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62 lines
4.7 KiB
Markdown
62 lines
4.7 KiB
Markdown
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# SMTP Smuggling
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{% hint style="success" %}
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Learn & practice AWS Hacking:<img src="../../.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="../../.gitbook/assets/arte.png" alt="" data-size="line">\
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Learn & practice GCP Hacking: <img src="../../.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="../../.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Support HackTricks</summary>
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
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* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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{% endhint %}
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## Basic Information
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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.
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### Why
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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:
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<figure><img src="../../.gitbook/assets/image (8) (1) (1) (1) (1).png" alt=""><figcaption><p><a href="https://sec-consult.com/fileadmin/user_upload/sec-consult/Dynamisch/Blogartikel/2023_12/SMTP_Smuggling-Overview__09_.png">https://sec-consult.com/fileadmin/user_upload/sec-consult/Dynamisch/Blogartikel/2023_12/SMTP_Smuggling-Overview__09_.png</a></p></figcaption></figure>
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### How
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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**.
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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.\
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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.
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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.
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Potential desynchronization data:
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* `\n.`
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* `\n.\r`
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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`.**
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## **References**
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* [https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/](https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/)
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{% hint style="success" %}
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Learn & practice AWS Hacking:<img src="../../.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="../../.gitbook/assets/arte.png" alt="" data-size="line">\
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Learn & practice GCP Hacking: <img src="../../.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="../../.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Support HackTricks</summary>
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
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* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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{% endhint %}
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