Formula Injection in Echo Go with Hmac Signatures

Formula Injection occurs when user-controlled data is interpreted as a formula by spreadsheet or document processing libraries, leading to unintended code execution or data manipulation. In Echo Go, when Hmac Signatures are used to validate webhook payloads or API requests, a common pattern is to include a signature header that is computed over selected request parameters. If the logic that builds the signature string incorporates user-supplied values without strict type and format validation, an attacker can inject formula-like content (for example, strings starting with =, +, or -) into a parameter that is later rendered in a spreadsheet export or processed by a templating engine. This can trigger remote code execution or data leakage when the exported file is opened by an application that evaluates formulas.

Consider an Echo Go handler that computes an Hmac Signature over a data query parameter and a timestamp query parameter. If the developer concatenates the raw query string to the signing key, an attacker can supply a data value such as =cmd|' /C calc'!A0. The signature will verify successfully because the server uses the same logic, but when the backend writes this value into a CSV or Excel file and the file is opened, the injected formula may execute system commands or cause unwanted side effects. This is especially dangerous when combined with weak input validation and when the server trusts the signature as proof of authenticity, bypassing contextual checks. The vulnerability is not in the Hmac algorithm itself, but in how the signed data is constructed, interpreted, and later consumed downstream.

In Echo Go, if middleware uses the request query parameters to generate the string-to-sign without canonicalization or strict type constraints, the attack surface expands. For example, a signature computed over amount=100;type=credit becomes risky if an attacker can inject newline characters or formula indicators into one of the values. The server may still compute a valid Hmac Signatures match, but the downstream consumer of that data treats the injected content as executable instructions. This highlights the need to treat signed payloads as untrusted at the point of rendering or parsing, even when the integrity check passes.

To mitigate Formula Injection in Echo Go while preserving Hmac Signatures integrity, enforce strict input validation, canonicalize data before signing, and avoid direct concatenation of raw user input into structured output formats. Use explicit type conversion and reject values that contain formula indicators when the data will be rendered in spreadsheets or interpreted by templating engines.

Below is a secure example of computing and verifying an Hmac Signature in Echo Go. The code uses crypto/hmac and crypto/sha256 to sign a canonical representation of the payload, avoiding raw query concatenation and ensuring that only expected parameters are included.

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "fmt"
    "net/url"
    "sort"
    "strings"
)

func computeSignature(params url.Values, secret string) string {
    // Canonicalize: only include known safe keys, sorted
    keys := []string{"amount", "timestamp", "currency"}
    var parts []string
    for _, k := range keys {
        if v := params.Get(k); v != "" {
            // Reject formula indicators at validation boundary
            if strings.HasPrefix(v, "=") || strings.HasPrefix(v, "+") || strings.HasPrefix(v, "-") {
                return "" // reject suspicious input
            }
            parts = append(parts, k+"="+v)
        }
    }
    message := strings.Join(parts, "|")
    key := []byte(secret)
    mac := hmac.New(sha256.New, key)
    mac.Write([]byte(message))
    return hex.EncodeToString(mac.Sum(nil))
}

func verifySignature(params url.Values, secret, receivedSig string) bool {
    expected := computeSignature(params, secret)
    return expected != "" && hmac.Equal([]byte(expected), []byte(receivedSig))
}

// In handler
func myHandler(c echo.Context) error {
    query := c.Request().URL.Query()
    sig := c.QueryParam("sig")
    if !verifySignature(query, "your-secret", sig) {
        return c.String(401, "invalid signature")
    }
    // safe to use query.Get("amount") etc.
    return c.String(200, "ok")
}

Key remediation steps encoded in the example:

• Canonical parameter ordering and whitelisting prevent attackers from adding extra fields that affect the signature.
• Rejecting values that start with =, +, or - at the validation layer blocks common formula injection patterns.
• Using a structured join with a delimiter (e.g., |) reduces ambiguity compared to raw concatenation.
• Always compare signatures with hmac.Equal to avoid timing attacks.

Additionally, when exporting signed data to spreadsheets, sanitize cell values by prepending a single quote ' or formatting cells as text to prevent formula evaluation. In Echo Go middleware, you can integrate these checks before constructing responses that may be consumed as files.