Spring4shell in Buffalo with Hmac Signatures

The Spring4shell vulnerability (CVE-2022-22965) targets applications using Spring MVC and Spring WebFlux with Java on JDK 9+ when data binding allows untrusted input to reach an object’s properties. When this vulnerability intersects with Buffalo—a Go web framework—and Hmac Signatures used to protect webhook or API request integrity, the combination can expose validation bypass and tampering paths if Hmac verification is applied only to a subset of fields or is performed after unsafe deserialization-style binding.

In Buffalo, Hmac Signatures are commonly implemented by generating a signature over selected headers, the request body, or a canonical string (e.g., timestamp + method + path + body) and sending it in a header like X-Signature. If the server reconstructs the expected Hmac using attacker-influenced data that is bound into structs or maps before verification, and if the underlying JSON or form binding in Buffalo (via the standard net/http mechanisms or an added binding step) allows property injection, an attacker may be able to tamper with critical fields—such as an amount or user identifier—while still producing a valid Hmac if the server recomputes the signature using the attacker-supplied values.

Consider a scenario where a Buffalo handler binds JSON into a struct and then verifies an Hmac computed over the raw body. If the binding step modifies or ignores certain fields, or if the server uses a permissive binding policy that allows unknown fields to populate the struct, an attacker can inject additional parameters that shift behavior (for example, changing a user ID or switching a transaction type). If the Hmac verification logic recomputes the signature using the modified struct or uses selective field inclusion without canonicalization, the attacker can craft a modified request that passes signature checks while altering business logic. This becomes especially risky when the signature is computed over only a subset of fields, such as omitting sensitive ones like price or elevation flags, enabling privilege escalation or unauthorized operations.

The interplay is notable when Buffalo applications consume external webhooks or APIs that rely on Hmac Signatures for integrity. If the signature is verified after loosely bound input is used—for example, to select a discount code or to determine a redirect URL—an attacker who can control bound properties might bypass intended checks. The OWASP API Top 10 highlights injection and broken object level authorization as common API risks; in this context, Hmac misuse or incomplete coverage can inadvertently neutralize integrity protections, enabling tampering that appears authenticated.

To stay within the scope of detection and reporting, tools like middleBrick can scan such endpoints for input validation weaknesses, authentication issues, and property authorization flaws across 12 parallel checks, including Hmac-related integrity failures. While middleBrick does not fix code, it provides findings with severity, reproduction steps, and remediation guidance, helping teams identify whether their Hmac coverage is incomplete or whether binding logic exposes mutable fields that should be verified before use.

Remediation centers on canonicalizing the data before signing, verifying the signature before any business logic or binding side effects, and ensuring that Hmac checks cover all fields that influence authorization or behavior. Below are concrete steps and code examples for secure Hmac handling in Buffalo.

1. Compute Hmac over the raw request body and critical headers

Always compute the Hmac on the exact bytes the sender used, avoiding re-encoding differences. Include a timestamp and a nonce to prevent replay attacks.

// Example: Compute Hmac over method + path + timestamp + raw body
package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "time"
)

func computeSignature(secret, method, path string, body []byte, timestamp string) string {
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(method))
    mac.Write([]byte("|"))
    mac.Write([]byte(path))
    mac.Write([]byte("|"))
    mac.Write([]byte(timestamp))
    mac.Write([]byte("|"))
    mac.Write(body)
    return hex.EncodeToString(mac.Sum(nil))
}

func VerifySignature(secret, method, path string, body []byte, r *http.Request) bool {
    expected := r.Header.Get("X-Signature")
    if expected == "" {
        return false
    }
    ts := r.Header.Get("X-Timestamp")
    if ts == "" {
        return false
    }
    actual := computeSignature(secret, method, path, body, ts)
    return hmac.Equal([]byte(actual), []byte(expected))
}

2. Verify before binding and reject unknown fields

Parse and verify the signature before deserializing into structs. Use strict binding to prevent property injection.

// Buffalo-style handler with strict verification
func OrdersHandler(secret string) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Read raw body once
        body := http.MaxBytesReader(w, r.Body, 1000000)
        raw, err := io.ReadAll(body)
        if err != nil {
            http.Error(w, "request too large", http.StatusRequestEntityTooLarge)
            return
        }
        // Restore body for downstream use if needed
        r.Body = io.NopCloser(bytes.NewBuffer(raw))

        if !VerifySignature(secret, r.Method, r.URL.Path, raw, r) {
            http.Error(w, "invalid signature", http.StatusUnauthorized)
            return
        }

        // Now bind safely with a strict decoder or explicit parsing
        var payload struct {
            OrderID string `json:"order_id"`
            Amount  int    `json:"amount"`
            Currency string `json:"currency"`
        }
        if err := json.NewDecoder(bytes.NewReader(raw)).Decode(&payload); err != nil {
            http.Error(w, "invalid JSON", http.StatusBadRequest)
            return
        }
        // Proceed with business logic using payload.OrderID, payload.Amount, etc.
    }
}

3. Use canonical serialization for cross-language compatibility

When multiple services produce or verify signatures, use a canonical form such as sorted key-value pairs or a deterministic JSON representation to avoid discrepancies.

// Canonical JSON using a sorted key encoder can be implemented with libraries like stretch/smartapi
// For simplicity, here is an approach with fixed field order:
func canonicalSign(secret, method, path string, body []byte, timestamp string) string {
    // Ensure body is canonical JSON; in production, use a library that sorts keys
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(fmt.Sprintf("%s|%s|%s|%s|", method, path, timestamp, string(body))))
    return hex.EncodeToString(mac.Sum(nil))
}

4. Apply replay protection and scope limiting

Include a nonce or short-lived token in the signed string and maintain a short cache of recently used nonces to prevent replay attacks. Scope the signature to a particular operation or resource ID where feasible.

5. Audit with automated scans

Use middleBrick to validate that your endpoints correctly enforce Hmac verification, check input validation, and avoid property authorization issues. The scanner can surface findings related to authentication, BOLA/IDOR, and data exposure that may stem from incomplete signature coverage.