Email Injection in Gin with Api Keys

Email injection in the Gin web framework typically occurs when user-controlled data is concatenated into email headers without validation or sanitization. Attackers can inject additional headers or malformed content by supplying newline characters (e.g., %0D%0A or \r\n) in input fields such as email addresses or message bodies. When api keys are involved, the risk pattern changes: developers sometimes embed or log api keys in email headers or bodies for debugging or transactional purposes, inadvertently creating data exposure paths.

Consider a Gin handler that builds a welcome email using query parameters and an api key passed via a header. If the email subject or recipient is constructed by string concatenation, newline injection can alter the header structure, enabling header smuggling or unauthorized redirection. Moreover, if the api key is included in the email body or subject line, the scan may detect unintended data exposure because the key could be leaked in logs or forwarded to unintended recipients. Unauthenticated endpoints that accept user input for email composition are especially vulnerable, as attackers do not need credentials to exploit the injection vector.

During a black-box scan, middleBrick tests input validation and data exposure by probing endpoints that handle email-related parameters and api key usage. It checks whether newline characters are sanitized, whether api keys are logged in plaintext in email context, and whether crafted payloads can modify email routing or headers. Findings often map to OWASP API Top 10:2023 A03:2023 Injection and A05:2023 Security Misconfiguration, and may align with PCI-DSS requirements around data exposure. Because Gin does not provide built-in email header sanitization, developers must explicitly validate and encode user-supplied content before using it in mail libraries.

To remediate email injection when using api keys in Gin, treat all user input as untrusted and apply strict validation before using it in email headers or bodies. Avoid embedding api keys in emails; if necessary, reference them by opaque identifiers instead of raw values. Use Go’s standard library functions to sanitize headers and ensure newline characters are removed or encoded.

Secure Gin handler example

//go
package main

import (
	"net/http"
	"strings"

	"github.com/gin-gonic/gin"
)

// sanitizeHeader removes carriage return and line feed characters.
func sanitizeHeader(value string) string {
	return strings.ReplaceAll(strings.ReplaceAll(value, "\r", ""), "\n", "")
}

func sendWelcomeEmail(c *gin.Context) {
	email := sanitizeHeader(c.Query("email"))
	userName := sanitizeHeader(c.Query("name"))
	// Do NOT use raw api key in email; use an opaque reference if needed.
	apiKeyID := c.Query("api_key_id")

	if email == "" || userName == "" || apiKeyID == "" {
		c.JSON(http.StatusBadRequest, gin.H{"error": "missing required parameters"})
		return
	}

	// Example: pass apiKeyID to mail service, not the key itself.
	// mailService.Send(email, "Welcome", buildTemplate(userName, apiKeyID))

	c.JSON(http.StatusOK, gin.H{"status": "email prepared", "email": email})
}

func main() {
	r := gin.Default()
	r.GET("/send-welcome", sendWelcomeEmail)
	r.Run()
}

In this example, sanitizeHeader removes \r and \n to prevent header injection. The handler avoids placing the api key in the email content and instead uses an identifier (api_key_id). For production, integrate a mailing library with strict input validation and ensure logs do not capture raw api keys or sensitive email fields.

Middleware approach to reject unsafe input

//go
package main

import (
	"net/http"
	"regexp"

	"github.com/gin-gonic/gin"
)

// rejectNewline rejects inputs containing \r or \n.
func rejectNewline() gin.HandlerFunc {
	return func(c *gin.Context) {
		for key, values := range c.Request.URL.Query() {
			for _, v := range values {
				if regexp.MustCompile(`[\r\n]`).MatchString(v) {
					c.JSON(http.StatusBadRequest, gin.H{"error": "invalid input: newline characters not allowed in " + key})
					c.Abort()
					return
				}
			}
		}
		c.Next()
	}
}

func main() {
	r := gin.Default()
	r.Use(rejectNewline())
	r.GET("/send-welcome", sendWelcomeEmail)
	r.Run()
}

Using middleware to reject newline characters across all query parameters adds a defense-in-depth layer. Combine this with secure handling of api keys—store them server-side, reference them by ID, and never echo them in responses or email content—to reduce both injection and data exposure risks.