Abstract:
A digital filter correlation engine, wherein the correlation engine combines N arbitrary digital filter states based on the weights and along with a threshold generate a network incident. This network incident in turn can be feedback to another digital filter. This multi-layering capability allows the creation of higher level event detections that are time-based for a cyber security analyst to analyze, thereby reducing the amount of manual work the analyst has to do in inspecting behaviors within the network.

Description:
CROSS-RELATED APPLICATIONS 
       [0001]    This application is a Continuation-In-Part of U.S. patent application Ser. No. 12/719,621 filed Mar. 8, 2010 and claims rights under 35 U.S.C. 119(e) from U.S. Application Ser. No. 61/298,310 filed Jan. 26, 2010, and U.S. Application Ser. No. 61/298,314 filed Jan. 26, 2010, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to network behavioral monitoring and more particularly to network behavioral monitoring using time-based behavioral pattern extraction relating to login attacks. 
       BACKGROUND OF THE INVENTION 
       [0003]    As is common, login attacks are used quite frequently to take over the control of computers for capturing information and to be able to use the computers in whatever way the attacker deems desirable. It will be appreciated that login attacks usually involve a burst of different login passwords, with the attacker hoping to hit the appropriate password and therefore gain access to the targeted computer. 
         [0004]    Burst attacks are usually detected due to the burst nature of these attacks which can occur as many as 10,000 a second. However, because of the ready visibility of such brute force attacks, attackers have taken a more stealthy approach to attacking a computer by spacing out the password attacks over days, months and years. This involves launching a single password attack at spaced apart time intervals which are not usually detectable by the normal types of password attack detection. For instance, it is very common to shut down access to a computer when there are for instance three or more incorrect passwords attempted, for instance in an hour. However, by launching single word password attacks spaced out over days and months, they will go unnoticed by this type of detection system. 
         [0005]    Key to the login attack is the Secure Shell login which is a Unix standard security protocol used to log into a system securely. The SSH login attack is one in which the attacker attempts to guess the password that will gain him access to the computer. 
         [0006]    As mentioned above, sophisticated SSH login attacks need to be more subtle because it is obvious when an attacker attacks a computer by throwing thousands of password attempts at the computer in a short period of time. The more subtle attack would be to send a password today and see if it succeeds, followed by the sending of another password on a different day if there is no success. 
         [0007]    It is noted that these subtle attacks have largely gone undetected and is a technique used by foreign governments to break into government computers on a regular basis. 
         [0008]    Note, that there have been various strategies involving neural networks that have tried counting attacks. Also other models are used to ascertain whether the same attackers are coming back to the computer using different IP addresses. Moreover, as mentioned above, cyber analysts have utilized frequency-based approaches, but they are relegated to the simplest brute force approaches that ascertain if a large number of attacks occur over a short period of time. 
         [0009]    The SSH login detection problem is compounded when these low frequency attacks take place over weeks or months which make them very difficult to resolve. 
         [0010]    There is therefore a need to provide a system that detects fairly low frequency probes that occur, for instance over a number of days, weeks, or months. 
         [0011]    What is therefore necessary is some way to correlate over multiple hours, days, weeks and months how many attacks from a given attacker occur consecutively over differing time periods. Moreover, there is a need to have various levels of attacks detected. For instance if one has a short time period of for instance hours, then exceeding a predetermined threshold for the number of probes per hour, one could take this as an event and generate an event indicator transmitted to a next longer period detection device. 
         [0012]    Thus, what is necessary is a detection system that segments the number of probes occurring on a daily, weekly monthly and yearly basis and then correlates all of the different levels of detection to robustly ascertain if a login attack is occurring. 
         [0013]    Most importantly, it is important to be able to detect attackers that can probe for a few days and stop for months and then go onto probe at some other infrequent time interval. For instance, one would like to be able to detect attacks at over for instance three sample periods which may be days or weeks and have credible evidence of an attack even when for other sample periods no attacks are detected. The failure to detect attacks in any sample period is not counter productive because one could then move up to the next longer time period and still record frequency characteristics that are required for positive detection. 
         [0014]    By way of further background, in terms of network security, oftentimes a network is under attack for those intent on hacking into a computer system to either disrupt the system or to take over control of the computers residing on the system. Networks employ spam filters and Malware detection as well as detecting login attacks. Most of these approaches involve either signature-based approaches or anomaly-based detection to see if a computer system is under attack. 
