Emails Vulnerabilities
Payloads
Ignored parts of an email
The symbols: +, - and {} in rare occasions can be used for tagging and ignored by most e-mail servers
E.g. john.doe+intigriti@example.com → john.doe@example.com
Comments between parentheses () at the beginning or the end will also be ignored
E.g. john.doe(intigriti)@example.com → john.doe@example.com
Whitelist bypass
inti(;inti@inti.io;)@whitelisted.com
inti@inti.io(@whitelisted.com)
inti+(@whitelisted.com;)@inti.io
IPs
You can also use IPs as domain named between square brackets:
john.doe@[127.0.0.1]
john.doe@[IPv6:2001:db8::1]
Other vulns
Third party SSO
XSS
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.
Reply-To
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**.
References
Hard Bounce Rate
Some applications like AWS have a Hard Bounce Rate (in AWS is 10%), that whenever is overloaded the email service is blocked.
A hard bounce is an email that couldn’t be delivered for some permanent reasons. Maybe the email’s a fake address, maybe the email domain isn’t a real domain, or maybe the email recipient’s server won’t accept emails) , that means from total of 1000 emails if 100 of them were fake or were invalid that caused all of them to bounce, AWS SES will block your service.
So, if you are able to send mails (maybe invitations) from the web application to any email address, you could provoke this block by sending hundreds of invitations to nonexistent users and domains: Email service DoS.
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