Full Report
SMTP, the Simple Mail Transfer Protocol, is the base email protocol that helps run the world today. Finding emails in servers could allow for terrible email spoofing and mass havoc being caused. The SMTP protocol is newline based, similar to HTTP. Unlike HTTP, it has commands. These commands include setting the sender, recipient, subject and more. The ending sequence is \r\n.\r\n and is the ending of the data. HTTP smuggling is a vulnerability where two different interpreters of the protocol (nginx vs. apache) see the data coming in separately. This leads to an attack where one server may think it's one thing while the other sees it as another. Being able to break the underlying parser in this way can allow for smuggling or adding of unintended information by an attacker. The authors decided to look for a similar type of bug but in SMTP to see commands within SMTP. SMTP servers support SMTP pipelining for a series of requests. Breaking out of these would be amazing to change the information being used on the next set of emails. They decided to try various different ending sequences that were typically invalid to the specification but may be supported. \n.\n, \r\n and many other things were tried. GMX was vulnerable to the \n.\r\n method. On Microsoft Exchange, \n.\r\n broke the parsing as well. They were using BDAT, where the size is specified. However, if the server doesn't support the BDAT then it will default to DATA once its came from Outlook. This worked on their own server as well as Amazon, Paypal, eBay and many others. This was on the outbound that caused problems btw. They started fuzzing various servers on the inbound size. If it timed out, then the EOL was not accepted. Otherwise, they had figured something out. This was useful for testing the interpretation more quickly than just sending emails and looking at the responses. It turns out that \r.\r got accepted by Cisco Secure email Many of the protections on email are bypassed from this method. I found that particularly interesting. The response from Microsoft was very sad. They claimed it wasn't a big deal since it required a non-standard sequence understanding on the other side of this. Honestly, going forward, I expect to see more server vulnerable to SMTP smuggling, similar to how it got popular once HTTP smuggling became popular.
Analysis Summary
# Vulnerability: SMTP Smuggling (Zero-Day Exploitation via Non-Standard Endings)
## CVE Details
- CVE ID: *Specific CVEs were not explicitly listed in the summary, but multiple 0-days were discovered and disclosed.*
- CVSS Score: *Not specified in the provided text.*
- CWE: CWE-20 (Improper Input Validation) or similar parser-related weakness.
## Affected Systems
- Products: Microsoft Exchange, GMX/IONOS, Cisco Secure Email, Amazon SMTP services, PayPal SMTP services, eBay SMTP services, Postfix (pre-patch/configuration).
- Versions: Not specified, but affected various inbound and outbound services across these vendors.
- Configurations: Systems that process SMTP commands where the parser incorrectly interprets non-standard End-of-Line (EOL) sequences meant to terminate the `DATA` command.
## Vulnerability Description
This vulnerability, dubbed **SMTP smuggling**, is analogous to HTTP smuggling. It exploits inconsistencies in how different components (e.g., a frontend receiving gateway vs. the backend mail relay/parser) interpret the termination sequence for an SMTP `DATA` command.
SMTP servers use `\r\n.\r\n` to indicate the end of the message body data. Researchers found that using non-standard EOL sequences, such as `\n.\r\n` (vulnerable in GMX) or `\r.\r` (vulnerable in Cisco Secure Email), could cause the primary parser to treat the sequence differently than a subsequent interpreter, allowing an attacker to effectively smuggle or append commands intended for the *next* transaction into the current one. This applies primarily to outbound paths when servers default from BDAT to DATA commands based on input interpretation. This flaw allows attackers to spoof the sender address (`MAIL FROM`) for subsequent emails processed by the hijacked session.
## Exploitation
- Status: Multiple 0-days discovered and disclosed responsibly in 2023. Exploitation allows for email spoofing.
- Complexity: The context suggests that finding the vulnerable, non-standard termination sequences requires research, but once a specific sequence is known for a target, exploitation based on similarity to HTTP smuggling might be relatively straightforward for an attacker targeting multiple vendors.
- Attack Vector: Network (SMTP protocol interaction).
## Impact
- Confidentiality: Potential impact if spoofing is used for credential harvesting or sensitive data exfiltration via social engineering.
- Integrity: High. Allows sending malicious emails appearing to originate from arbitrary and trusted addresses (spoofing).
- Availability: Lower direct impact, though wide-scale successful spoofing campaigns could degrade trust in email systems.
## Remediation
### Patches
- **Microsoft Exchange & GMX/IONOS:** Promptly fixed the identified issues (specific patch versions not detailed, but remediation occurred quickly).
- **Postfix:** Developers released short-term workarounds available at the official Postfix documentation site related to SMTP smuggling.
### Workarounds
- Administrators should consult vendor advisories (or the Postfix link for Postfix servers) for specific configuration changes until official patches are fully deployed.
- For Cisco Secure Email, the vendor initially claimed the behavior was a feature, not a bug. Users should review configurations impacting how non-standard EOL sequences are handled, especially if they handle inbound transactions from systems like Outlook that may use non-standard DATA termination.
- **General Mitigation:** Implement strict validation and normalization of EOL sequences (`\r\n`) at the protocol termination layer to prevent parser ambiguity.
## Detection
- Indicators of Compromise: Unexpected termination of an SMTP `DATA` command sequence by the client, followed by commands that should logically belong to the next session (e.g., unsolicited `MAIL FROM` or `RCPT TO` commands occurring immediately after what should have been the final message transmission).
- Detection methods and tools: The article mentions that official testing tools were released following the research, which can likely be used to scan SMTP servers for this ambiguity. (Links to these tools are implied to be on `smtpsmuggling.com`).
## References
- SEC Consult Blog: hxxps://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/
- Postfix Fixes/Workarounds: hxxps://www.postfix.org/smtp-smuggling.html
- Research Summary Site: hxxps://smtpsmuggling.com/
- Portswigger Top 10 mention: hxxps://portswigger.net/research/top-10-web-hacking-techniques-of-2023