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Wham Bam, the Cutwail/Blackhole Combo

Over the past few weeks we have seen a resurgence of malicious spam with links leading off to the Blackhole exploit kit. Last week about 2% of spam hitting our traps fell into this category, which is pretty significant given that many people still consider 'spam' annoying but harmless. The spam typically originates from the ubiquitous Cutwail spambot variants, but other botnets are also involved. The campaigns vary widely from day to day according to the attackers' whims. The message templates are based on mimicking high-profile brands, for example:

American Express

Today, I'll focus on a Verizon campaign. Below is a sample message, which as is typical, displays some aspect of an account bill or statement. Merely hovering over the link shows that the URL (underlined in red) is not associated with Verizon in any way:


In this case, the link simply leads directly to a site hosting the Blackhole exploit kit. (In other examples I've seen there are one or more redirects via other compromised hosts.) I didn't get the screen grab in time before the site went down, but here is an identical one from a similar campaign, showing the obfuscated JavaScript typical of Blackhole:

Of course, the goal of an exploit kit is to exploit. So how successful is this campaign? As it happens, a colleague of mine happened to 'find' his way into the admin panel of this particular Blackhole server and take a screenshot, which reveals some interesting information indeed:


I love looking at admin panels! At a glance you can see what is driving the bad guys. If you somehow can't read Russian, I have annotated the four columns representing Hits, Hosts, Downloads (successful exploits), and Download Rate (percentage of successful exploits). You can see the highlighted domain from the original spam message had a 10% success rate, 17 installs out of 167 unique host visits. Overall success rates for the kit were 13.06% for this particular day, and 7.77% overall. Is it just me, or does anyone else think that a 10% success rate is higher than it should be?

The bottom section displays the array of exploits used. Notably, over 75% of successful installs were accomplished using some type of PDF exploit, with over 50% resulting from an exploit targeting the Adobe Reader PDF LibTiff vulnerability (CVE-2010-0188). The "PDF ALL" refers to a bundle of known PDF exploits including GetIcon (CVE-2009-0927), CollectEmailinfo (CVE-2007-5659), printf (CVE-2008-2992), and newPlayer (CVE-2009-4324).

Some of the other exploits used in this kit are the trusty MDAC (CVE-2006-0003), Java AtomicReferenceArray (CVE-2012-0507), Microsoft Help Center URL Validation (CVE-2010-1885), and a Flash exploit, which is most likely SWF File Remote Memory Corruption (CVE-2011-0611), given past Blackhole analysis.

On the installs by country, while the United States had the lowest install rate (6.16%), it had the highest total number of installs by far (616), reflecting its higher number of hits. This is not really surprising given that the spam campaigns, and the brands used, seem specifically targeted at US users. The operator of this kit is most likely affiliated with a pay-per-install program, and these programs typically pay more for US-based computers. Another interesting stat was the 13 successful installs in Russia, representing a high 35% exploit rate. Hmmm, I wonder whether some of these are the kit operator's test machines?

So what can we learn from this little analysis?

  • Malicious spam campaigns are large, widespread and change templates daily. It can be 'Verizon' one day and 'LinkedIn' or 'Facebook' the next.
  • The spam may look pretty convincing if you happen to be a customer of the brand being spammed (otherwise it should be pretty darned suspicious!) Check out URLs by hovering over links. Most email clients will preview the URL.
  • All the exploits targeted by Blackhole are public and patches are available. Simply making sure all your software is up to date will provide a lot of protection. This should include third party apps and browser plugins, and especially Adobe Reader, Java, and Flash.
  • Go and ensure your PDF Reader is up to date. Now!

Trustwave Secure Web Gateway and MailMarshal Secure Email Gateway provide protection against Blackhole and other exploit kits, and these spam campaigns, respectively.

Thanks to fellow SpiderLabs colleague Daniel Chechik for his input and for wandering into this particular Blackhole kit.