Email Injection on Azure

Email injection attacks in Azure environments typically exploit the way email services process and validate input parameters. In Azure, this vulnerability often appears in applications using Azure Logic Apps, Azure Functions with SendGrid bindings, or custom SMTP configurations with Azure services.

The most common attack vector involves manipulating email headers through crafted input. For example, when an Azure Logic App receives user input for 'To', 'Subject', or 'Body' fields, an attacker can inject newline characters (%0A or %0D) to add additional headers or modify existing ones. This allows attackers to:

• Send emails to arbitrary recipients without authorization
• Modify email subjects to impersonate legitimate communications
• Add CC or BCC recipients to leak sensitive information
• Change the sender address to appear as a trusted source
• Inject spam content or phishing links

In Azure Functions using SendGrid bindings, the vulnerability manifests when the function accepts dynamic email parameters without proper sanitization. Consider this vulnerable Azure Function code:

module.exports = async function (context, req) {
    const message = {
        to: req.query.to,
        from: '[email protected]',
        subject: req.query.subject,
        text: req.query.body
    };
    context.bindings.sendGridMessage = message;
    context.done();
};

An attacker could exploit this by calling the function with:

[email protected]%0ACc: [email protected]%0A%0ABCC: [email protected]%0A%0AContent-Type: text/html%0A%0A<h1>Phishing Alert</h1>

This would send the email to the intended recipient while also CC'ing the attacker and including HTML content that bypasses simple text filtering.

Azure Logic Apps are particularly vulnerable when using HTTP triggers with email actions. If the Logic App doesn't validate input parameters, an attacker can craft requests that modify the email workflow. For example:

POST /api/logicapp-trigger HTTP/1.1
Host: mylogicapp.azurewebsites.net
Content-Type: application/json

{
    "to": "[email protected]\nCc: [email protected]",
    "subject": "Important Update\nContent-Type: text/html",
    "body": "<script>alert('XSS')</script>"
}

The Logic App would process these headers literally, sending the email with unauthorized recipients and potentially malicious content.

Detecting email injection in Azure environments requires both static code analysis and runtime scanning. For Azure Functions and Logic Apps, middleBrick provides specialized detection capabilities that scan the unauthenticated attack surface without requiring credentials or code access.

middleBrick's Azure-specific scanning identifies email injection vulnerabilities by testing for:

• Header injection points in HTTP-triggered functions
• Parameter manipulation in Logic App email actions
• SMTP configuration vulnerabilities in custom Azure services
• SendGrid binding misconfigurations

The scanner tests for common injection patterns including newline characters, header splitting, and parameter manipulation. It also checks for proper input validation and sanitization mechanisms that should be in place.

For Azure Logic Apps specifically, middleBrick analyzes the workflow definition to identify email actions that accept dynamic input without validation. The scanner checks for:

• HTTP trigger actions with email destinations
• Dynamic content references in email fields
• Missing validation actions before email sending
• Hardcoded credentials or API keys in email configurations

Here's how you can use middleBrick to scan an Azure Function for email injection vulnerabilities:

npx middlebrick scan https://myazurefunction.azurewebsites.net/api/send-email

# Or integrate into CI/CD
npx middlebrick scan --fail-on-high https://mylogicapp.azurewebsites.net/api/trigger

The scan results provide a security risk score (A-F) with specific findings about email injection vulnerabilities, including the exact parameters that are vulnerable and remediation steps.

For Azure Logic Apps, middleBrick can analyze the published endpoint and workflow structure to identify injection points. The scanner tests various injection payloads and reports on the application's resilience to email header manipulation.

Additional detection methods include:

1. Monitoring Azure Application Insights for unusual email patterns
2. Using Azure Security Center to identify misconfigured email services
3. Implementing custom logging to track email parameter modifications
4. Regular penetration testing focused on email functionality

Remediating email injection vulnerabilities in Azure requires implementing proper input validation and sanitization at the application layer. Here are Azure-specific approaches to secure your email functionality:

For Azure Functions using SendGrid bindings, implement strict input validation:

const validator = require('email-validator');

module.exports = async function (context, req) {
    // Validate email addresses
    if (!validator.validate(req.query.to)) {
        context.res = { status: 400, body: 'Invalid email address' };
        return;
    }
    
    // Sanitize subject and body to prevent header injection
    const sanitize = (input) => {
        return input.replace(/\r/g, '').replace(/\n/g, ' ').trim();
    };
    
    const message = {
        to: req.query.to,
        from: '[email protected]',
        subject: sanitize(req.query.subject),
        text: sanitize(req.query.body)
    };
    
    context.bindings.sendGridMessage = message;
    context.done();
};

For Azure Logic Apps, implement validation workflows before email actions:

1. Add HTTP trigger with input parameters
2. Insert a validation action using Data Operations - Compose
3. Use expressions to validate email format: if(empty(triggerBody().to), false, contains(triggerBody().to, '@'))
4. Add conditional logic to block invalid inputs
5. Only proceed to email action if validation passes

Here's an example Logic App structure that prevents email injection:

HTTP Request Trigger
    ↓
Validate Input (Data Operations - Compose)
    ↓
Condition: isValidEmail(triggerBody().to) AND isValidSubject(triggerBody().subject)
    ↳ Yes: Send Email Action
    ↳ No: Response with error

For custom SMTP implementations in Azure App Service, use parameterized email libraries that automatically handle header injection:

const nodemailer = require('nodemailer');

module.exports = async function (context, req) {
    const transporter = nodemailer.createTransport({
        host: 'smtp.sendgrid.net',
        port: 587,
        secure: false,
        auth: {
            user: process.env.SENDGRID_USER,
            pass: process.env.SENDGRID_PASS
        }
    });
    
    // Use the library's built-in validation and sanitization
    const mailOptions = {
        from: '[email protected]',
        to: req.query.to,
        subject: req.query.subject.substring(0, 998), // Limit subject length
        text: req.query.body
    };
    
    try {
        await transporter.sendMail(mailOptions);
        context.res = { body: 'Email sent successfully' };
    } catch (error) {
        context.res = { status: 500, body: 'Email sending failed' };
    }
};

Additional Azure-specific security measures:

• Use Azure Key Vault to store email service credentials
• Implement Azure AD authentication for email API endpoints
• Use Azure Application Gateway to add an additional security layer
• Enable Azure DDoS Protection for email services
• Configure Azure Monitor alerts for unusual email sending patterns

For enterprise deployments, consider using Azure Communication Services instead of direct SMTP, as it provides built-in security features and better integration with Azure's security ecosystem.