         [0015]    The problem with prior approaches to network security is that while they are relatively robust in detecting massive attacks that occur for instance within milliseconds, they are unable to detect attacks which are sporadic or take place over long periods of time. In some attacks, the attacker tries to access system passwords through trial and error. However, if passwords are tried over days, weeks and months, as opposed to seconds, then very cumbersome manual approaches to detecting the attack are sometimes tried. The result however, is that such attacks are not detected at all or are detected too late. 
         [0016]    Moreover, while there are a number of reasons to detect different types of attacks, there are nonetheless large numbers of false alarms. These can be due to minor spam attacks or system noise and it is only with difficulty that one can isolate a major high level attack from mere annoyances. 
         [0017]    While math models to represent potential behaviors in a network such as artificial intelligence, neural networks and the like use statistical models to ascertain behavior, these models are insufficient for the more stealthy approach of attacking a system periodically over a long period of time. 
         [0018]    Thus, there are detectable events when attackers seek to scan a network that because of their complexity are difficult to detect either by a manual inspection of logs or by using models of potential behaviors. Thus, complex attack models involving sophisticated patterns, frequencies or repetitions have heretofore not been automatically detectable, or are masked by false alarms. 
         [0019]    As will be appreciated, attackers oftentimes try to determine if they can recognize a system and characterize it by detecting the type of operating system that is utilized and by detecting the type of applications running on the system. This type of scan is called a TCP scan which is an acronym for the Transmission Control Protocol. The TCP protocol is a standard protocol involving a three-way handshake in which a client tries to connect to a server, the server acknowledges the attempt and then the client responds with a final thank you in a three-way handshake. If the scan involves an incomplete transaction, the attacker will try to connect to a server and the server will respond. However the client will never acknowledge the servers response, thereby leaving the server hanging and open. This is what is referred to as an incomplete TCP connection and is one mechanism for denying service. 
         [0020]    Moreover, not only are single incomplete sessions an indication of an attack, when there are a large number of incomplete sessions, that also is a symptomatic of a scan. 
         [0021]    Note, not only are passwords attempted in order to enter into a system, network sensors can include packet and protocol content inspections. Content inspections look at various parameters such as an http web page and the actual words on a page. 
         [0022]    Malware relates to looking for known signatures or known content and behavior models may be employed to ascertain when a system is connecting to a large number of internal hosts. Typically if the system reaches out to a web server, the server may be instructed for instance to double click on a page and go to another site for more information. If for instance the network experiences ten connections per second after one connects to a page for a couple of seconds, this may indicate a Malware attack. 
         [0023]    As to network flow detection, flow rates which involve connections to different hosts, can if certain thresholds are exceeded indicate a scan. 
         [0024]    More particularly, if an entity seeks to scan a system utilizing an attack mechanism, for instance three times in a row within a short time period, then for simple attacks one can quickly recognize the short-duration repetitive behavior. Such attacks such as SSH login attacks which occur as many as a thousand times a second are readily detectable. 
         [0025]    However, if the attack occurs for instance a couple of times throughout a day or perhaps a few times in the succeeding days over extended periods of time, it is very difficult to recognize such attacks with present methods. 
         [0026]    Those systems which react to the sensing of an attack so as to report it every time it occurs are plagued with false alarms, called noise. This means that attacks spanning long periods of time are not very easy to pick off. Thus, if an entity seeks to break into a computer with an automated password attack by trying a large number of passwords; if the attack occurs with a massive number of passwords per second, it is quite easy to detect such an attack. 
         [0027]    However, if the attacker tries the various passwords over an extended period of time, then it is very likely that such attacks will go on unnoticed. 
         [0028]    So-called nation state attacks which seek to gain control over sensitive computers needs only to have one correct password recognized in order to be able to control the computer. Thus, if the attacks occur for instance over a number of months, the instant that a password is successful, the computer is compromised. 
         [0029]    Those events which indicate that a scan or attack is in process include incomplete TCP handshaking in which there are no answer-backs. Also the number of TCP resources invoked, if large, is also an indication of the presence of an attack. Moreover, one can analyze packet and protocol contents to sense an attack, as well as utilize conventional Malware detection systems and/or network flow detection. 
         [0030]    As noted above, if there are certain Malware attacks, one seeks to identify these attacks due to the particular recognized signature of the attack. These signatures can include the transmission of malicious code and one may not be sure utilizing conventional Malware detection whether a virus or Trojan attack is in fact taking place. 
         [0031]    If one looks at the suspected malicious code over a long time period, one may for instance see five occurrences in ten minutes. It is noted that if one sees five occurrences in ten minutes, one is potentially seeing half an occurrence per minute and one might decide that half an occurrence per minute indicate the presence of a Malware attack. 
