
• Do not use request parameters, headers, or deserialized struct fields to construct authorization decisions without independent validation.
• Align your OpenAPI/Swagger spec definitions with actual input constraints; use middleBrick’s spec analysis to detect mismatches between documented schemas and runtime behavior.