RFC 2047 encoded-word Email Smuggling Exploit

By /

Listen to this Post

Featured Image
The RFC 2047 “encoded-word” mechanism allows attackers to smuggle encoded payloads into email addresses. Shockingly, some email parsers decode these encoded words before validation, leading to potential security vulnerabilities.

You Should Know:

How RFC 2047 Encoding Works

RFC 2047 defines a way to encode non-ASCII text in email headers using the format: 

=?charset?encoding?encoded-text?=

Example:

=?utf-8?B?dGVzdEBleGFtcGxlLmNvbQ==?=

(Decodes to `[email protected]`)

Exploiting Email Parsers

Some systems improperly decode these strings before validation, allowing:
– Email header injection
– Bypassing email filters
– Phishing attacks with spoofed addresses

Practical Exploitation Example

1. Crafting a malicious email address:

=?utf-8?B?cGhpc2hAZXhhbXBsZS5jb20=?=

(Decodes to `[email protected]`)

2. Testing parser behavior:

echo "From: =?utf-8?B?cGhpc2hAZXhhbXBsZS5jb20=?=" > test.eml 
swaks --to [email protected] --from "=?utf-8?B?cGhpc2hAZXhhbXBsZS5jb20=?=" --server mail.target.com -tls

3. Checking if filters decode before validation:

import email 
msg = email.message_from_file(open("test.eml")) 
print(msg["From"])  Check if decoded or still encoded

Mitigation Techniques

  • Decode after validation
  • Use strict regex validation: 
    ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}$
  • Sanitize headers before processing: 
    from email.header import decode_header 
decoded_header = decode_header(encoded_header)[bash][0] 
if isinstance(decoded_header, bytes): 
decoded_header = decoded_header.decode('utf-8')

Tools to Test Email Parsers

  • Swaks (Swiss Army Knife for SMTP): 
    swaks --to [email protected] --from "=?utf-8?B?YXR0YWNrZXJAZXhhbXBsZS5jb20=?=" --server mail.example.com
  • Python `email` library: 
    import email 
msg = email.message_from_string(raw_email) 
print(msg.get("From"))

What Undercode Say

This exploit demonstrates how improper parsing of RFC 2047 encoded words can lead to email-based attacks. Always:
– Decode after validation
– Use strict input filtering
– Test email parsers for such edge cases

Expected Output:

A secure email system should reject or properly handle encoded-word syntax before processing to prevent header injection and phishing attacks.

Reference: PortSwigger Research: Splitting the Email Atom

References:

Reported By: 0xacb Rfc – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

Join Our Cyber World:

💬 Whatsapp | 💬 Telegram

Related Posts: