In the last few months, we have seen an increase of generic emails (e.g.
regarding unpaid invoices) being sent with links to infected legitimate
websites hosting malware. This malware often comes in the form of docs
with macros e.g. https://pastebin.com/VHz41RUL
In a lot of cases, neither the sender nor the URL are listed in any
blacklists at send time, and we are looking into ways to deal with these
links. We have developed some heuristics based on the text but this is
more reactive than proactive and the spams often are very similar to
legitimate emails. Ideally we would be able to see what is /really/
behind these links.
The technologies we know exist are:
a) Link following
Whether it is only for url shorteners or for all links, simulating a
click could give us info on what will happen, but has implications when
the website interprets that like a click from the user and updates their
database in some way such as unsubscribing a user.
b) Link rewriting
Rewrite the link so that it is analysed by the anti-spam provider at
click-time. Costly to implement and breaks message integrity/DKIM. Even
after 24h, a lot of these infected websites are not listed on
blacklists. This method also has privacy implications.
c) DNS-based approaches
Similar to link rewriting, use a dns-firewall such as Cisco Umbrella to
block queries to malicious websites. Our tests indicate that this does
not work very well for the aforementioned infected websites. It might
work well for C&C servers but we feel like that is a bit late to avoid
Are there other solutions that we have not thought of? Are any of you
having trouble with these types of links?