         [0032]    Thus, two problems occur in intrusion detection systems. The first is a false firing on a signature, noting that the false firing may occur hundreds to thousands of times a second, accounting for millions of events during a day, all of which need to be analyzed. Often such attacks involve low quality events that can be ignored. 
         [0033]    Without frequency or recurrence filtering, it is very difficult to arrive at a model of behavior characterizing attacks that does not result in an abundance of false-positives. 
         [0034]    While password attacks historically utilize a brute force approach. A brute force attack is very clear and obvious when one gets thousands of bad password attacks on an account. However, it is extremely difficult to ascertain that attacks have occurred if for instance the attacker tries one password a minute, or one password a day which tends to fall below the firewall rules that are for instance set to see if three failed password attempts have been made. 
         [0035]    Due to the sophistication of attackers, most attackers now extend their attacks over much longer durations such as days, weeks or months and even years, and it is now a necessity to be able to track such attacks with sophisticated analysis. 
         [0036]    It is possible to take a look at log histories in order to derive the information necessary to ascertain that an attack has occurred or is process. However, it is clear that such manual approaches or even simple automatic approaches are incapable of countering such long term behavior. 
         [0037]    In summary, artificial intelligence and neural networks when properly trained still cannot indicate attacks that occur over very long periods of time. Moreover, simple approaches do not work well and intrusion detection systems will have to require a very high level of computer automation in order to detect the various patterns that an attacker might utilize. Thus, there is a requirement for a threat detection system that depends on a sequence of many events happening over a long period of time to achieve a low false alarm rate. 
         [0038]    Note for purposes of the following discussion the following definitions apply: 
         [0039]    anomalous packet=a network packet that is in some way unusual compared to other packets. 
         [0040]    unknown network flows=a network communication which cannot be deciphered, or categorized as a known communication protocol 
         [0041]    shell code detection=‘shel code’ refers to a malicious piece of code used in part or whole to compromise or takeover a computer system. 
         [0042]    DOS response=DOS is a Denial of Service attack, as when an attacker sends a lot of connection requests to a web server, hoping to overload the server. A DOS response is something you would do to mitigate the denial of service, for instance block the attackers communications at a firewall. 
         [0043]    TCP flows=a TCP flow is a network communication using the TCP network protocol, also known as a TCP session. 
         [0044]    Host=IP, IP is short for an interne protocol address. A Host is a desktop, or a server, each are hosts. 
         [0045]    Port=a connection node. 
         [0046]    IP flow=IP to IP communication, one computer talking to another in any manner, shape or form. 
         [0047]    TCP Flow=Two hosts communicating using the TCP protocol. Tracking this requires a Client IP (the initiator), and the Server IP and Server Port. When one goes to google.com this is over a TCP flow. 
         [0048]    UDP flow=IP:Port to IP:Port 
         [0049]    TCP Server=a Server IP and the Servers Port 
         [0050]    Track options include all, host, port, IP flow, TCP flow, UDP flow and TCP server. 
       SUMMARY OF INVENTION 
       [0051]    According to the present invention, the manual effort heretofore used to extract the existence of a login attack over substantial periods of time is automatically detected using digital filtering and an associated correlation engine that detects login attacks occurring over long periods of time. Thus, as one feature, the digital filtering removes noise caused by detection of individual events, since the filters can be set to recognize sophisticated login attack patterns including the hard-to-detect infrequent attacks that might otherwise be missed. As a result, the subject invention reduces the manual work required by the application of correlation to digital event filters. 
         [0052]    It is a feature of the digital filters that they can extract time-based behavioral patterns of interest, specifically login attacks, and by correlating different digital filter states the higher level meaning desired is known. Because of the ability to set filter parameters, the resultant flexibility of the correlation engine makes it possible for a cyber analyst to obtain automated detection of sophisticated long term login attacks. 
         [0053]    More particularly, assuming a single attack vector such as a login attack vector, the number of events occurring in various time periods may be analyzed and thresholded so as to be able to provide alarm indications for events occurring over long periods of time. For instance a digital event filter may set to detect the number of occurrences of a particular event, per second, per hour, per day, per week, per month or per year, where exceeding the threshold for any one of these time periods results in either an alarm condition being indicated or passage of the thresholded result to the next longer-period digital event filter. Thus, if for instance there are 100 occurrences of an event in an hour, this could trigger an alarm, or the thresholded result can be coupled to the next digital event filter in the sequence, set for instance to accumulate the number of events in a day. The number of events in a day in turn can be thresholded by a threshold set to ten, and exceeding this threshold triggers an alarm. Alternatively, the fact of the exceedance of this ten per day threshold may be coupled to the input of a follow-on digital event filter, for instance set to accumulate the number of events in a week. 
