NGSSoftware Insight Security Research wrote:
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Technical Details
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java.util.random implements a linear congruential generator, of the
following form:
synchronized protected int next(int bits) {
seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);
return (int)(seed >>> (48 - bits));
}
Jetty generates a 64-bit session id by generating two 32-bit numbers in
this way, so we end up with an encoded 64-bit integer. By decoding the
integer and splitting it into its two component 32-bit integers, we can
easily brute-force the generator's internal state.
So it outputs the full 64 bit integer (encoded), huh? consider yourself
lucky ;-)
With Apache JServ, I had to deal with a session ID constructed in a
similar manner, yet only the last 6 symbols were output (~31 bits out of
the 64).
You can read about this in my "Hacking Web Applications Using Cookie
Poisoning" (April 2002) -
http://www.cgisecurity.com/lib/CookiePoisoningByline.pdf
Apache JServ is "example #2" in that text. You may find part of my
analysis relevant to this (Jetty) case as well (BTW - do you plan to
make your tool/source available?)
Regards,
-Amit
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