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frojack writes:

Robots.txt files are simple text files that website owners put in directories to keep web crawlers like Google, Yahoo, from indexing the contents of that directory. It's a game of trust, web masters don't actually trust the spiders to not access every file in the directories, they just expect these documents not to appear in search engines. By and large, the bargain has been kept.

But hackers have made no such bargain, and the mere presence of robots.txt files are like a X on a treasure map. And web site owners get careless, and, yes, some operate under the delusion that the promise of the spiders actually protects these documents.

The Register has an article that explains that hackers and rogue web crawlers, actually use robots.txt files to find directories worth crawling.

Once a hacker gets into a system, it is standard reconnaissance practice to compile and update detailed lists of interesting sub directories by harvesting robots.txt files. It requires less than 100 lines of code.

If you watch your logs, you've probably seen web crawler tracks, and you've probably seen some just walk right past your robots.txt files. If you are smart there really isn't anything of value "protected" by your robots.txt. But the article lists some examples of people who should know better leaving lots of sensitive information hiding behind a robots.txt.