         [0054]    By this sequential digital event filtering one can detect attacks that occur over long periods of time, while at the same time selectively ignoring low level occurrences that are too unreliable or too uncertain to be declared an attack. 
         [0055]    The use of serial digital event filters provides exceptional flexibility for a cyber analyst to set up an automated system for recognizing various long term attack patterns. This is because the analyst can set the time periods for the recurrence levels to tailor the response of the system to a selected recurrence level based not on the event occurrences themselves, but rather on the number of events for pre-selected time periods. 
         [0056]    This type of event detection may also be used for vectors other than a login attack vector to give a robust indication of a sophisticated attack in which multiple digital event filters can be run in parallel to detect different event vectors, with the results in one embodiment weighted as to severity. 
         [0057]    For instance, in one embodiment, for multiple attack vectors, the correlation engine combines the outputs of a number of digital filters, each detecting a different type of attack or attack vector. When the system detects different type attacks occurring during the same time period, then the presence of an attack can be robustly ascertained. 
         [0058]    As mentioned above, it is possible to weight the sensor results to give more weight to more important attacks. In this case the correlation engine may be based on N arbitrary digital filter states. These states may be weighted and thresholded such that a weighted network incident can be generated. Moreover, this network incident in turn can be combined with other digital filtering. 
         [0059]    This multi-layering capability allows the creation of higher level event detections that are time-based for a cyber security analyst to consider, thereby reducing the amount of manual work the analyst has to do in inspecting behaviors within the network. 
         [0060]    Note that each of the above digital filters is set to accumulate a predetermined number of events that occur in a predetermined time period. The result is an extremely flexible system for the cyber analyst to be able to address not only the various attempts that can occur, but also to be able to recognize these events over longer and longer periods of time without manual log inspections. 
         [0061]    In summary, the utilization of a login event sensor and a digital event filter results in a digital event filter correlation engine, called a DEFCON engine, which takes into account the occurrence time lines of the particular events sensed. With the use of the digital filter correlation engine, false alarming is dramatically reduced, while at the same time providing an exceptionally robust login attack detection modality. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0062]    These and other features of the subject invention will be better understood in connection with the Detailed Description, in conjunction with the Drawings, of which: 
           [0063]      FIG. 1  is a block diagram of a digital event filter correlation engine for detecting the presence of an attack on a computer system; 
           [0064]      FIG. 2  is a graph showing an occurrence timeline for event occurrences that occur within an hour, within a day, within seven days and within thirty days showing respective alarm thresholds of 100, 10, 4 and 1; 
           [0065]      FIG. 3  is a block diagram of the subject system showing filters related specifically to anomalous packet events, untrusted flow packet events and half open TCP connections, with the filtered output for occurrences occurring within a predetermined equation reference goes heretime period being weighted and summed, wherein the weighted result indicates an attack event; 
           [0066]      FIG. 4  is a difference equation that describes the operation of the filters of  FIG. 3 ; 
           [0067]      FIG. 5  is a chart showing DEFCON configuration tokens; 
           [0068]      FIG. 6  is a chart showing DEFCON event rule struct configuration tokens; 
           [0069]      FIG. 7  is a chart showing DEFCON filter struct configuration tokens; 
           [0070]      FIG. 8  is a program listing of an example of a DEFCON configuration for an example run; 
           [0071]      FIG. 9  is a program listing for an example run showing the status of the subject system; 
           [0072]      FIG. 10  is a program listing of an example run showing detecting anagram and logger attacks; 
           [0073]      FIG. 11  is an event log showing a test log file; and, 
           [0074]      FIG. 12  is a table showing a filter sample file for an example run. 
       
    
    
     DETAILED DESCRIPTION 
       [0075]    While the subject case is directed to stealthy login attack detection, the subject system is described generally, noting that the login attack detection is a subset of the capabilities of the DEFCON engine. 
         [0076]    Referring now to  FIG. 1 , a Digital Event Correlation Engine, DEFCON,  10  is described which can detect sophisticated login attacks. The DEFCON engine incorporates an event detection sensor  12  in this case a login attack sensor which is designed to detect a particular type of attack generated by an attacker scan  14  and delivered over the internet  16 . 
