In the wake of any high-profile tragedy, scammers quickly spring into action. That's continued to be the case in the wake of Monday's Boston Marathon bombing, as attack campaigns backed by spam and malicious websites have used the tragedy as a lure for infecting PCs with malware.

By Wednesday, Cisco reported that 40% of all spam emails seen crossing the Internet were name-dropping the Boston Marathon bombing.

But related malicious online activities began just hours after the bombing, with more than 125 potentially fake domains that tied to the event registered Monday, according to TheDomain.

By Tuesday, that number had grown to 234, reported John Bambenek at Internet Storm Center. "Some of these are just parked domains, some are squatters who are keeping the domains from bad people. A couple are soliciting donations (one is soliciting bitcoins, oddly enough)," he said. "So far, there has been no reports of any spam related to this but there have been a few fake twitter accounts which are fairly quickly getting squashed."

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Also Tuesday, "the first of two botnets began a massive spam campaign to take advantage of the recent Boston tragedy," said Craig William, a technical leader in Cisco's security intelligence operations group, in a blog post. "The spam messages claim to contain news concerning the Boston Marathon bombing. The spam messages contain a link to a site that claims to have videos of explosions from the attack. Simultaneously, links to these sites were posted as comments to various blogs."

Related email subject lines included "Explosion at Boston Marathon" and "Boston Explosion caught on Video," and many of the emails contained a link that included a numeric IP address with "news.html" or "boston.html" at the end. "Once visited, the page redirects to three other URLs which try to drop a JAR [Java archive] file on your system, if they detect that the computer has a vulnerable Java installation installed," said security researcher Sorin Mustaca at antivirus vendor Avira, in a blog post.

Another string of malicious websites used iFrames to display related videos of the Boston explosions from YouTube while attempting to silently install a malicious Java app that exploits a known Java vulnerability. According to some reports, these sites would later push a malicious Windows executable (.exe) file, too.

Wednesday, a second botnet began sending Boston bombing-related spam, including a link that was falsely labeled as having come from CNN. "In reality, the link takes users to a compromised website that contains an instant HTTP meta-refresh redirect to an attacker controlled site that we believe is attempting to install the Blackhole Exploit Kit," said Cisco's Williams. Crimeware toolkits such as Blackhole target known vulnerabilities on a PC. If successfully exploited, the toolkit drops additional malware onto the system, allowing attackers to turn the PC into a spam relay or node in a distributed-denial-of-service attack. In addition, any sensitive data stored on the PC, such as banking credentials, may be transmitted to attackers.

Jason Hill, a researcher at Websense Security Labs, also reported seeing a bombing-themed exploit that uses the RedKit Exploit Kit to "exploit an Oracle Java 7 Security Manager Bypass vulnerability (CVE-2013-0422)" that was patched by Oracle in January.

In any of the above attack scenarios, criminals are using the bombing as a lure to trick users into executing malicious files or visiting sites that launch drive-by attacks. "This social engineering technique is not new. We see this every time there is something happening in the world (war, natural catastrophes, social events) that is potentially interesting for a lot of people," said Avira's Mustaca.

"Clearly, there are no depths to which cybercriminals are not prepared to stoop in their hunt for victims," Graham Cluley, senior technology consultant at Sophos, said in a blog post. "The sick truth is that malware authors and malicious hackers lose no sleep about exploiting the deaths of innocent people in their attempt to infect computers for the purposes of stealing money, resources and identities."