This post is an unpublished review for Subscription Bombing • Email under Attack

One of the most important inputs one can have when designing a response strategy against a security attack is a good characterization. This article describes a relatively newly described attack mode (subscription bombing), hypothetizes on the motivations that can lie behind it, and presents some countermeasures that can be taken by different actors to reduce its impact.

At its core, suscription bombing is a classical reflection attack: it uses a third party service so that the answer to a relatively simple request is amplified and results in a distributed denial of service (DDoS) for the victim. And, as with most DDoS attacks, its effectivity lies in that there is not much a person can do against traffic coming from seemingly random different providers all around the world.

The core differentiatof for subscription bombing is that the attack’s victim is not a network port, but an individual’s e-mail address. The attacker builds a database of service providers that allow interested users to sign up for newsletter on their activities, or a mailing list, or even just to create a new account on a given Web system. This action will generate a (seemingly legitimate) confirmation mail sent to the victim. But the attacker scripts together hundreds of thousands of such request, creating a deluge of confirmation mails sent to the unsuspecting victim.

The authors explain the goals an attacker might pursue by performing this kind of attack. They suppose this can be due to harassment (a disgruntled employee being denied a salary raise, a political adversary, or even a romantic ex-partner wanting to inconvenience the victim’s use of their e-mail). More worryingly, the attack can be used as a distraction: by sending a high volume of mails in a controlled timeframe, the attacker can reduce the probability of the victim noticing a specific attack warning them of, i.e., financial fraud, unwanted purchases, or break-in attempts into their accounts. Attacks targetting mailboxes at private mail servers can also lead to overloading an account’s limit, causing it to reject mails after the attack is delivered and before the folder is cleaned. And it can also pave the way for follow-up, targetted deception attacks, where the attackers call the victim pretending to be the company’s IT department, and get them to install a remote desktop monitoring and management tool, with which they can effectively seize control of the victim’s data.

To do this, they present a study they made over 24 cases of victims, from which 47,970 total e-mails were received between October and December 2024, with individual attacks receiving between 81 and 3,387 e-mails per hour, from where they presented several descriptive analysis.

The authors explored cyber criminal’s offers on underground websites, comparing flooding services and pricing schemes.

Finally, mitigation strategies are discussed. Mitigation is quite problematic, as none of the mail servers is acting in either a hostile way or lacking permissions — they are performing just the task they should. The authors suggest four mitigation strategies for mail server operators to reduce the burden on their users, although none of them is easily automatizab (rate-limit the number of emails a given inbox can receive from previously unseen senders; educate users about this kind of attacks; group similar newsletter or account reset mails during active attacks; and automatically unsubscribe or bounce newsletter messages when a surge is detected). They also recommend newsletter providers and services accepting the unrestricted creation of user accounts to provide some hardening to increase the effort wrongdoers need to spend to abuse their services, such as requiring CAPTCHAs or requiring users to take several steps before requesting a subscription, although they recognize this adds friction to the process providers are most interested in providing; filtering and triaging known-good and known-bad domains, although this is hard to implement on a preemptive fashion, and adhering to easy unsubscription standards, such as easily identifiable headers with which mass unsubscription could be performed more easily victims, instead of hunting for the right places to click, potentially even in mails written in an unknown language.

The described problem is interesting, and properly tackling it can be a game changer for many users who will suffer this kind of abuse, and the article is easy to read and soundly supports its claims.