         [0077]    For purposes of illustration only, a single type of event detector or sensor is first described. The event can be indicated by for instance the detection of a login attack, or for instance anomalous packet detection, unknown network flow detection, shellcode detection, and denial of service attack detection; and through the use of metrics including excessive shellcode Ngram matches to a TCP server that is under attack. Note, the term shell code refers to small pieces of code used as a payload in an exploitation, with Ngram referring to phonemes, syllables, letters, words, or other base pairs that are for instance utilized in password attacks. 
         [0078]    As will be discussed hereinafter, various types of intrusions or events can be detected. 
       Single Vector Sensing 
       [0079]    In one embodiment, a digital event filter  20  can be characterized as having an accumulator  21  section set to a particular number of occurrences per time period for the sensed event. In this example the time period is one hour. As part of the digital filter  20 , accumulator  21  is coupled to a threshold circuit  22  section. When, as illustrated by threshold circuit  22 ,  100  detected events within an hour exceed a T=100 threshold an alarm condition is initiated by activating an alarm  24 . 
         [0080]    The output of threshold circuit  22  can also be coupled to a follow-on digital event filter  26  characterized by an accumulator  27  section which is set up to count the number of occurrences per day, as opposed to per hour. In this characterization of filter  26 , accumulator  27  is coupled to a threshold circuit  28  section. The number of events per day is thresholded by threshold circuit  28  having its threshold set to T=10, such that when 10 detected events occur in a day, an alarm condition is initiated by activating an alarm  30 . 
         [0081]    The output of threshold circuit  28  in one embodiment is coupled to an even further follow-on digital event filter  32  characterized as having an accumulator section  33  which is set to count the number of events per week. In this characterization, accumulator  33  is coupled to a threshold circuit  34  section which has a threshold T=4 set, such that when the number of detected events per week exceeds four, an alarm condition is initiated by activating an alarm  36 . 
         [0082]    Finally, the output of threshold circuit  34  is coupled to a further follow-on digital event filter  40  characterized as having an accumulator section  41  set to count the number of detected events per month. Accumulator  41  is coupled to a threshold circuit  42  section of filter  40  and has a threshold set to T=1 such that if one such occurrence occurs during a month, an alarm condition is initiated to activate an alarm  44 . 
         [0083]    What will be seen is that each of the digital event filters is configurable to count the number of events detected over a predetermined time period that exceed a preset threshold to provide an alarm when the number of occurrences for that time period exceed this threshold. 
         [0084]    In one embodiment, if one is looking not for the number of occurrences per hour nor per day; but rather per week in order to establish the existence of an intrusion, then if the attack takes place over a week as opposed to days, hours, minutes or seconds, then the initiation of an alarm condition robustly indicates the occurrence of the event. 
         [0085]    It will be appreciated that the digital event filters can be programmed for the time period over which they are to operate. These units therefore provide a filter which inhibits outputs until such time as the threshold number of events per time period is exceeded. 
         [0086]    While it is possible to provide an indication of the gross number of events per time period, the subject thresholding fires an alarm only when the number of occurrences per time period exceed a predetermined threshold. 
         [0087]    Of course it is possible to indicate the gross number of occurrences per hour, day, per week and per month or any specified time period which will give the analyst some indication of the type of attack that is underway for the particular sensor or event detection that is utilized. 
         [0088]    However, when these events occur over long periods of time the analyst can set the thresholds to correspond to the longest time period that the analyst expects an attacker to use. 
         [0089]    Thus, the flexibility of a single event detection system utilizing multiple digital event filters provides the analyst with a large number of ways to view incoming attacks and to sort them based on various time periods. As a result, the analyst can tailor the system by tailoring the time periods and the thresholds. 
         [0090]    Referring to  FIG. 2 , in one embodiment assuming that successive digital event filters operate with time periods of one hour, one day, seven days and thirty days, it can be seen that if there are 100 event occurrences in a given hour then an alarm signal such as illustrated by arrow  50  can be generated. Likewise, for a given day assuming there are ten event occurrences during the day, as illustrated by arrow  52 , the corresponding alarm can be generated. With respect to a weeks worth of events, meaning seven days, if the number of events are greater than for instance four during the week, an alarm can be generated as illustrated at  54 , whereas if one is looking at a thirty day or one month time period, then if for instance there is one event in that time period, then an alarm as illustrated at  56  will be generated. 
         [0091]    By setting the time constants for the particular digital event filters one has one type of control over the filtering of the incoming data, whereas setting a particular threshold of the number of events that occur during this time period offers another level of sophistication to be able to ignore lower level attacks. 
