Email Injection in Buffalo with Hmac Signatures

Email Injection in the Buffalo web framework occurs when user-controlled input is directly interpolated into email headers or templates without validation or encoding. If an application builds email headers (e.g., To, From, Subject) by concatenating user input, an attacker can inject additional headers or control message routing. For example, a newline character (\n) in a username or email field can enable header injection, allowing an attacker to add CC, BCC, or custom headers like X-Mailer. This is a classic web security issue, aligned with injection vectors described in the OWASP API Top 10 and broader input validation weaknesses.

Combining Email Injection with Hmac Signatures can expose a vulnerability if the signature is computed over a subset of the data that excludes user-controlled fields subject to injection, or if the signature is included in an email header or body without proper safeguards. For instance, if a request contains both an email parameter (e.g., email) and an Hmac signature of selected fields, an attacker might inject a newline and forge an additional parameter that is not covered by the signature, thereby bypassing integrity verification. The signature may be generated from a sanitized or canonical representation on the server, but if the runtime data used for verification diverges from the signed representation due to injection, the signature no longer guarantees integrity. This creates a trust boundary issue where the application may trust the signed payload while processing altered, user-supplied headers or body content.

Moreover, if the Hmac Signature is transmitted in an email header (e.g., X-Api-Signature) and the email content is user-influenced, an attacker could leverage injection to manipulate headers in a way that interferes with signature validation or logging. While Hmac Signatures themselves are cryptographically strong, their security depends on ensuring that the signed data is exactly what is verified at runtime. In Buffalo, if form values or query parameters used to recompute the Hmac are modified via injection before verification, the mismatch may either cause false validation failures or, worse, be silently ignored depending on how the comparison is implemented. This underscores the importance of validating and sanitizing all inputs before they participate in cryptographic operations and of ensuring that the canonicalization process used for signing matches the validation logic exactly.

To remediate Email Injection in Buffalo when using Hmac Signatures, you must ensure that user input is strictly validated and encoded before being used in email headers or bodies, and that the data covered by the Hmac is canonicalized consistently between signing and verification. Use Buffalo’s built-in validation features and secure string handling to prevent injection, and structure your Hmac computation over a deterministic, minimal set of trusted data.

Below are concrete code examples demonstrating secure handling of email fields and Hmac Signatures in Buffalo.

1. Validate and sanitize email input

Use a whitelist validation rule for email fields and reject inputs containing unexpected newline or carriage return characters.

package actions

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/buffalo/validate"
    "github.com/gobuffalo/validate/v3/rules"
)

func ValidateEmail(c buffalo.Context) error {
    rules := validate.Params{
        "email": []rules.Rule{
            rules.Required,
            rules.Email,
            rules.NotContains{
                Values: []string{"\n", "\r"},
                Message: "email: invalid characters",
            },
        },
    }
    errs := validate.Validate(c, rules)
    if !errs.Any() {
        return nil
    }
    return c.Render(422, r.JSON(H{"errors": errs}))

2. Compute Hmac over canonical, limited fields

Sign only the fields you intend to verify, using a canonical format (e.g., sorted key-value pairs with a delimiter). Avoid including user-controlled headers or body content in the signed string.

package main

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

func ComputeHmac(fields map[string]string, secret string) string {
    // Canonicalize: sort keys to ensure deterministic ordering
    keys := make([]string, 0, len(fields))
    for k := range fields {
        keys = append(keys, k)
    }
    sort.Strings(keys)

    var payload string
    for i, k := range keys {
        if i > 0 {
            payload += "|"
        }
        payload += k + "=" + fields[k]
    }

    key := []byte(secret)
    mac := hmac.New(sha256.New, key)
    mac.Write([]byte(payload))
    return hex.EncodeToString(mac.Sum(nil))
}

// Example usage:
// fields := map[string]string{"email": "user@example.com", "action": "invite"}
// sig := ComputeHmac(fields, "my-secret-key")

3. Verify Hmac before processing email actions

Recompute the Hmac using the same canonicalization and compare using a constant-time function to avoid timing attacks. Do not trust any headers or parameters that were not part of the signed set.

package main

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

func VerifyHmac(receivedSig string, fields map[string]string, secret string) bool {
    expected := ComputeHmac(fields, secret)
    return hmac.Equal([]byte(expected), []byte(receivedSig))
}

// Ensure that fields used for verification are extracted from a trusted source,
// not from user-supplied headers that may have been injected.
func SafeEmailHandler(c buffalo.Context) error {
    // Assume email and action come from validated form input, not raw headers
    email := c.Param("email")
    action := c.Param("action")

    fields := map[string]string{
        "email": email,
        "action": action,
    }
    receivedSig := c.Request().Header.Get("X-Api-Signature")
    if !VerifyHmac(receivedSig, fields, "my-secret-key") {
        return c.Error(401, fmt.Errorf("invalid signature"))
    }

    // Proceed with email logic using the validated, canonicalized data
    return nil
}

4. Encode email headers and avoid direct concatenation

When constructing email headers, use proper encoding (e.g., Q-encoding for non-ASCII) and avoid building headers via string concatenation with user input. Use dedicated libraries for header assembly if available.

package main

import (
    "net/mail"
)

func BuildHeader(name, value string) (string, error) {
    addr := mail.Address{Name: name, Address: value}
    return addr.MarshalText()
}