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Let's create a simple directive so you can see correlation in action. As an example, let's use a simple directive to monitor suspicious access to the Web server using two different plugins. In order to do so, first turn down the values for your OSSEC plugin. From the OSSIM management page, go to the Plugins section under Configuration. Scroll through the tables to find Plugin ID 7010, and click on the ID column to edit the plugin's values. On the resulting page, change the reliability values for the SIDs 5503 and 5716 from 5 to 1 (Figure 8). If you left these values at 5, they would send an alarm before the rule is processed. Because the goal is to observe correlation, you need to turn them down.
Click on the Directives link found under the Correlation section of the navigation pane. From here, you get a brief description of how directives are ordered and processed. Click on the Add Directive line in the top left of the page. In the resulting fields, enter “Unauthorized Access to Web Server” as the Name. In the blank field next to Id, enter 101, which places your directive in the Generic directives group. Set the Priority to 2 and click Save. On the next page (Figure 9), click on the + symbol to add a rule to your new directive. In the Name field, type “NMAP Scan on Web Server from Foreign Host”. Enter 1001 as the Plugin Id (snort). In the Plugin Sid field, type “2000537, 2000545”, and under the Network section in the To field, type in the IP address of your CentOS server and the Port to List 22. In the Risk field, set Occurrence to 3, Reliability to 1. Set the Sticky field to True and Sticky Different to SRC_IP. Click the Save button at the bottom of the page.
In theory, you have a directive that will send an alarm when a host runs an Nmap scan against port 22 on your Web server. However, you won't receive alerts yet. In order for a directive to send an alarm, the risk of the directive being tripped must be greater than 1.
Although I have not talked much about risk until now, it is integral to the function of correlation. Risk is the primary factor used by the correlation engine to determine when alarms are generated. It is calculated using a series of subjective numerical values assigned by the agents and administrators. Expressed in mathematical form, the formula for risk looks like this:
Risk = (priority x reliability x asset) / 25
Priority is the number OSSIM uses to prioritize rules. It is set at the Directive level. Priority can have a value of 0–5. 0 means OSSIM should ignore the alert. A value of 5 means OSSIM should treat this as a serious threat. Reliability refers to how reliable a rule is based on the chance that it's a false positive. It is set at the individual rule level and can be cumulative if there is more than one rule in a directive. Possible values for reliability are 1–10, and they equate to percentages, so 6 would mean a rule is reliable 60% of the time. Asset is the value that represents the importance of a host. You assigned the highest possible priority (5) to your CentOS server in the Policies section earlier in the article.
At this point, you have one rule under your directive, but no correlation, so you need to add another rule. Click on the + symbol on your directive. Give the new rule a name of “Too Many Auth Failures”. Set the Plugin ID to 7010 (OSSEC), and set the From field to the IP address of your Web server as the OSSEC agent will show the Web server as the source of the events. Set Occurrence to 4 and Reliability to 0 for now. Click Save. After adding the second rule, navigate to the row of the new rule and move the mouse over the directional arrows that control how rules are treated inside the directive. The up and down arrows are similar to AND statements, meaning both rules must match, and the left and right arrows nest rules within each other like nested IF statements. Move your second rule to the right. Open the second rule back up and change the reliability to +2, which will increase the reliability by 2 over the previously processed rule (3 if the first rule is met). Now, if both rules are met, the risk will be > 1 and an alarm will be generated. Listing 1 shows the directive in XML format.
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