       Multiple Vector Sensing 
       [0092]    While the digital filters may be used by themselves to sense a single vector, the use of a digital filter by itself may be incapable of detecting sophisticated attacks. For this purpose multiple digital filters addressing different vectors present a better picture of what is happening. There are a number of different vectors that can indicate an attack. For instance, the high occurrence of an anomalous packet content (anagram) can indicate an attack. Detecting this vector can be accomplished by configuring the associated filter to be a high pass filter on anomalous packets. Another indication of an attack is the presence of untrusted data flows. Thus, a different filter may be provided to ascertain that the same packets contain untrusted data flows relating to attack behavior and the associated filter is configured to be a pass through filter on untrusted data flows. 
         [0093]    Finally a still further filter may be one that detects when a network appliance experiences large spikes in denial of service, synchronize and acknowledge floods or other network metrics, with the filter serving as a high pass filter for instance on half-open TCP connections. 
         [0094]    In order to detect different vectors and referring now to  FIG. 3 , an event rule can contain N filters, each detecting a different vector and each with an assigned weight, a threshold and a minimum incident firing, where minimum incident firing means The filter satisfied its thresholds a specific number of times. The result is a linear combination of the current number of detected events for each filter is summed and thresholded. If the threshold is exceeded then it can be reliably determined that an attack has occurred involving one or more of the vectors and one can dump the associated track queues for review by a security analyst. 
         [0095]    In one embodiment there are three structures involved, namely an event rule, a filter and a track, with the event rule containing N filters and the filter containing P tracks. Here a track refers to a particular vector. 
         [0096]    In one embodiment, the memory architecture of the subject engine includes an event rule which contains a filter ID array, a filter reference array and a filter weight array. The filters contain an infinite impulse response array (IIR), a finite impulse response (FIR) array, a track hash table, and an event rule ref linked list. 
         [0097]    Finally in terms of the memory architecture, tracking includes a key, a y[n] array, an x[m] array, an event and packet linked list and a filter reference, where y[n] refers to a filter event-result for an x[n] sample. And x[m]=input data samples. 
         [0098]    More particularly, when for instance different types of sensors are used to detect different types of attacks or vectors and referring again to  FIG. 3 , a filter function  60 ,  62  and  64  can be set to detect for instance an anomalous packet event  66 , an untrusted flow packet event  68 , or a half open TCP connection  70 . 
         [0099]    These events are detected and the digital filter characteristic is set up to detect the number of such events for the particular time period set up for the filter. Thus h[n] is the specific filter function, whereas x[n] and y[n] are filter track specific. The input samples to the filters having the filter characteristics indicated at  60 ,  62  and  64  are respectively labeled x[n], whereas the number of events occurring within the time period for each of the filters is labeled y[n]. 
         [0100]    For event correlation, the outputs of these filters are combined in an event rule module  70  which in one embodiment multiplies weights W 1 , W 2  and W 3  with the numbers output respectively from filters  60 ,  62  and  64 . These weights are multiplied as illustrated at  72 ,  74  and  76 , with the numbers associated with the outputs of the aforementioned filters. Thus, in one embodiment, the weights are cross multiplied with the filter outputs, with the outputs of multipliers  72 ,  74  and  76  being summed at a summation node  78  as illustrated. 
         [0101]    The raw number associated with the output from summation node  78  is an indication of an attack, along with its severity provided by the weighting functions. 
         [0102]    Thus what is shown in  FIG. 3  and in fact by the Equation 1 of  FIG. 4  is that filter output y[n] is related to the number of occurrences of a specific event over a specific period of time. If there were no weighting involved and in fact only one vector were involved, then the diagram of  FIG. 3  would devolve into the diagram of  FIG. 1 . However, by providing numbers of different sensors for different types of attacks and providing different granularities, as well as different weights, the indication of an attack ranked in accordance with severity can provide the analyst with the information necessary to understand that an attack has in fact occurred, that the attack is important and that he or she should look into the logs to try to determine the source of the attack. 
         [0103]    What is provided is the application of digital signal processing theory to network sensors and metrics, along with a correlation engine to extract and react to time-based behavioral events relating to potential attacks. 
         [0104]    What is now described is a rigorous mathematical characterization of the system of  FIGS. 1 and 3  through the use of a general digital filtering equation. However, prior to describing the general digital filtering equation of  FIG. 4 , the events to be analyzed by the sensed system are those that occur at non-equidistant time intervals. Thus, the events do not occur for instance every two seconds for instance. The events that are measured occur when a rule fires and this does not usually occur at a uniform rate. In order for the subject system to detect an attack, the system has to figure out when one has a current sample how long ago the last sample was. To do this it is possible to characterize the subject filtering system using the general digital filter equation (Equation 1) of  FIG. 4 . 
