# SMTP Smuggling {% hint style="success" %} Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\ Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
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{% endhint %} ## 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:

https://sec-consult.com/fileadmin/user_upload/sec-consult/Dynamisch/Blogartikel/2023_12/SMTP_Smuggling-Overview__09_.png

### 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`. But if the Inbound SMTP server also supports `\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.\r` 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/) {% hint style="success" %} Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\ Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
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