What's with the accesses to $random_existing_file/cache/df.php?

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http://serverfault.com – Occasionally I eyeball Apache's access_log and lately I've been noticing these accesses to URLs that I don't serve. They're correctly 404'ed, but I'd like to know just who and what is involved here. "Obviously" it's some sort of vulnerability probing; I'd like to know which. (Not that it affects me, but I like to know the score.) Here's an example: - - [28/Nov/2012:17:36:34 +0200] "GET /cvfull.pdf/cache/df.php HTTP/1.1" 404 489 "-" "-" Oddly, all 26 attempts are to either /cache/df.php, or to /cvfull.pdf/cache/df.php - they come in pairs. A few weeks ago it was zx.php, now it's (HowTos)