         [0105]    Note that for the time independent components and for an infinite impulse response, one simply takes the previous state of the system and adds the effects of the current event independent of time. This is what the b k x[n] term represents in the Equation 1. The time component effect is represented by the a k x[n] term. 
         [0106]    What is done is to create a digital filter which is a set of coefficients that describe a curve that represents the shape of the curve one is looking for, i.e. the curve that one would draw around the sequence of events that occur over time that would indicate an attack. This process is similar to what one obtains with a Fast Fourier transform. In the subject case the digital filter is established by specifying the shape of the desired curve and the coefficients needed to describe that curve. When one generates those coefficients, then all that is needed is to apply these coefficients to the current input samples. 
         [0107]    If the current input sample of events times those coefficients generates the curve corresponding to an attack, then one has found the data one is looking for. If the coefficients generated from the theoretical curve times the actual physical event data does not generate the expected curve corresponding to an attack, then one knows that data set does not represent an attack. 
         [0108]    The subject system can therefore be characterized by defining a curve that looks a shape denoting an attack and then generating the coefficients. This can be done by hand if one has a simple curve. 
         [0109]    Once one generates the coefficients then it is only a matter of determining if the data looks like the data that is described by the posited curve. In the subject case, the filter used is a simple digital filter that says by entering fields in a configuration language to specify for instance “detect 3 events every 2 seconds”, an attacking-indicating curve is posited. This curve is one that one could draw around the event. If one draws a curve over time, in the above example one could draw 3 spikes every 2 seconds and that would represent the shape of the attack-indicating curve. 
         [0110]    However, in the subject system one does not have to go through the effort of defining a curve. Rather one can use a simple configuration language. 
         [0111]    The appended source code describes the custom configuration language data structure to provide for the necessary filter functions. (See Appendix). 
         [0112]    This source code defines the Configuration Data Language (CDL) grammar which is a simple token name space value language that supports data structures. 
         [0113]    By way of example, referring to  FIG. 8 , a data structure is shown. Here a token name is used followed by a left curly brace, with the data located inside the left curly braces and then a right curly brace. Thus, the curly braces encapsulate the data associated with this data structure. 
         [0114]    Other than that, all entries involve a name, space and a value. Thus, the data language is simply a name value language, i.e. names followed by a value, where the value can be a number, a string, or an array of numbers whose name value pairs are contained inside data structures that are identified by a name and a left curly brace, data and then a right curly brace. 
         [0115]    Referring now to  FIG. 5 , what are shown are example configuration tokens that refer to the fields entered in the subject custom data structure. The same is true for  FIG. 6  which specifies fields for the Event Rule data structure, one example of which is in  FIG. 8 . Note these tables specify all configurable items in the configuration file. As to  FIG. 7 , this specifies the filter structure configuration tokens that are enterable items for the particular filters. 
       Digital Event Filter Correlation 
       [0116]    More specifically, Equation 1 of  FIG. 4  is an equation which describes the operation of the filters. Note also that sensors and metrics are sampled to produce filter input samples, i.e. x[n] and involve a time-based sample period in which the sensor events are cumulative within the sample period. 
         [0117]    The filter state is defined by y[n] which is the response of the input samples to the system response or filter. Thus y[n]=h[n]*x[n], where h[n] is the filter response. 
         [0118]    As will be appreciated, design and analysis of a filter is commonly instantiated by using the z transform and input samples from the filters, in one embodiment are analyzed by using a discrete Fourier transform (DFT). Thus input samples are analyzed based on a time-based 1/frequency view of the input samples, with the magnitude of a plot displaying how many sensor events occur with different frequencies. The time-based view is extracted from the sampling period and the spectral content at a frequency F in the magnitude plot. As will be appreciated, the filters can be set for example to be two events every five seconds, twelve events every fifteen seconds, twenty five events every fifty seconds or indeed span much longer periods of time. 
         [0119]    As the metric data flows from point A to point B, a filter state y[n] and input stream x[n] must be unique to that data flow, with the filter state being applicable to network sensors not metrics. On the other hand security events of interest, i.e. incidents, occur in data flows. 
         [0120]    It is noted that different scenarios require tracking different data flows. The different data flows to track include all activity that falls under a network intrusion defense and response (NIDAR) system including IP table filter, host-IP, port, IP Flow—IP to IP, TCP flow Client to/from Server:Port, UDP flow—IP:Port to IP:Port, and TCP Server meaning data flow Server:Port, where these terms are defined as follows: 
         [0000]    IP table filter=A filter with IP addresses, it is used to filter IN or OUT packets with the specific IP addresses contained in the filter.
