Insecure Deserialization in Buffalo with Hmac Signatures

Insecure deserialization in Buffalo applications that rely on Hmac Signatures for session or payload validation can occur when deserialization is performed on attacker-controlled data before the Hmac signature is verified, or when the signature is applied to a serialized object in a way that does not protect the integrity of the deserialization process. Buffalo uses signed cookies and form parameters to carry state, and if these are deserialized (for example, using Gob, JSON, or XML) without first validating authenticity and integrity, an attacker can tamper with the serialized structure to trigger gadget chains during unmarshaling.

Consider a scenario where a Buffalo app stores user preferences in a signed cookie. The cookie value is serialized (for example, as Gob or JSON), then an Hmac is computed over the serialized bytes and appended or embedded in the cookie. If the application deserializes the cookie value before verifying the Hmac, or uses a weak comparison that does not protect the canonicalization of the serialized bytes, an attacker can modify the serialized payload and observe whether the server behaves differently (e.g., different error paths or timing), leading to insecure deserialization. The Hmac signature is intended to prevent modification, but if verification is skipped or performed after deserialization, the protection is bypassed. This pattern maps to OWASP API Top 10:2023 A05:2025 Security Misconfiguration and A07:2025 Identification and Authentication Failures, and can be leveraged for privilege escalation or remote code execution when the deserialization library supports gadget chains (e.g., via certain Java or Go libraries that process unsafe formats).

In practice, an attacker might intercept a session cookie, replace serialized objects with malicious gadget-laden equivalents, and rely on the server’s deserialization logic to execute code when the object is reconstructed. Even when Hmac Signatures are used, if the signature covers only a subset of the payload or is computed over an intermediate representation, the mismatch between what is signed and what is deserialized creates an exploitable gap. Because Buffalo does not enforce a default secure order of operations, developers must explicitly verify signatures prior to any deserialization and ensure that the serialized representation is not processed with unsafe unmarshalers. MiddleBrick scans can detect such insecure workflows by correlating API spec definitions with runtime behavior, highlighting endpoints where deserialization occurs without prior integrity checks.

Remediation centers on verifying the Hmac signature before any deserialization and ensuring the signature covers the exact bytes that are later unmarshaled. In Buffalo, this means structuring handlers so that raw input (cookies, headers, form values) is authenticated with Hmac before being passed to unmarshalers. Use constant-time comparison to avoid timing attacks, and prefer standard libraries for Hmac and serialization to reduce implementation risk.

Below are concrete code examples for securing Hmac-based session handling in Buffalo. The first example shows a safe pattern: verify the signature on the raw serialized data, then deserialize only after successful authentication.

// Safe: verify Hmac over the raw serialized payload before deserialization
package actions

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/base64"
    "encoding/json"
    "net/http"

    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/buffalo/middleware"
)

// SessionClaims represents the data stored in the signed cookie
 type SessionClaims struct {
    UserID int    `json:"user_id"`
    Role   string `json:"role"`
}

// VerifySignedSession decodes and validates an Hmac-signed JSON cookie
func VerifySignedSession(raw string, secret []byte) (*SessionClaims, error) {
    // Expect raw format: base64(payload).base64(hmac)
    parts := splitOnce(raw, '.')
    if len(parts) != 2 {
        return nil, http.ErrAbort{StatusCode: http.StatusBadRequest}
    }
    payloadB64, sigB64 := parts[0], parts[1]
    sig, err := base64.StdEncoding.DecodeString(sigB64)
    if err != nil {
        return nil, err
    }
    payload, err := base64.StdEncoding.DecodeString(payloadB64)
    if err != nil {
        return nil, err
    }

    // Verify Hmac using constant-time comparison
    mac := hmac.New(sha256.New, secret)
    mac.Write(payload)
    expected := mac.Sum(nil)
    if !hmac.Equal(sig, expected) {
        return nil, http.ErrAbort{StatusCode: http.StatusUnauthorized}
    }

    // Safe deserialization only after signature verification
    var claims SessionClaims
    if err := json.Unmarshal(payload, &claims); err != nil {
        return nil, err
    }
    return &claims, nil
}

// Example handler in a Buffalo route
func SessionShow(c buffalo.Context) error {
    cookie, err := c.Request().Cookie("session")
    if err != nil {
        return c.Render(401, r.String("unauthorized"))
    }
    claims, err := VerifySignedSession(cookie.Value, []byte("your-32-byte-secret-here-123456"))
    if err != nil {
        return c.Render(401, r.String("invalid signature"))
    }
    // Use claims.UserID and claims.Role safely
    return c.Render(200, r.JSON(map[string]interface{}{"user_id": claims.UserID, "role": claims.Role}))
}

The second example demonstrates how to apply this pattern to form parameters, where the client sends a JSON body and an Hmac header. Buffalo middleware can be used to enforce verification globally or per-route.

// Safe: Hmac verification for JSON request bodies in Buffalo
package actions

import (
    "crypto/hmac"
    "crypto/sha256"
    "io/ioutil"

    "github.com/gobuffalo/buffalo"
)

func VerifyHmacHeader(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        expectedMAC := c.Request().Header.Get("X-Payload-Mac")
        if expectedMAC == "" {
            return c.Render(400, r.String("missing MAC header"))
        }
        body, err := ioutil.ReadAll(c.Request().Body)
        if err != nil {
            return c.Render(400, r.String("invalid request"))
        }
        // Restore body for downstream handlers (Buffalo clones or rewinds as needed)
        c.Request().Body = ioutil.NopCloser(bytes.NewBuffer(body))

        secret := []byte("your-32-byte-secret-here-123456")
        mac := hmac.New(sha256.New, secret)
        mac.Write(body)
        expected := mac.Sum(nil)
        remoteMAC, err := base64.StdEncoding.DecodeString(expectedMAC)
        if err != nil || !hmac.Equal(remoteMAC, expected) {
            return c.Render(401, r.String("invalid MAC"))
        }
        return next(c)
    }
}

// Usage in app.go: app.GET("/profile", actions.ProfileShow, VerifyHmacHeader)

Key practices: always verify Hmac before deserialization, include all canonical bytes that affect state in the Hmac input, use a strong secret and rotate keys periodically, and rely on standard library functions for Hmac and serialization. MiddleBrick can validate that your endpoints follow these patterns by checking spec definitions and highlighting routes where deserialization precedes integrity checks.