# SMTP Smuggling
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## Basic Information
This type of vulnerability was [**originally discovered in this post**](https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/) were it's explained that It's possible to **exploit discrepancies in how the SMTP protocol is interpreted** when finalising an email, allowing an attacker to smuggle more emails in the body of the legit one, allowing to impersonate other users of the affected domain (such as admin@outlook.com) bypassing defenses such as SPF.
### Why
This is because in the SMTP protocol, the **data of the message** to be sent in the email is controlled by a user (attacker) which could send specially crafted data abusing differences in parsers that will smuggle extra emails in the receptor. Take a look to this illustrated example from the original post:
### How
In order to exploit this vulnerability an attacker needs to send some data that the **Outbound SMPT server thinks that it's just 1 email but the Inbound SMTP server thinks that there are several emails**.
The researchers discovered that different **Inboud servers considers different characters as the end of the data** of the email message that Outbound servers doesn't.\
For example, a regular end of the data is `\r\n.\r\n`. But if the Inbound SMTP server also supports `\n.\n`, an attacker could just add **that data in his email and start indicating the SMTP commands** of a new new ones to smuggle it just like in the previous image.
Ofc, this could only work if the **Outbound SMTP server doesn't also treat this data** as the end of the message data, because in that case it will see 2 emails instead of just 1, so at the end this is the desynchronization that is being abused in this vulnerability.
Potential desynchronization data:
* `\n.\n`
* `\n.\r\n`
Also note that the SPF is bypassed because if you smuggle an email from `admin@outlook.com` from an email from `user@outlook.com`, **the sender is still `outlook.com`.**
## **References**
* [https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/](https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/)
Learn AWS hacking from zero to hero withhtARTE (HackTricks AWS Red Team Expert)!
Other ways to support HackTricks:
* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
 (1).png)