 
host-IP=IP Address of a host computer
 
port=A TCP or UDP port number, between 0 and 65535
 
IP Flow=an IP to IP communication, and includes any packets sent between two IP host addresses.
 
TCP flow=A communication of any TCP packets flowing from a TCP Client to and from a TCP Server on a specific server port
 
UDP flow=A communication of any UDP packets flowing from one host IP and Port to another host IP an Port
 
TCP Server=A computer acting as a Server, ie. Providing a computer service, such as a Web Server providing the HTTP web service. The TCP server is identified by its host IP and the particular port  1  uses to communicate over. IN the case of Web traffic, the HTTP service uses Port  80 , for instance.
 
         [0121]    Note that each data flow track contains y[n] queue filter states x[mq] input samples, a time stamp of the current sample period and an event queue for instance that stores events and associated packet for each x[m]&gt;zero. 
         [0122]    What is now described is the customization handled by the subject system utilizing the above custom configuration data language, followed by a number of example runs. 
         [0123]    Referring again to  FIGS. 5 ,  6  and  7 , a listing of configuration tokens is described to define the meaning of various terms and the type of string utilized. 
         [0124]    In  FIG. 5 , the DEFCON configuration tokens are described which when set describe the indicated functions. For instance, log events log the detected events, whereas filter-events activate the serial digital filtering functions, Note the event rules for a particular event are specified by the event rule creation. 
         [0125]    With respect to  FIG. 6 , the event rule structure configuration tokens are described in which for instance the model token describes for the user the particular event rule used, the threshold defines the threshold that is used to compare against the accumulator sum with weights, whereas a minimum threshold describes the time period over which events are collected. Filters are defined by numbers, and filter weights are described by a filter weight number. Moreover, filter thresholds are a list of thresholds for each filter, with “sample-file” token referring to the file name to store the internal calculations utilized in an event rule to permit analysis, whereas “max-samples” refers to the maximum number of samples to collect or store in the sample file. 
         [0126]    Finally,  FIG. 7  specifies filter structure tokens in which filters are identified, in which the discrete sampling periods for the accumulators of  FIG. 1  are identified, and in which a track identifies or defines what input sample granularity the associated filter operates on. Note, in one embodiment, the sample token specifies two options, namely a “real” option or a “logical” option. The sample file token relates to the storage of samples for post processing and the “max sample” again specifies the maximum number of samples that can be written to the sample-file name. 
         [0127]    Note that an infinite impulse response (IIR) array has coefficients that can be specified that apply to the current and previous filter states, whereas a finite impulse response array (FIR) specifies coefficients that apply to the cumulative sum of events in a sample period. Finally, a queue size token is used to specify how many events are queued up so that associated events can be presented for further inspection. 
         [0128]    How these tokens are utilized in the subject digital filter correlation engine can be seen by the previously described example configuration of  FIG. 8 . 
         [0129]    In one example of the subject system, in  FIG. 9  the system status is described for the network intrusion defense and response system using the above custom configuration language.  FIG. 10  shows an example run involving anagrams and logger attacks, whereas  FIG. 11  shows an event log file. Finally,  FIG. 12  shows the filter sample file for an example run. 
         [0130]    The aforementioned digital filters filter sensor events and metrics assume that time based signatures correlate with network behavior. Note that sensor events include such things as anomalous packet detection, unknown filter flow detection, shellcode detection, DoS detection and the detection of TCP SYN or synchronize acknowledge floods, and TCP flows. 
         [0131]    Note also that the subject utilizes tracking granularities for network sensor filters including filter state and input samples queued per track, as well as track options that include all, host, port, IP flow, TCP flow, UDP flow, TCP server and other options. 
         [0132]    Note also there are various options available to a user to track values across a series of network sessions or TCP flows. 
         [0133]    The subject digital event filter correlation engine is therefore able to correlate one to N digital filter results to produce meaningful events of interest. The subject system is therefore uniquely able to detect small monthly IP flow with high anomalous scores which would for instance indicate a nation state attack. The system can also detect excessive shellcode Ngram matches to a TCP server to indicate a server is under attack and can detect high frequency content with shellcode detected sensor events, or can apply high pass filters to shellcode detected sensor event samples. 
         [0134]    Those skilled in the art will appreciate that the correlation engine of the present invention can work as a linear or time based tools, but also allows frequency based behavior, layered frequency behaviors, and feedback mechanism typical of digital filters. 
         [0135]    While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications or additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.