(Cross-posted from Official Google Blog)

We recently began two new series of posts. The first, which explains how we harness data for our users, started with this post. The second, focusing on how we secure information and how users can protect themselves online, began here. This post is the second installment in both series.- Ed.

We sometimes get questions on what Google does with server log data, which registers how users are interacting with our services. We take great care in protecting this data, and while we've talked previously about some of the ways it can be useful, something we haven't covered yet are the ways it can help us make Google products safer for our users.

While the Internet on the whole is a safe place, and most of us will never fall victim to an attack, there are more than a few threats out there, and we do everything we can to help you stay a step ahead of them. Any information we can gather on how attacks are launched and propagated helps us do so.

That's where server log data comes in. We analyze logs for anomalies or other clues that might suggest malware or phishing attacks in our search results, attacks on our products and services, and other threats to our users. And because we have a reasonably significant data sample, with logs stretching back several months, we're able to perform aggregate, long-term analyses that can uncover new security threats, provide greater understanding of how previous threats impacted our users, and help us ensure that our threat detection and prevention measures are properly tuned.

We can't share too much detail (we need to be careful not to provide too many clues on what we look for), but we can use historical examples to give you a better idea of how this kind of data can be useful. One good example is the Santy search worm (PDF), which first appeared in late 2004. Santy used combinations of search terms on Google to identify and then infect vulnerable web servers. Once a web server was infected, it became part of a botnet and started searching Google for more vulnerable servers. Spreading in this way, Santy quickly infected thousands and thousands of web servers across the Internet.

As soon as Google recognized the attack, we began developing a series of tools to automatically generate "regular expressions" that could identify potential Santy queries and then block them from accessing Google.com or flag them for further attention. But because regular expressions like these can sometimes snag legitimate user queries too, we designed the tools so they'd test new expressions in our server log databases first, in order to determine how each one would affect actual user queries. If it turned out that a regular expression affected too many legitimate user queries, the tools would automatically adjust the expression, analyze its performance against the log data again, and then repeat the process as many times as necessary.

In this instance, having access to a good sample of log data meant we were able to refine one of our automated security processes, and the result was a more effective resolution of the problem. In other instances, the data has proven useful in minimizing certain security threats, or in preventing others completely. In the end, what this means is that whenever you use Google search, or Google Apps, or any of our other services, your interactions with those products helps us learn more about security threats that could impact your online experience. And the better the data we have, the more effectively we can protect all our users.