It's the Google for the Internet of Things, a playground for hackers and terrorists -- and, maybe, a useful tool for companies looking to lock down their own environment.
Shodan founder John Matherly launched the search engine more than five years ago as a marker intelligence tool, designed to provide technology companies with information about where and how their products was being used.
"And of course the same type of information can be queried about competitors, to better understand how they're positioned in the market using empirical data," said Matherly.
Since its launch, however, the search engine has taken on a life of its own, he admits.
"It has become a tool by security experts to gain a better understanding of the Internet," he said.
The public website is actually a small piece of what Shodan offers. Enterprise clients can buy raw, real-time access to all the data it collects.
For example, a company can use Shodan to search its own networks.
"It is very common for large companies to have a random computer laying around running Telnet or having a building automation system online that wasn't properly configured by a contractor," he said. "And with the advent of cloud computing, I've seen a big increase in the number of publicly-accessible cloud servers that don't have any authentication enabled and therefore leak their entire contents to the Internet."
A company can also use Shodan to check the security of companies they're considering acquiring or doing business with.
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Another use of Shodan is to find the malware command and control servers used by cybercriminals, which is normally a very time-intensive process.
"However it is very straight-forward to identify them with Shodan once a fingerprint has been established," Matherly said.
"I don't see it as a threat," said Leonard Jacobs, president and CEO at Minneapolis-based managed security service provider Netsecuris Inc. "For our practice, we see it as a good thing for our customers, we use it to confirm what we find through other techniques."
In theory, a company would know about all the devices and systems it has exposed on the Internet, he said.
But sometimes, for the sake of convenience, corners are cut.
"You'll be surprised what you'll find out there," he said.
A force for evil
Shodan allows attackers to quickly identify specific devices, or specific software, on a very large scale.
"For example, every web-connected Furby could be identified quickly by going to Shodan and looking for the appropriate signature, versus the hacker having to scan the entire Internet," said Shane MacDougall, a partner at Canadian security consultancy Tactical Intelligence Inc.
Cybercriminals, terrorists, rogue nation-states, even rival companies can use Shodan to identify critical infrastructure and cause it to malfunction, said Jean Taggart, security researcher at San Jose-based Malwarebytes Corp.
And fixing these problems isn't always a high priority for organizations, he added.
"I worry that we won't see movement until a serious enough event occurs," he said. "A government-led effort to identify the owners of these devices and secure them is in high order."
Using Shodan also means that a hacker doesn't set off any warning bells at a targeted company.
According to Michael Baucom, vice president of R&D at Columbia, Md.-based Tangible Security, the true power of Shodan is that all the scanning has already been done -- the user is simply querying the results without revealing their address or actions to the target, and with minimal effort.
"Scans of the magnitude of Shodan would take a long time and would be very noisy," he said.
But Shodan doesn't actually create any vulnerabilities, said Hagai Bar-El, CTO at Sansa Security, an Israel-based security firm focusing on the Internet of Things.
"It merely points them out," he said. "Unarguably, detecting weak nodes is an important part of an attack, but hackers have been able to do this with automated crawlers, long before Shodan was developed."
In addition, most independent security researchers agree that once a vulnerability is discovered, the public is better off if it is publicized rather than kept secret, he said.
"The public that relies on the vulnerable system has a right to be made aware of its vulnerable state," he said. That way, they can do something about it, such as switching vendors.
"Publishing a flaw found in a commercial product is the only effective way of encouraging the vendor to actually fix it," he added. "History is full of examples of vendors that have ignored security issues in their products for months and years until those flaws are made public."
Meanwhile, every unfixed vulnerability is a "ticking timebomb," he said.
But it's not just that the bad guys already have these tools available, said Shodan's Matherly,. The Shodan website requires that users register for an account, and only the first set of search results is free.
There are also "numerous technical measures" to prevent abuse, Matherly added.
"In reality, it is much cheaper and effective for the bad guys to use a botnet or a compromised host running [open source network scanning tools] zmap or masscan than to search Shodan."