It doesn't take a lot of security savvy to realize that private keys used for things like SSH login probably should not be stored in the webroot of a web server. The physical world equivalent would be to place your house key under the doormat, and nobody does that, right?

Still, we are seeing an uptick of scans on web servers looking for such files that really shouldn't be present.

93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /dsa HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /id_dsa HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /id_dsa.old HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /identity HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /id_rsa HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /id_rsa.old HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /key HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /key.priv HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /rsa HTTP/1.1" 404 124 "-" "-"
[...]

The scan looks for about 50 different file names that are commonly associated with SSH keys. In addition, it also probes for the presence of common Unix shell history files:

93.95.yyy.xx - - [09/Jun/2014:17:39:43 +0100] "HEAD /.bash_history HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:43 +0100] "HEAD /.history HTTP/1.1" 404 124 "-" "-"
93.95.yyy.xx - - [09/Jun/2014:17:39:43 +0100] "HEAD /.sh_history HTTP/1.1" 404 124 "-" "-"

One signature that the scans so far had in common is that the first request is always to "checknfurl123".

93.95.yyy.xx - - [09/Jun/2014:17:39:41 +0100] "HEAD /checknfurl123 HTTP/1.1" 404 124 "-" "-"

This is most likely a test to determine how the scanned server responds to requests for files that do not exist, so that false positives can be eliminated in the subsequent attempts to read the SSH keys. I am currently running a honeypotty to see what the scanners do next if the "HEAD" request comes back with an okay (200). No luck yet, so if you already have that bit of intel, please share via the comments below.