On Tue, 13 Mar 2018, Olivier Coutu wrote:

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 an infection.

Are there other solutions that we have not thought of? Are any of you having trouble with these types of links?

d) Don't accept emails from outside your organization that link to hosted documents. The document needs to be attached, so that it can be scanned. Unfortunately this is not feasible if you're not a (at least semi-)monolithic organization where you can apply such policies.

e) if you are a (semi-)monolithic organization that uses a boundary proxy to control web access, then add such URLs to that proxy's blocklist until the contents can be scanned, or so that the proxy does the redirect-through-AV automatically (not sure if that will work, though).

 John Hardin KA7OHZ                    http://www.impsec.org/~jhardin/
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