It's a well-known fact that filtering emails using IP blocklists (DNSBLs) blocks the vast majority of malicious emails. It's effective and economical, using minimal computational power. So why should you also use domain and hash blocklists for filtering?

Data going into a filter

Basics of email filtering

Firstly, this article isn’t focused on where in the filtering process blocklists should be used. Generally, IP blocklists are used at the top of the filtering stack, at the point of SMTP connect. This should then be followed with content filtering, utilizing domain, and hash blocklists.

Nothing’s ever black and white, though; there are compelling reasons to use domain blocklists at SMTP connect, and other compelling reasons to use IP blocklists at the content filtering stage. But let’s leave those technicalities for now and look at why you should be utilizing domain, zero reputation, and hash blocklists, in addition to IP ones.

IP and domain economics 

Cybercriminals are, for the majority of the time, interested in making money, as easily and quickly as possible. One of our aims is to try and increase the costs associated with these illegal activities, so they are no longer profitable because once the bad guys are not making money, they will look to other ventures.

Why use domain blocklists?

IP addresses are an exceptionally cheap commodity. It’s simple for a miscreant to spin-up a virtual private server (VPS) at a hosting company and get a new IP at minimal cost. Meanwhile, domains cost resources, both in terms of money to buy them and the time it takes to set them up. Hence the supply is never unlimited.

Additionally, domains need registering before they can be used. This brings the following unique opportunity; if we know a domain exists, and it’s malicious before use, our users can block the initial malicious email that is sent, and there is no time-to-respond to the threat anymore. It keeps you ahead of the threat.

Why use newly registered domain blocklists?

Has your organization immediately sent emails from a domain that it has just registered? The probable answer is “no.” Legitimate organizations can take weeks, if not months or even years, to utilize domains in emails. 

However, there’s a high probability that those who are registering domains, with malicious intent, will immediately use the domain before there’s any chance of bad reputation being associated with it. That’s why filtering your email against a list of domains that have been registered very, very recently provides additional protection.

Why use hash blocklists?

The’ bad guys’ always try to outsmart those trying to protect the internet, and what better way to gain a good reputation than to hijack the one of an existing, legitimate account? When the account in question belongs to a large provider, for example, Gmail, it’s not possible to block it by IP or domain….and miscreants know this.

Here’s where email hash blocklists come in useful. A list of cryptographic hashes of `bad` email addresses can be queried; the use of hashes addresses privacy concerns about sending people’s email addresses around. Those running email infrastructure, who are willing to use the hash blocklist, can generate a hash of the email addresses to check and query the service against that. If there’s a match, the email is flagged as malicious.

Cryptowallet hash blocklists

Another current trend are “sextortion” emails. The commonality among these is the inclusion of wallet IDs, for payment. These cryptowallet IDs are hashed and listed in the hash blocklist, so even if these emails are sent from accounts that appear to be legitimate, they can still be successfully blocked. 

Malware hash blocklists

Last but not least are malware files. Yes, you guessed it. These too can be hashed, blocking emails containing dodgy attachments, that evade IP and domain filtering, or are being forwarded from legitimate mailservers or accounts.  

While this is not supposed to replace your standard antivirus protection layer, it can help fill the gap for newly observed malware binaries, not yet identified by your AV engine.

All hash blocklists should be integrated with your content filter, for example, SpamAssassin or Rspamd.

Additional automatic protection

Yes, IP blocklists can filter the majority of spam; however, what about the remaining percentage of email-borne threats that make their way through? With some simple changes to your email infrastructure, you can automatically protect against them. No additional hardware is required, and datasets automatically update with the latest intelligence from our researchers…. what’s not to like?

Related Products

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