Insecure Deserialization in Gin with Hmac Signatures

Insecure deserialization in a Gin application that uses Hmac Signatures can occur when application logic deserializes attacker-controlled data, such as JSON, XML, or gob payloads, without first validating integrity or authenticity. Even when requests carry an Hmac Signature meant to verify request origin and integrity, developers may deserialize the payload before verifying the signature, or they may verify the signature on a subset of data while deserializing a broader structure that has been tampered with.

For example, an API might accept a JSON body containing a serialized object and an hmac header. If the handler reads the raw body into a generic interface{} or into a struct with nested fields, then passes that object to an insecure unmarshaler before computing the Hmac over the canonical request, an attacker can modify the serialized content and potentially bypass signature checks if the signature does not cover the entire effective serialized representation. This becomes a deserialization path where malicious payloads can trigger unintended object creation, leading to arbitrary code execution or logic bypass.

Gin’s binding mechanisms can inadvertently encourage this pattern. When using c.ShouldBindJSON or similar binding methods, Gin unmarshals the request body into the target struct. If the developer then performs Hmac verification on only selected fields or on a transformed representation, the binding step has already created objects from attacker-controlled input. Attack patterns such as injection of specially crafted objects can exploit language-specific unmarshal behavior, leading to security issues that align with the OWASP API Top 10’s Broken Object Level Authorization and can be surfaced by checks such as BOLA/IDOR or Property Authorization in middleBrick scans.

An additional risk arises when Hmac signatures are computed over serialized forms that include references to type information or metadata used by the deserializer. If the signature scheme does not canonicalize the serialized representation, different but semantically equivalent payloads can produce different byte sequences, allowing signature bypass through alternative encodings. middleBrick’s OpenAPI/Swagger spec analysis can detect mismatches between declared request shapes and runtime binding behavior, highlighting where deserialization occurs without prior integrity checks.

Because this combination ties deserialization directly to authenticated or integrity-checked requests, the impact can be severe: attackers may achieve unauthorized actions, escalate privileges, or access sensitive data. Instrumentation and testing, including active probes for injection and authorization bypass, are necessary to uncover these subtle ordering and coverage issues.

To remediate insecure deserialization in Gin when Hmac Signatures are used, ensure signature verification is performed before any deserialization and that the signature covers the exact byte sequence used for binding. The canonical approach is to read the raw body, compute the Hmac over that raw body, validate the signature, and only then bind to a struct.

Below is a secure Gin handler pattern using Hmac Signatures. The example uses Hmac-SHA256 with a shared secret, reads the request body as a byte slice, computes the signature, compares it in constant time, and then deserializes only after successful verification.

//go
package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"io/ioutil"
	"net/http"

	"github.com/gin-gonic/gin"
)

const sharedSecret = "your-secure-shared-secret"

func verifyHmacSignature(body []byte, receivedSig string) bool {
	h := hmac.New(sha256.New, []byte(sharedSecret))
	h.Write(body)
	expected := hex.EncodeToString(h.Sum(nil))
	return hmac.Equal([]byte(expected), []byte(receivedSig))
}

func secureHandler(c *gin.Context) {
	rawBody, err := ioutil.ReadAll(c.Request.Body)
	if err != nil {
		c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "failed to read body"})
		return
	}
	// restore body for subsequent reads if needed
	c.Request.Body = ioutil.NopCloser(bytes.NewBuffer(rawBody))

	sentSig := c.GetHeader("X-API-Signature")
	if sentSig == "" || !verifyHmacSignature(rawBody, sentSig) {
		c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "invalid signature"})
		return
	}

	var payload MyRequest
	if err := c.BindJSON(&payload); err != nil {
		c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "invalid JSON"})
		return
	}

	// process validated payload
	c.JSON(http.StatusOK, gin.H{"status": "ok"})
}

type MyRequest struct {
	Action string `json:"action"`
	Value  int    `json:"value"`
}

Key points in this pattern:

• Read the raw body before binding; do not bind first.
• Compute the Hmac over the raw bytes exactly as transmitted to avoid encoding discrepancies.
• Use hmac.Equal for constant-time comparison to prevent timing attacks.
• Re-inject the body or use a copy if you need to bind after verification, ensuring binding occurs only after integrity is confirmed.

If you rely on query parameters or headers for part of the authenticated context, include them in the signature computation to prevent parameter confusion attacks. middleBrick’s CLI can be used to validate that your endpoints enforce signature verification before deserialization by running middlebrick scan <url> and reviewing the Authentication and BOLA/IDOR findings.

For ongoing safety, adopt strict content-type handling and avoid generic interface{} unmarshaling. In the Pro plan, continuous monitoring can alert you if response payloads risk exposing sensitive data, and the GitHub Action can fail builds when risk thresholds are exceeded.