Network Security Fundamentals: Egress Filtering

As we wrap up our initial wave of Network Security Fundamentals, we’ve already discussed Default Deny, Monitoring everything, Correlation, and Looking for Not Normal. Now it’s time to see if we can actually get in the way of some of these nasty attacks. So what are we trying to block? Basically a lot of the issues we find through looking for not normal. The general idea involves implementing a positive security model not just to inbound traffic (default deny), but to outbound traffic as well. This is called egress filtering, and in practice is basically turning your perimeter device inside out and applying policies to outbound traffic. This defensive tactic ensures that non-standard ports and protocols don’t make their way out of your network. Filtering can also block reconnaissance tactics, network enumeration techniques, outbound spam bots, and those pesky employees running Internet businesses from within your corporate network. Amazingly enough this still happens, and too many organizations are none the wiser. Defining Egress Filtering Policies Your best bet is to start with recent incidents and their root causes. Define the outbound ports and protocols which allowed the data to be exfiltrated from your network. Yes, this is obvious, but it’s a start and you don’t want to block everything. Not unless you enjoy being ritually flayed by your users. Next leverage the initial steps in the Fundamentals series and analyze correlated data to determine what is normal. Armed with this information, next turn to the recent high-profile attacks getting a lot of airtime. Think Aurora and learn how that attack exfiltrates data (custom encrypted protocol on ports 443). For such higher-probability attacks, define another set of egress filtering rules to make sure you block (or at least are notified) when you have outbound traffic on the ports used during the attacks. You can also use tighter location-based filtering policies, like not allowing traffic to countries where you don’t do business. This won’t work for mega-corporations doing business in every country in the world, but for the other 99.99% of you, it’s an option. Or you could enforcing RFC standards on Port 80 and 443 to make sure no custom protocol is hiding anything in a standard HTTP stream. Again, there are lots of different ways to set up your egress filtering rules. Most can help, depending on the nature of your network traffic, none are a panacea. Whichever you decide to implement, make sure you are testing the rules in non-blocking mode first to make sure nothing breaks. Blocking or Alerting As you can imagine, it’s a dicey proposition to start blocking traffic that may break legitimate applications. So take care when defining these rules, or take the easy way out and just send alerts when one of your egress policies is violated. Of course, the alerting approach can (and probably will) result in plenty of false positives, but as you tune the policies, you’ll be able to minimize that. Which brings up the hard truth of playing around with these policies. There are no short cuts. Vendors who talk about self-defending anything, or learning systems, or anything else that doesn’t involve the brutal work of defining policies and tuning them over time until they work in your environment, basically doesn’t spend enough time in the real world. ‘nuff said. To finish our discussion of blocking, again think about these rules in terms of your IPS. You block the stuff you know is bad, and you alert on the stuff you aren’t sure about. Let’s hope you aren’t so buried under alerts that something important gets by, but that’s life in the big city. No Magic Bullets Yes, we believe egress filtering is a key control in your security arsenal, but as with everything else, it’s not a panacea. There are lots of attacks which will skate by undetected, including those that send traffic over standard ports. So once again, it’s important to look at other controls to provide additional layers of defense. These may include outbound content filtering, application-aware perimeter devices, deep packet inspection, and others. More Network Security Fundamentals I’m going to switch gears a bit and start documenting Endpoint Security Fundamentals next week, but be back to networks soon enough, getting into wireless security, network pen testing, perimeter change control, and outsourced perimeter monitoring. Stay tuned. Share:

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Friday Summary: March 19, 2010

