2. The
class file verifier
(which includes the
bytecode verifier
) checks that the
program obeys the rules of the Java Virtual Machine (but note that this
does not necessarily mean that it obeys the rules of the Java language).
3. The
security manager
imposes local restrictions on the things that the
program is allowed to do. It is perfectly possible to customize this to allow
code limited access to carefully controlled resources. This could mean
allowing no access to the local file system, and network access only to the
location from which the code, or its Web page, came.
You may wish, for example, to print something from an applet. You are
unlikely to want your security manager to allow anyone to do that, but you
might allow access to especially trustworthy people. So you download the
applet; discover that it is located at a trustworthy URL address and encrypted
with someone's private key; check the accompanying public key certificate to
make sure it is valid, and identify someone especially trustworthy; decrypt the
applet with that public key, and then allow it the necessary access.
One important thing that distinguishes Java from other forms of executable
content is that it has
both
the web of trust that signatures bring
and
the three
security components to validate the downloaded code. These precautions are
taken, not because Java users are less trustful than others, but because even
the most trusted of code suppliers sometimes make mistakes, or can have
their systems compromised. Without the validation, a web of trust can
become a web of corruption if any one trusted site is successfully cracked.
1.3.3 Java as a Threat to Security
So, in the absence of implementation errors, either on the part of the browser
vendors
or on the part of computer operators, administrators and systems
programmers
, Java should be safe. The browser vendors have a good
reputation for responding to reports of flaws in their implementations, and one
of the key purposes of this book is to help you avoid any slips in your
installation.
If something does go wrong, then the most severe threat you face is
system
modification
, the result of what are sometimes called
attack applets
. This is
worse than someone's being able to read data from your system, because
you have no idea what has been left behind. There could be a virus on your
computer, or on any computer to which you are connected. Alternatively,
some of your business data could have been modified so that it is no longer
valid.