Email Injection in APIs

Email injection is a security vulnerability that occurs when an API accepts user-controlled input and incorporates it into email headers or message bodies without proper validation. This allows attackers to manipulate email parameters, send unauthorized messages, or exploit the server's mail functionality.

The vulnerability typically manifests when APIs accept parameters like recipient addresses, CC/BCC fields, subject lines, or message content through URL parameters, form data, or JSON payloads. If the API directly passes these values to an email library without sanitization, attackers can inject additional email headers or modify message routing.

For example, an API endpoint like /api/[email protected]&subject=Hello&body=Message becomes vulnerable if it doesn't validate that the to parameter contains only a single email address. An attacker could submit [email protected]%0A%0DCc:[email protected] to add a CC recipient, or use [email protected]%0ABcc:[email protected] to add a blind carbon copy recipient without the original sender knowing.

Email injection vulnerabilities in APIs can lead to several serious security impacts. The most common scenario involves attackers using the compromised API to send spam or phishing emails, effectively turning the API provider's infrastructure into a spam relay. This can damage the organization's reputation, trigger IP blacklisting, and result in service disruptions.

Attackers can exploit email injection to perform social engineering attacks by sending emails that appear to come from legitimate sources. By manipulating the From, Reply-To, or Return-Path headers, they can impersonate trusted entities and trick recipients into revealing sensitive information or clicking malicious links.

More sophisticated attacks involve email header injection to bypass security controls. An attacker might inject Content-Type: text/html headers to send HTML emails with embedded tracking pixels or malicious scripts. They could also manipulate Date or Message-ID headers to interfere with email threading or filtering systems.

Supply chain attacks become possible when email injection affects notification systems. An attacker could intercept order confirmations, password reset emails, or payment notifications by injecting themselves as a CC or BCC recipient, gaining access to sensitive business information or customer data.

Detecting email injection vulnerabilities requires systematic testing of API endpoints that handle email functionality. The primary approach involves sending specially crafted payloads that attempt to inject additional email headers or modify existing ones.

Start by identifying API endpoints that accept email-related parameters. Look for endpoints with parameters like to, cc, bcc, subject, from, or replyTo. Test each parameter by submitting values containing newline characters (%0A or %0D in URL encoding) followed by additional header fields.

For example, test with payloads like:

to: [email protected]%0A%0DSubject: Injection Test%0ABcc: [email protected]%0A%0DContent-Type: text/html%0A%0D%0A<script>alert('XSS')</script>

Monitor the actual emails sent to verify if the injection succeeded. Check if additional recipients were added, if HTML content was rendered, or if the subject line was modified.

middleBrick automatically scans for email injection vulnerabilities as part of its Input Validation security check. The scanner tests API endpoints by submitting payloads with newline characters and additional header fields, then analyzes the responses to determine if the injection succeeded. This black-box approach requires no credentials and can identify vulnerabilities in seconds.

The scanner also checks for other related issues like insufficient email validation (allowing invalid email formats), lack of rate limiting on email endpoints (enabling spam), and improper handling of email content that could lead to XSS in HTML emails.

Preventing email injection requires a defense-in-depth approach that validates and sanitizes all email-related input. The most critical step is implementing strict input validation for all email parameters.

For email address validation, use robust regex patterns or built-in email validation libraries rather than simple string checks. Verify that addresses match the RFC 5322 standard and reject any input containing newline characters, carriage returns, or other control characters. Most programming languages provide email validation functions that handle edge cases correctly.

Implement whitelist validation for email headers. Only allow specific, expected headers and reject any attempt to add new headers through user input. For example, if your API only needs to and subject fields, explicitly reject any input that tries to add CC, BCC, or other headers.

Use parameterized email APIs when available. Many email libraries provide methods that separate email addresses from headers, preventing injection by design. For example, instead of building raw email strings, use functions like send_mail(to=[...], subject=..., body=...) that handle sanitization internally.

Sanitize all user input by removing or encoding newline characters and other control characters. Replace %0A, %0D, and similar sequences with empty strings or safe alternatives before processing.

Implement rate limiting on email endpoints to prevent spam abuse. Set reasonable limits on the number of emails that can be sent per minute, hour, or day, and require API keys or authentication for higher-volume usage.

Here's a Python example of secure email handling:

import re
import smtplib
from email.message import EmailMessage

def is_valid_email(email):
    # Basic RFC 5322 compliant validation
    pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    return re.match(pattern, email) is not None

def sanitize_input(value):
    # Remove control characters and newlines
    return re.sub(r'[
]+', '', value)

def send_secure_email(to, subject, body):
    if not is_valid_email(to):
        raise ValueError('Invalid email address')
    
    msg = EmailMessage()
    msg['To'] = to
    msg['Subject'] = sanitize_input(subject)
    msg.set_content(sanitize_input(body))
    
    with smtplib.SMTP('localhost') as server:
        server.send_message(msg)

This approach validates email addresses, removes dangerous characters, and uses the email library's built-in security features rather than constructing raw email strings.

Email injection vulnerabilities have caused significant damage to organizations across various industries. In 2018, a vulnerability in a popular PHP contact form library allowed attackers to inject arbitrary email headers, leading to widespread spam campaigns originating from compromised websites. The vulnerability affected thousands of sites and resulted in numerous IP blacklistings.

A 2020 incident involved a SaaS company whose API allowed customers to send notification emails. Due to insufficient input validation, attackers discovered they could inject BCC headers, silently copying themselves on all customer notifications. This led to massive data exposure as attackers gained access to order confirmations, account updates, and other sensitive communications across the platform's customer base.

The CVE-2021-3176 vulnerability in PHPMailer, a widely-used PHP email library, demonstrated how email injection could be combined with other attacks. The vulnerability allowed remote code execution through crafted email headers, affecting applications that passed user input directly to the library without proper sanitization.

Financial services companies have faced regulatory penalties when email injection vulnerabilities allowed unauthorized access to customer communications. In one case, a bank's API for sending transaction alerts was exploited to intercept fraud notifications, enabling attackers to bypass security controls and complete unauthorized transactions.

These incidents highlight why email injection is listed in the OWASP API Security Top 10 as a critical vulnerability. The combination of reputational damage, regulatory compliance issues, and direct financial losses makes email injection a high-priority security concern for any API that handles email functionality.