Email Injection in Gin with Bearer Tokens

Email Injection occurs when user-controlled data is improperly sanitized before being used in email headers or message bodies, enabling an attacker to inject additional headers or content. In the Go Gin framework, this often arises when building email messages using user input for fields such as To, From, Subject, or message body without validation. When Bearer Tokens are used for API authentication, developers sometimes embed or log these tokens in email payloads or error messages, unintentionally creating paths for token leakage or further exploitation.

Consider a Gin handler that accepts a JSON payload containing an email address and an optional authorization token for session context:

{
  "email": "[email protected]",
  "token": "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." // example Bearer token
}

If the handler constructs an email using string concatenation or formatting without escaping newlines or special characters, an attacker can inject extra headers. For example, if the email parameter contains a newline followed by CC: [email protected] or additional headers, those injected lines may be processed by the mail server or logged in plaintext. Bearer Tokens used for authorization may also appear in logs when debugging errors, and if those logs are emailed or exposed via an insecure channel, token exposure becomes likely.

Gin does not automatically sanitize email inputs. Developers must treat all incoming data, including Bearer Tokens extracted from Authorization headers, as untrusted. Common patterns that increase risk include:

• Using user input directly in SMTP client calls without validation.
• Logging request payloads that include Authorization headers, which may expose Bearer Tokens in centralized logging or error emails.
• Returning detailed errors to clients that include email or token context, aiding reconnaissance for injection or token harvesting.

Because Bearer Tokens are often long-lived credentials, any leakage can lead to unauthorized access to protected APIs. Attackers may combine Email Injection with token leakage to pivot within a system, leveraging exposed tokens to escalate abuse. The combination of Gin routing, user-supplied email fields, and Bearer Token usage increases the impact of improper input handling and logging practices.

Remediation focuses on strict input validation, avoiding logging of sensitive credentials, and using structured email libraries that enforce header separation. Below are concrete patterns for handling email and Bearer Tokens safely in Gin.

1. Validate and sanitize email inputs using a dedicated package, and avoid concatenating raw user input into email headers:

import (
    "net/mail"
    "github.com/gin-gonic/gin"
    "net/smtp"
)

func sendEmail(to, subject, body string) error {
    // Validate and parse the recipient address to prevent header injection
    _, err := mail.ParseAddress(to)
    if err != nil {
        return err
    }
    auth := smtp.PlainAuth("", "[email protected]", "SMTP_PASSWORD", "smtp.example.com")
    msg := []byte("To: " + to + "\r\n" +
        "Subject: " + subject + "\r\n" +
        "Content-Type: text/plain; charset=utf-8\r\n" +
        "\r\n" +
        body)
    return smtp.SendMail("smtp.example.com:587", auth, "[email protected]", []string{to}, msg)
}

func handleEmail(c *gin.Context) {
    var payload struct {
        Email string `json:"email" binding:"required,email"`
        Token string `json:"token"`
    }
    if err := c.ShouldBindJSON(&payload); err != nil {
        c.JSON(400, gin.H{"error": "invalid request"})
        return
    }
    // Use the validated email; do not include raw token in email headers or body
    if err := sendEmail(payload.Email, "Welcome", "Thank you for signing up."); err != nil {
        c.JSON(500, gin.H{"error": "failed to send email"})
        return
    }
    c.JSON(200, gin.H{"status": "email sent"})
}

2. Avoid logging or exposing Bearer Tokens. When inspecting Authorization headers, extract the token but do not persist or echo it in responses or logs:

func authMiddleware() gin.HandlerFunc {
    return func(c *gin.Context) {
        auth := c.GetHeader("Authorization")
        if auth == "" {
            c.AbortWithStatusJSON(401, gin.H{"error": "authorization header required"})
            return
        }
        // Do NOT log the full Authorization header which may contain Bearer token
        // Instead, validate presence and scope without recording the token value
        if len(auth) < 7 || auth[:7] != "Bearer " {
            c.AbortWithStatusJSON(401, gin.H{"error": "invalid authorization format"})
            return
        }
        // Token validation logic would go here (e.g., JWT verification)
        c.Next()
    }
}

3. Use structured logging that redacts sensitive fields. If you must log requests, ensure tokens are masked:

import "github.com/sirupsen/logrus"

func loggingMiddleware() gin.HandlerFunc {
    return func(c *gin.Context) {
        // Custom logging that excludes Authorization header
        entry := logrus.WithFields(logrus.Fields{
            "method": c.Request.Method,
            "path":   c.Request.URL.Path,
        })
        entry.Info("request received")
        c.Next()
    }
}

These practices reduce the likelihood of Email Injection and protect Bearer Tokens by ensuring inputs are validated, sensitive credentials are not included in messages or logs, and error handling does not expose internal context.