Practical Threat Analysis and Risk Management
The whole point of threat analysis is to try to determine what level of defenses are called for against the various things to which your systems seem vulnerable.
There are three general means of mitigating risk. Defenses can be categorized as means of reducing an asset's value to attackers mitigating specific vulnerabilities and neutralizing or preventing attacks.
Reducing an asset's value may seem like an unlikely goal, but the key is to reduce that asset's value to attackers, not to its rightful owners/users. The best example of this is encryption: all of the attacks described in the examples earlier in this article would be made irrelevant largely by proper use of e-mail encryption software.
If stolen e-mail is effectively encrypted, it can't be read easily by thieves. If it's digitally signed (also a function of e-mail encryption software), it can't be tampered with without the recipient's knowledge, regardless of whether it's encrypted too. A physical world example is dye-bombs: a bank robber who opens a bag of money only to see himself and his loot sprayed with permanent dye will have some difficulty spending that money. Asset-devaluation techniques like these don't stop attacks, but they have the potential to make them unrewarding and pointless.
Another strategy to defend information assets is to eliminate or mitigate vulnerabilities. Software patches are a good example of this: every single sendmail bug over the years has resulted in its developers distributing a patch that addresses that particular bug.
An even better example of mitigating software vulnerabilities is defensive coding. By running your source code through filters that parse, for say, improper bounds checking, you can help insure that your software isn't vulnerable to buffer-overflow attacks. This is far more useful than releasing the code without such checking and simply waiting for the bug reports to trickle back to you.
The defensive approach we tend to focus on the most (not that we should) is heading off attackers before they reach vulnerable systems. The most obvious example is firewalling; firewalls exist to stymie attackers. No firewall yet designed has any intelligence about specific vulnerabilities of the hosts it protects or of the value of data on those hosts. A firewall's function is to mediate all connections between trusted and untrusted hosts and minimize the number of attacks that succeed in reaching their intended targets.
Access control mechanisms such as username/password schemes, authentication tokens and smart cards also fall into this category since their purpose is to distinguish between trusted users and untrusted users (i.e., potential attackers). Note, however, that authentication mechanisms also can be used to mitigate specific vulnerabilities (e.g., using SecurID tokens to add a layer of authentication to a web application with inadequate access controls).
And with that, I bid you adieu for the next couple of months. Due to the demands of a book on Linux security I'm writing for O'Reilly & Associates, the Paranoid Penguin temporarily will be covered by others. Have no fear; they'll maintain the high level of paranoia and vigilance you've come to expect here. See you again in the April 2001 issue.
Mick Bauer (firstname.lastname@example.org) is a network security consultant in the Twin Cities area. He's been a Linux devotee since 1995 and an OpenBSD zealot since 1997, and enjoys getting these cutting-edge OSes to run on obsolete junk.
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