defensive lines in anti-malware revisited

now, i realize i've covered the topic of defensive lines in anti-malware before, about 2 years ago, but i've refined my thinking since then and now i've found the motivation to share (guess i was just being lazy before)... this thread on wilders security forums made me realize that there are some misconceptions about defensive lines in anti-malware... specifically the ideas of what can be the first or last line of defense seem to require some refinement so here goes...

if you're thinking about multiple lines of defense then it makes sense to consider that there is a sequence of defensive lines at various 'distances' from the final outcome of complete system compromise... that is to say that there are various opportunities (some earlier, some later) during the course of a piece of malware's progression towards compromising a system at which you can prevent a system from becoming compromised, and each of these points a defense can be mounted... the first line of defense, therefore, should be the one furthest away (or at the earliest opportunity) from that possible outcome... likewise the last line of defense necessarily must be the last chance for preventing that outcome...

so what is the first possible line of defense then? well, first opportunity you have for avoiding having your computer get infested with malware is the point of initial exposure - if you don't encounter the malware in the first place then there's nothing else to say about it... when it comes to defending oneself the main defense here is actually risk aversion behaviours like not going to so-called 'dodgy' sites, not plugging strange flash drives you found on the ground into your computer, etc... when it comes to defending others, any malicious site take-down effort will also help prevent people from coming across the malware on malicious sites that get taken down... somewhere in the middle are tools like mcafee's site advisor, or google's malware warning functionality that can help users determine which sites to avoid...

the next logical point at which you can prevent compromise is preventing the transfer of the malware onto the machine we're trying to protect... for automatic transfers (such as drive-by downloads or vulnerability exploiting worms) the primary defense is closing the avenues by which such automated transfer can take place, usually by applying security patches (at least where browser exploits are concerned)... a basic NAT enabled router can also protect machines behind it from certain types of automatic transfer by virtue of preventing unsolicited traffic to those machines... for manual transfers, risk aversion behaviours again play an important role - only downloading software from reputable sites (preferably direct from the vendor's site) being a prime example... a somewhat after-the-fact but related defense is on-demand scanning all incoming materials (whether you think they could carry malware or not) before allowing them to be accessed for any other purpose (sort of like a mandatory quarantine period)...

after potential malware has successfully been transferred onto the system the next opportunity to prevent it's progression to full compromise is preventing access to it... this is where on-access scanners come into play... if the user can't access the malware then there should be no way they can trigger it...

if your malware access controls fail to prevent access (say because the malware is packaged within some obfuscating wrapper, or perhaps the malware is just new) then the following point at which full compromise can be avoided is by preventing execution of the malware... on-access known-malware scanning can prevent execution as well, at least for known malware (if it's in some obfuscated wrapper like a dropper, then when the malware is 'dropped' it will no longer be obfuscated and that camouflage will no longer be protecting it from the scanner)... application whitelisting can also prevent a great deal of malware (even that which is too new to be considered 'known' yet) from executing (by virtue of preventing everything that isn't already explicitly allowed to execute from executing) so long as the user doesn't give the malware permission to run...

after malware has successfully begun to execute on a system, any certainty about preventing that malware from compromising that system is gone, but that doesn't mean there aren't still possibilities for prevention... it may still be possible to prevent compromise by preventing one or more of the malware's bad behaviours using behavioural blacklisting, behavioural whitelisting, or some combination thereof (all of which falling under the general umbrella of behaviour blocking)... in so far as certain behaviours involve accessing system resources that can have their access restricted, operating as a user who doesn't have access to those resources (ie. running as a non-administrative user, following the principle of least privilege) will prevent a great deal of existing malware from being able to operate properly and thus can prevent compromise...

finally, if malware gets past all those previously described defensive lines, there's still one possible opportunity to full system compromise... if you can avoid the consequences of the malware's behaviour by being lucky enough (or by managing the circumstances well enough) that the malware was actually running in a sandbox rather than on the main host system then it will be the sandbox that gets compromised rather than the main host system... sandboxes are such that their compromise is usually inconsequential because they can be regenerated easily - though extrusion of sensitive data may still be a problem, if the malware was contained withing a sandbox then compromise of the host will have been avoided... otherwise the system will have become compromised, prevention will have completely failed, and it would now be an issue of detecting the preventative failure, diagnosis to determine the extent of that failure, and recovery from it...

you can make any one of these stages your own first line of defense, but only by ignoring earlier opportunities to defend yourself... likewise, you can make any of these your last line of defense, but only by ignoring subsequent opportunities to defend yourself... think about what that statement means: ignoring opportunities to defend yourself... is that really something you want to do? does it sound wise? it certainly doesn't sound that way to me...