SMTP Smuggling

SMTP Smuggling


SMTP smuggling is a novel vulnerability that allows e-mail spoofing by exploiting interpretation differences of the SMTP protocol in vulnerable server constellations. More specifically, different understandings of so called "end-of-data" sequences between outbound (sending) and inbound (receiving) SMTP servers may allow an attacker to smuggle - hence SMTP smuggling - spoofed e-mails (see Figure 1). Threat actors can abuse this to send malicious e-mails from arbitrary e-mail addresses, allowing targeted phishing attacks.

Affected software identified on the outbound side includes Exchange Online and GMX, which are hosting millions of domains. On the inbound side, over a million SMTP instances including Postfix, Sendmail, Cisco and others are affected. This allowed spoofing e-mails from millions of domains (e.g.,,, to millions of SMTP servers.

As a result of miscommunications in the vulnerability disclosure, millions of inbound SMTP servers are left vulnerable to SMTP smuggling. Further cases of SMTP smuggling vulnerabilities in outbound servers would again allow spoofing e-mails from affected domains.
Also due to a severe case of SMTP smuggling in Cisco Secure Email and missing vendor support, it is highly advised to change the vulnerable default configuration (see affected software).

In-depth information on SMTP smuggling can be found in the original blog post as well as in the referenced resources. The official CERT/CC vulnerability note (VU#302671) can be found here.

Figure 1: SMTP smuggling overview


Here are some tools for identifying SMTP smuggling issues that are available on GitHub!

"Official" SMTP smuggling tools:

View SMTP Smuggling Tools on GitHub

Thank you Hanno Böck for smtpsmug!

View smtpsmug on GitHub

I'm vulnerable! What now?

The Affected Software section may include the information you are looking for. Furthermore, there may already be fixes and workarounds for your specific SMTP software online.
If you don't find any solutions online, please create an issue on GitHub.

Affected Software

Official vendor statements can be found in CERT/CC's vulnerability note. However, keep in mind that there are "different views" on the vulnerability (e.g., Cisco).

Software Description Fix/Workaround Smuggling direction CVE
Cisco Secure Email (Cloud) Gateway On-premise and cloud versions of Cisco Secure Email Gateway using the default configuration are vulnerable to severe inbound SMTP smuggling, since end-of-data sequences like "<CR>.</CR>" are accepted. This allows spoofing e-mails from millions of domains. More information can be found in the original blog post. The option "CR and LF Handling" must be reconfigured to "Reject" or "Allow" (deprecated). However, due to known drawbacks, we are currently evaluating other workarounds. This issue will most likely not be addressed by Cisco in the near future. Inbound -
Postfix Postfix was identified to accept non RFC-compliant end-of-data sequences like "<LF>.<CR><LF>". This is not as severe of an issue like with Cisco Secure Email, however this still allows SMTP smuggling, if outbound servers (as identified with Exchange Online and GMX) do not filter such "<LF>.<CR><LF>" sequences. More information can be found in the original blog post. Detailed information can be found on the Postfix website.
Thanks to Wietse and Viktor for providing quick solutions!
Inbound CVE-2023-51764
Sendmail Sendmail and Postfix handle end-of-data sequences the same way. As with Postfix, pipelined SMTP commands by default get executed after a "fake" end-of-data sequence is processed. For now, Sendmail snapshot can be used. More information can be found here. Inbound CVE-2023-51765
Exim Some configurations of Exim are vulnerable to SMTP smuggling similar to Postfix and Sendmail. Upgrading to Exim version 4.97.1 or later should fix this issue. More information can be found on the Exim website Inbound CVE-2023-51766
SurgeMail SurgeMail is vulnerable to SMTP smuggling similar to Postfix and Sendmail. More information on fixing SurgeMail can be found on the SurgeMail website. Inbound -
Exchange Online Outbound Exchange Online servers allowed to send "<LF>.<CR><LF>" sequences to inbound/receiving servers due to insufficient filtering of message data. This behavior is a violation of RFC 5322 and, as of now, was only identified for Exchange Online and GMX (see below). This vulnerability allowed smuggling spoofed e-mails from millions of domains hosted at Exchange Online (e.g.,,,, etc.) to millions of e-mail servers running Postfix, Sendmail, Exim and possibly more. Microsoft has already fixed this issue. Outbound -
GMX As Exchange Online, GMX also allowed to send "<LF>.<CR><LF>" sequences to millions of inbound/receiving servers due to insufficient filtering of message data. GMX promptly fixed this issue. Outbound -
Possibly more! Please help us to identify more vulnerable SMTP software by using the provided tools. If you're vulnerable and using software that is not already in this list, please create an issue on GitHub.

Unaffected Software

Here is a list of software that was confirmed to be unaffected by SMTP smuggling.
However, keep in mind that constellations with multiple SMTP servers could again introduce SMTP smuggling issues.

Software Description
qmail Vanilla qmail was confirmed to be unaffected by SMTP smuggling.



The scanner is currently under review! In the meantime, check out the other available tools.


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