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Scanning the Internet in 45 Minutes


The Internet is a big thing. Or, more accurately, a big collection of things. Figuring out exactly how many things, and what vulnerabilities those things contain has always been a challenge for researchers, but a new tool released by a group from the University of Michigan that is capable of scanning the entire IPv4 address space in less than an hour.

There have been a handful of Internet-wide scans done by various organizations over the years, but most of them have not had a security motivation. And they can take days or weeks, depending upon how the scan is done and what the researchers were trying to accomplish. But the new Zmap tool built by the Michigan researchers has the ability to perform an Internet-wide scan in about 45 minutes while running on an ordinary server. The tool, which the team presented at the USENIX Security conference last week, is open-source and freely available for other researchers to use.

To demonstrate the capabilities of Zmap, the Michigan team, which comprises J. Alex Halderman, an assistant professor, and Eric Wustrow and Zakir Durumeric, both doctoral candidates, ran a scan of the entire IPv4 address space, returning results from more 34 million hosts, or what they estimate to be about 98 percent of the machines in that space. Zmap is designed specifically to bypass some of the speed obstacles that have slowed down some of the previous large-scale scans of the Internet. The researchers removed some of the considerations for machines on the other end of the scan, for example assuming that they sit on well-provisioned networks and can handle fast probes. The result is that the tool can scan more than 1,300 times faster than the venerable Nmap scanner.

“While Nmap adapts its transmission rate to avoid saturating the source or target networks, we assume that the source network is well provisioned (unable to be saturated by the source host), and that the targets are randomly ordered and widely dispersed (so no distant network or path is likely to be saturated by the scan). Consequently, we attempt to send probes as quickly as the source’s NIC can support, skipping the TCP/IP stack and generating Ethernet frames directly. We show that ZMap can send probes at gigabit line speed from commodity hardware and entirely in user space,” the researchers say in their paper, “Zmap: Fast Internet-Wide Scanning and Its Security Implications”.

“While Nmap maintains state for each connection to track which hosts have been scanned and to handle timeouts and retransmissions, ZMap forgoes any per-connection state. Since it is intended to target random samples of the address space, ZMap can avoid storing the addresses it has already scanned or needs to scan and instead selects addresses according to a random permutation generated by a cyclic multiplicative group.”

That stateless scanning, the researchers said, allowed Zmap to get both faster response times and better coverage of the target address space. As for practical applications of the tool, the researchers already have found several. In the last year, the team ran 110 separate scans of the entire HTTPS infrastructure, finding a total of 42 million certificates. Interestingly, they only found 6.9 million certificates that were trusted by browsers. They also found two separate sets of mis-issued SSL certificates, something that’s been a serious problem in recent years.

The Zmap team also wrote a custom probe to look for the UPnP vulnerability that HD Moore of Rapid 7 discovered in January. After scanning 15.7 million devices, they found that 3.3 million were still vulnerable. That bug can be exploited with a single packet.

“Given that these vulnerable devices can be infected with a single UDP packet [25], we note that these 3.4 million devices could have been infected in approximately the same length of time-much faster than network operators can reasonably respond or for patches to be applied to vulnerable hosts. Leveraging methodology similar to ZMap, it would only have taken a matter of hours from the time of disclosure to infect every publicly available vulnerable host,” the researchers say in the paper.

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