Email Injection in Fiber with Dynamodb

Email Injection in a Fiber application that uses DynamoDB typically arises when user-controlled input is reflected into email headers or message bodies without validation, and the data is subsequently stored in or retrieved from DynamoDB. In a black-box scan, middleBrick tests input vectors such as query parameters, JSON payload fields, and header values that flow into the application logic and then into DynamoDB operations. If user input is concatenated into email headers (e.g., To, CC, Subject) or into templated messages without sanitization or strict allowlisting, an attacker can inject additional headers or control message routing. Because DynamoDB is often used as a user data store, a compromised registration or profile update flow can persist malicious input and later feed it into email generation, turning stored data into a persistent injection vector.

Consider a registration endpoint that stores user-supplied display name and email in DynamoDB and then sends a welcome email. If the display name is used in the email Subject or as part of the message body without sanitization, an attacker can inject newline characters (%0a or \r\n) to add extra headers such as Cc: or Bcc:. middleBrick’s checks for Input Validation and Unsafe Consumption highlight cases where untrusted data reaches outputs like email messages. In addition, because DynamoDB does not enforce email format constraints, stored data may appear valid in storage but become malicious when rendered by downstream email-generation code. This cross-context contamination—user input entering DynamoDB and later influencing email composition—is the specific risk pattern that creates and exposes the vulnerability in this stack.

During a scan, middleBrick maps findings to frameworks such as OWASP API Top 10 and checks like Property Authorization to ensure that data retrieval from DynamoDB does not leak sensitive fields used in email construction. The scanner also tests for BFLA (Broken Function Level Authorization) where an IDOR-prone endpoint allows an attacker to modify another user’s profile data in DynamoDB to inject malicious email content. Because DynamoDB often stores user profile and contact preferences, an attacker who can tamper with these records can influence future email messages. middleBrick’s LLM/AI Security checks are particularly relevant here: they verify that unauthenticated endpoints do not expose mechanisms where an attacker could cause the application to send emails with injected recipients or content derived from manipulated DynamoDB records.

Remediation centers on strict input validation, separation of data from control information, and safe construction of email messages. In Go Fiber, validate and sanitize all user inputs before using them in DynamoDB operations and before incorporating them into email headers or bodies. Use allowlists for known-safe characters for display names and enforce email formats for email fields. When storing to DynamoDB, avoid placing user-controlled strings into header keys; instead, store raw input as data attributes and use a controlled template for email composition.

Example: a safe registration flow in Fiber with DynamoDB that avoids injection into email headers.

app.Post("/register", func(c *fiber.Ctx) error {
    type RegisterRequest struct {
        Name  string `json:"name" validate:"required,printascii"`
        Email string `json:"email" validate:"required,email"`
    }
    var req RegisterRequest
    if err := c.BodyParser(&req); err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid body"})
    }
    // Validate and sanitize: allow only printable ASCII for name, strict email format
    if !isValidName(req.Name) || !isValidEmail(req.Email) {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid input"})
    }
    // Store sanitized data in DynamoDB
    svc := dynamodb.NewFromConfig(cfg)
    _, err := svc.PutItem(context.TODO(), &dynamodb.PutItemInput{
        TableName: aws.String("Users"),
        Item: map[string]types.AttributeValue{
            "UserID": &types.AttributeValueMemberS{Value: uuid.NewString()},
            "Name":   &types.AttributeValueMemberS{Value: req.Name},
            "Email":  &types.AttributeValueMemberS{Value: req.Email},
        },
    })
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "unable to store user"})
    }
    // Compose email using controlled template; do not inject user data into headers
    subject := "Welcome to Our Service"
    body := fmt.Sprintf("Hello %s,\n\nWelcome! Your email on file is %s.", req.Name, req.Email)
    if err := sendEmail(req.Email, subject, body); err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "email failed"})
    }
    return c.SendStatus(fiber.StatusCreated)
})

func isValidName(name string) bool {
    // Allow letters, spaces, hyphens, apostrophes; reject newlines and control chars
    matched, _ := regexp.MatchString(`^[\p{L} \-']+$`, name)
    return matched
}

func isValidEmail(email string) bool {
    // Basic strict email validation
    matched, _ := regexp.MatchString(`^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$`, email)
    return matched
}

func sendEmail(to, subject, body string) error {
    // Use a trusted email service; keep subject and body templated
    msg := mail.NewMessage()
    msg.SetHeader("From", "no-reply@example.com")
    msg.SetHeader("To", to)
    msg.SetHeader("Subject", subject)
    msg.SetBody("text/plain", body)
    return smtp.SendMail("smtp.example.com:587", nil, "no-reply@example.com", []string{to}, msg)
}

Key points: user input is validated before DynamoDB operations, stored as data fields only, and never concatenated into email headers. The email subject and recipients are controlled constants or safely derived from a verified email field, not from user-provided strings that could contain newline characters. This approach mitigates both injection into DynamoDB-stored data and subsequent injection into email generation.