Your Facebook account gets compromised. Your browser flags your favorite sports site as a malware distributor. Your Twitter account is hacked through a phishing scam. You get AV pop-ups on your machine, but cannot tell which are real and which are scareware. Your identify gets stolen. You try to repair the damage and make sure it doesn’t happen again, only to get ripped off by the credit agency (you know who I am talking about). Exasperated, you just want to go home, relax, and catch up on March Madness. But it turns out the bracket email from your friend was probably another phishing attempt, and your alma mater suspends a star player while it investigates derogatory public comments – which it eventually discovers were forged. Man, it sucks to be Generation Y. There has been an incredible cacophony over the last couple weeks across the mainstream media about social networks being manipulated for fun, personal satisfaction, and profit. Even the people my my semi-rural area are discussing how it has affected them and their children, so I know it is getting national attention. What I can’t figure out is how their behavior will change – if at all. RSnake discussed a Microsoft paper recently, expanding on its discussion of why training users on the dangers of unsafe browsing often does not make economic sense. Even if it was viable, people don’t want to learn all that stuff, as it makes web browsing more work than fun. So what gives? I believe that our increasing use of and dependency on the Internet, and the corresponding increases in fraud and misuse, require change. But will people feel differently, and will this drive them to actually behave differently? Will the changes be technological, legal, or social? We could see tighter or looser privacy rules on websites, or legal precedents or new laws – we have already seen dramatic shifts in what younger people consider private and are willing to publicize online. The paper asserts that “The wisdom of the crowd discerns that ignoring some threats brings little actual harm …” which I totally agree with, and describes Twitter phishing and Facebook hacks. Bank accounts being drained and cars being shut down are a whole different level of problem, though. I really don’t have an answer – or even an inkling – of what happens next. I do think the problem has gotten sufficiently mainstream that we will to see mainstream impacts and reactions, though. Interesting times! On to the Summary: Webcasts, Podcasts, Outside Writing, and Conferences Adrian on Database Dangers in the Cloud on Dark Reading. Video interview with Rich on endpoint security, agents, and best of breed technologies. Project Quant in Database Security Metrics Project Needs Community Input Favorite Securosis Posts Rich: FireStarter: IP Breach Disclosure, No-Way, No-How. I’m surprised this generated so little debate for a FireStarter. When I explain this verbally to people, it never fails to generate a vigorous response. David Mortman: FireStarter: IP Breach Disclosure, No-Way, No-How. Mike Rothman: Mogull’s Law. If Rich has the stones to name a law after himself, then I’m in. Not sure how proportional the causation is, but clearly users do whatever hurts the least. Adrian Lane: Network Security Fundamentals: Egress Filtering. Other Securosis Posts LHF: Quick Wins with DLP – the Conclusion Incite 3/17/2010: Seeing the Enemy Database Activity Analysis Survey LHF: Quick Wins in DLP, Part 2 Favorite Outside Posts Rich: Conversations With a Blackhat. The best takeaway from RSnake’s summary of talking with some bad guys is that at least some of what we are doing on the security side is actually working. So much for the “security is failing” meme… David Mortman: Three Steps to a Rational Security Budget. Mike Rothman: Why I’m Skeptical of “Due Diligence” Based Security. I have no idea what Alex is talking about, but he has a picture of Anakin, Obi-Wan and Yoda with the glowing ghosts of John Lennon and George Harrison. So it’s my favorite of the week. Adrian Lane: Walkthrough: Click at Your Own Risk. Analysis of privacy and the manipulation of public impressions through social media. An excellent piece of analysis from … a football statistics site. Long but very informative, and a perspective I don’t think a lot of people appreciate. Project Quant Posts Project Quant: Database Security – Configuration Management Top News and Posts What I thought was the biggest news of the week: HD Moore’s post on The Latest Adobe Exploit and Session Upgrading. – AL Penetrating Intranets through Adobe Flex Applications. A study highlights efforts to take down ISPs that allow malicious activity. This is a boon to reputation-based filtering. To be honest, I used to be skeptical of the idea but I’m slowly becoming a convert. –RM Zeus Trojan Now Has Hardware Licensing Scheme. Microsoft, security vendor clash over Virtual PC bug. Hacker Disables Over 100 Cars Remotely. Former employee using someone else’s login. Now where have we heard that before? Emerging Identity Theft Market. The $10 million number seems high to me, but the trends are not surprising. Facebook Password Scam. We have been talking about the Internet subsuming television for years. Google’s Set-top Box is an attempt to watch closely, because television is all about advertising, which is Google’s strong suit (although they have not been a TV player to date), and this would enable a new kind of advertising. Should be interesting! Blog Comment of the Week Remember, for every comment selected, Securosis makes a $25 donation to Hackers for Charity. This week’s best comment goes to Andy Jaquith, in response to RSA Tomfoolery: APT is the Fastest Way to Identify Fools and Liars. When a comments makes me laugh out loud, it usually gets my vote! I’ve been using the phrase “Advanced Persistent Chinese” lately. It sounds good, it’s more accurate, and it’s funny. What’s not to like? I completely agree that the displays of vendor idiocy around APT are far too widespread. You can’t have a carnival without the barker, apparently. Good seeing you, by the way, Any – albeit far too briefly. Share:

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