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Protecting Critical Resources with Target Value Ratings (TVRs)

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To counteract the potential of attacks against our network, whether they be reconnaissance attacks, access attacks, or denial of service (DOS) attacks, many companies have imple-mented an intrusion prevention system (IPS) to mitigate the risk of such attacks. This article dives into customizing Target Value Ratings (TVRs) to provide a better fortress of security for your critical resources in just minutes.
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One of the challenges of having a network that’s open or available to the public is that it’s available to the public. With any type of open or permitted access, there lies the potential for malicious traffic to enter our networks and to disrupt critical network services. To counteract the potential of attacks against our network, whether they be reconnaissance attacks, access attacks, or denial of service (DOS) attacks, many companies have implemented an intrusion prevention system (IPS) to mitigate the risk of such attacks. This article dives into customizing Target Value Ratings (TVRs), which provide a better fortress of security for your critical resources in just minutes.

Can we protect the network?

Simply purchasing an intrusion prevention system (IPS) and getting it installed in our network, using the defaults, is probably not enough to sufficiently protect our network. Many hours of tuning are required to optimize the response and countermeasures that are implemented by the intrusion prevention system. On the positive side, there are significant changes that we can make to immediately begin additional protection for our critical resources. One of these very effective and very quick changes that we can implement to increase or protection of our server farms and other critical resources is to modify something called the Target Value Ratings (TVR). Let’s take a look at what that is and why we care.

Which is more important to you, a printer or a Server?

To better understand the logic behind TVRs, let’s take a network printer that is used by the receptionist, and a network server that is used for e-commerce by thousands of customers, and see how we feel about the same exact traffic being sent to either device.

Normal transactions by an authorized user to either device isn’t a problem, and our heart rate doesn’t increase. On the other hand, if there is a port scan searching for a few specific well-known ports, this activity isn’t too alarming if it is done against the network printer, but we certainly are going to raise an eyebrow and be concerned about that same reconnaissance attack looking for open ports on a critical e-commerce server.

So what do we do?

Our first thought to correct the problem of malicious traffic reaching our important servers may be to modify the individual signature that’s looking for a port scan, and specify an aggressive counter-measure to be taken when that signature is matched by the IPS. An example of an aggressive response would be denying the packet in line. Now here is the rub: if we modify the signature to respond aggressively, then that signature will respond aggressively regardless of the target, which could be the printer or the critical server. An additional problem is that there are literally thousands of signatures to deal with, and manually going into change each and every one of those signatures to respond with aggressive actions such as denying packets is not only tedious, it’s also a double-edged sword, because now we’re applying aggressive actions to thousands of signatures regardless of whether the attack was launched at a printer and was harmless, or at a critical server. But there is a solution; read on.

Training the IPS to care more about attacks to the server

The magic solution is the combination of Target Value Ratings and Event Action Overrides. This solution can instantly provide the aggressive protection for our servers and at the same time not be overly aggressive for questionable traffic that doesn’t pose a significant risk when directed towards a printer. Let’s take a look at each one of these.

Every time a signature is matched, there is a Risk Rating (RR) assigned to that event. The calculated RR uses the following formula, with the final results that exceed 100 being rounded down to 100.

The other factors in the formula are Attack Severity Rating (ASR), Signature Fidelity Rating (SFR), Attack Relevancy Rating (ARR), Promiscuous Delta (PD), and Watch List Rating. Although these other factors play a part in the formula, the largest factor is the TVR. TVRs are assigned to hosts or subnets based on destination IP address.

By default, without any tweaking whatsoever, all IP hosts have a default TVR of 100. By assigning a target value rating of high or mission-critical, the target value rating, behind the scenes will increase based on the information outlined in Table 1.

Table 1

Target Value Rating

Value used in the RR calculation

Mission Critical








No Value


The bottom line is that when the TVR value is higher, the resulting Risk Rating will be higher as well. By assigning the mission-critical (200) Target Value Rating to be associated with the IP addresses of our critical resources such as servers in the server farm, any attacks that are sent to those IP addresses will trigger a risk rating that is significantly higher than the same attack going to a printer or some other IP destination address that’s not assigned a mission-critical target value rating. So the next question is how exactly does that help us? The answer to that is the second component that works in conjunction with the Risk Rating, and that is the clever feature called Event Action Overrides.

How the higher Risk Rating means more protection.

With Event Action Overrides, if a Risk Rating exceeds a certain level, such as 90 or higher, the IPS can be trained to automatically be more aggressive in its countermeasures (such as denying the attacker or denying the packet that triggered the signature). Doing this places our sensor on high alert regarding traffic destined for critical resources so that it can dynamically respond with aggressive behavior against malicious traffic destined to our critical servers.

Here is an example:

We have taken signature number 2004, which is looking for a simple PING request, we have enabled it, and set it to a severity of medium for our test.

When we send a ping through the network, the sensor will trigger a simple alert (because that is the only action assigned to this signature), and we can see the details of the event using the monitor option in IPS device manager as shown in Figure 2.

The pings were successful, and the sensor reports a Risk Rating value of 75.

Now let’s take a look at what the “Batman and Robin” team of Event Action Overrides and Target Value Ratings can do for us. First, Figure 3 shows the default event action override, which causes the sensor to deny any packets if the resulting risk rating of a triggered signature is 90 or higher.

Now we can modify the TVRs for a specific host or network. This would be the server, or sub network of servers that are critical to our business.

By setting the Target Value Rating for the host at to a value of Mission Critical (200), the resulting Risk Rating will be higher when signatures are matched, and the higher Risk Rating will trigger an Event Action Override to deny the attacker sending packets to that destination.

This set of PING requests didn’t make it because the Risk Rating was not high enough for the Event Action Overrides to kick in and deny each of the packets.

Here is the resulting alert information:

Notice that the Risk Rating is 100 (when previously it was only 75), and the additional information regarding dropping the packet is included in the alert.

Now any of the thousands of signatures that get triggered, based on traffic destined towards our critical hosts (those with the increased TVR), will have the aggressive action of deny packet applied to those packets.

Where do we go from here?

Additional options include adding more actions to the Event Action Overrides, as well as specifying the thresholds for when those overrides kick in. For more information on protecting your network with Cisco’s IPS, check out the IPS 7.0 books from Cisco Press.

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