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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 659
628
SECURITY
CHAP. 9
LOGIN: mitch
LOGIN: carol
LOGIN: carol
PASSWORD: FooBar!-7
INVALID LOGIN NAME
PASSWORD: Idunno
SUCCESSFUL LOGIN
LOGIN:
INVALID LOGIN
LOGIN:
(a)
(b)
(c)
Figure 9-17.
(a) A successful login. (b) Login rejected after name is entered.
(c) Login rejected after name and password are typed.
feedback about whether the login name itself is valid. All she learns is that the
login name plus password combination tried is wrong.
As an aside on login procedures, most notebook computers are configured to
require a login name and password to protect their contents in the event they are
lost are stolen. While better than nothing, it is not much better than nothing.
Any-
one who gets hold of the notebook can turn it on and immediately go into the
BIOS setup program by hitting DEL or F8 or some other BIOS-specific key (usual-
ly displayed on the screen) before the operating system is started. Once there, he
can change the boot sequence, telling it to boot from a USB stick before trying the
hard disk. The finder then inserts a USB stick containing a complete operating sys-
tem and boots from it. Once running, the hard disk can be mounted (in UNIX) or
accessed as the
D:
drive (Windows). To prevent this situation, most BIOSes allow
the user to password protect the BIOS setup program so that only the owner can
change the boot sequence.
If you have a notebook computer, stop reading now.
Go put a password on your BIOS, then come back.
Weak Passwords
Often, crackers break in simply by connecting to the target computer (e.g.,
over the Internet) and trying many (login name, password) combinations until they
find one that works. Many people use their name in one form or another as their
login name.
ForSomeone named ‘‘Ellen Ann Smith,’’ ellen, smith, ellen
smith,
ellen-smith, ellen.smith, esmith, easmith, and eas are all reasonable candidates.
Armed with one of those books entitled
4096 Names for Your New Baby
, plus a
telephone book full of last names, a cracker can easily compile a computerized list
of potential login names appropriate to the country being attacked (ellen
smith
might work fine in the United States or England, but probably not in Japan).
Of course, guessing the login name is not enough. The password has to be
guessed, too.
How hard is that?
Easier than you think. The classic work on pass-
word security was done by Morris and Thompson (1979) on UNIX systems. They
compiled a list of likely passwords: first and last names, street names, city names,
words from a moderate-sized dictionary (also words spelled backward), license
plate numbers, etc.
They then compared their list to the system password file to
see if there were any matches. Over 86% of all passwords turned up in their list.
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