Comment Email template injection (Score 1) 17
Original use case: Your site allows an anonymous visitor to register an account. They enter a USERNAME and an EMAIL ADDRESS. Your site sends them a verification code using the text: "USERNAME, thank you for registering, your verification code is XXXXXX". Your site's emails pass SPF and DKIM verification.
Guess what, now you are an accomplice in spamming, phishing, and fraud.
How so?
The attacker pretends to register with the EMAIL ADDRESS of victim@example.com and a USERNAME of "Congratulations, you are eligible for a gift, please claim it at malicious.site.example.net/UNIQUE_STRING". The site name is crafted to be a typosquatting attack on some site well known to the victim.
Your site dutifully sends that verification code. After template substitution, the text becomes: "Congratulations, you are eligible for a gift, please claim it at malicious.site.example.net/UNIQUE_STRING, thank you for registering, your verification code is XXXXXX". Which happens to be a grammatically correct sentence in English.
If the victim is gullible enough to click through, the unique string allows the attacker to deduce that the victim's address is in fact active, being read, and being clicked through. Then the malicious site can ask for the user's credentials for the original site, or for credit card number + exp date + pin, or other interesting stuff.