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This section is going to be based on **how to abuse this parameter supposing that an attacker controls it**.
This parameter is going to be added to the command line PHP will be using to invoke the binary sendmail. However, it will be sanitised with the function `escapeshellcmd($additional_parameters)`.
An attacker can **inject extract parameters for sendmail** in this case.
**sendmail** interface is **provided by the MTA email software** (Sendmail, Postfix, Exim etc.) installed on the system. Although the **basic functionality** (such as -t -i -f parameters) remains the **same** for compatibility reasons, **other functions and parameters** vary greatly depending on the MTA installed.
Depending on the **origin of the sendmail** binary different options have been discovered to abuse them and l**eak files or even execute arbitrary commands**. Check how in [**https://exploitbox.io/paper/Pwning-PHP-Mail-Function-For-Fun-And-RCE.html**](https://exploitbox.io/paper/Pwning-PHP-Mail-Function-For-Fun-And-RCE.html)
Some services like **github** or **salesforce allows** you to create an **email address with XSS payloads on it**. If you can **use this providers to login on other services** and this services **aren't sanitising** correctly the email, you could cause **XSS**.
### Account-Takeover
If a **SSO service** allows you to **create an account without verifying the given email address** (like **salesforce**) and then you can use that account to **login in a different service** that **trusts** salesforce, you could access any account.\
_Note that salesforce indicates if the given email was or not verified but so the application should take into account this info._
You can send an email using _**From: company.com**_ and _**Replay-To: attacker.com**_ and if any **automatic reply** is sent due to the email was sent **from** an **internal address** the **attacker** may be able to **receive** that **response**.
Certain services, like AWS, implement a threshold known as the **Hard Bounce Rate**, typically set at 10%. This is a critical metric, especially for email delivery services. When this rate is exceeded, the service, such as AWS's email service, may be suspended or blocked.
A **hard bounce** refers to an **email** that has been returned to the sender because the recipient's address is invalid or non-existent. This could occur due to various reasons, such as the **email** being sent to a non-existing address, a domain that isn't real, or the recipient server's refusal to accept **emails**.
In the context of AWS, if you send 1000 emails and 100 of them result in hard bounces (due to reasons like invalid addresses or domains), this would mean a 10% hard bounce rate. Reaching or exceeding this rate can trigger AWS SES (Simple Email Service) to block or suspend your email sending capabilities.
It's crucial to maintain a low hard bounce rate to ensure uninterrupted email service and maintain sender reputation. Monitoring and managing the quality of the email addresses in your mailing lists can significantly help in achieving this.
For more detailed information, AWS's official documentation on handling bounces and complaints can be referred to [AWS SES Bounce Handling](https://docs.aws.amazon.com/ses/latest/DeveloperGuide/notification-contents.html#bounce-types).
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