Firewall Fundamentals
| After the decision has been made to log events from your firewall, the next step is determining what you should be looking for in the logs and how you should properly perform log analysis. The most important thing to remember is that firewall logs are virtually worthless if no one ever looks at the logs. Logging is merely a means to an end, namely knowing what is going on with your firewalls so that you can respond accordingly. Review of the logs should not be reserved for only when an incident has occurred. It should be a part of the weekly, if not daily, tasks that the firewall administrators perform. To help reduce the time and effort required to review the logs, many of the enterprise security incident management products provide tools and utilities that assist the firewall administrator in separating the wheat from the chaff, allowing the firewall administrator to spend less time reviewing the logs, while still providing the information necessary to help identify situations before they become a problem. Another aspect of reviewing the logs that should not be overlooked is the need to define a log archive and normalization policy. Too many organizations do not store their firewall logs long enough to adhere to regulations (some of which such as Sarbanes-Oxley are generally accepted to require seven years of log data to be stored). This creates situations where data from the logs may be necessary, but the logs themselves have been destroyed. In conjunction with this, it is important to normalize your log data. Normalization just means converting your logs into a standard format that allows for easier review and correlation of data from different data sources (such as different firewall vendors). What to Look for in Firewall Logs
After you have collected the firewall logs and begun the process of analyzing the logs, determine the data that you should be looking for in the logs. With that said, it is important to remember not to fall into the trap of looking in your firewall logs only for "bad" events. Yes, firewall logs can be the key element in discovering security incidents and compromises, but that is only one of the reasons for analyzing your logs. You also want to be able to use the log information to assist in defining the baselines and normal operations of the firewall. After all, one of the easiest ways to know whether behavior that has been logged is malicious is to know what the good things are and then note the exceptions. The simple fact of the matter is that certain events should always raise suspicion when they are detected. Ten of the most common events that warrant further investigation are as follows:
The following sections explain these events in more detail. Authentication Allowed
Although it may seem rather innocuous at first glance, it is important to look for authentication-allowed events because they can identify situations where access was granted by the firewall when it should not have been allowed. The reasons can range from legitimate administrators logging on when they should not have to malicious users logging on after compromising the account and password that they are using. In addition, if your firewall is configured to authenticate user access, this event can be used to identify users who have been authenticated for whatever function they are attempting to perform. Traffic Dropped (Not Addressed to the Firewall)
Most firewalls will have some resources that they are protecting. Traffic addressed to these servers will typically be processed by the firewall and filtered accordingly. Although traffic-dropped messages can indicate that someone is attempting to access a protected resource in a manner other than what the firewall administrator has defined, a common cause of this event is a simple misconfiguration of the ruleset. Therefore, if users cannot access protected resources, it is important to review the logs to determine whether the firewall is dropping the traffic, thereby pointing you in the direction of what may need to be fixed to provide access to the resources requested. Firewall Stop/Start/Restart
The firewall should never stop, start, or restart without the firewall administrator knowing in advance that the situation is going to occur. This event can be caused by non-firewall-specific issues such as power failures as well as by firewall-specific issues such as the firewall crashing or a high-availability failover, and therefore it should always be investigated in more detail to ascertain the root cause. Firewall Configuration Changed
Almost all firewall configuration changes should be accompanied with the appropriate change control documentation. This event always warrants further investigation to ensure that the changes that were made are legitimate and in accordance with expected results. In fact, many SIM products can be configured to perform a comparison of the changed configuration against a "known good" configuration when a firewall configuration changed event occurs. In fact, some products such as NetIQ Security Manager can actually use that information to attempt to undo the changes that were made if they are found to be out of compliance with the known good configuration. Interface Up/Down Status Changed
Firewall interfaces transitioning from an up to a down status and vice versa can indicate problems with the underlying network configuration. This information can prove particularly helpful in situations where redundant firewalls are implemented, because the network interfaces transitioning to a down state could cause the firewall failover process to occur. Administrator Access Granted
Whenever administrator access is granted, the corresponding event should be investigated. Although similar to monitoring for authentication, in this case we are looking explicitly at gaining administrator access. Most likely the access is expected, and there is nothing suspicious or out of order that warrants further review. However, if that is not the case, this event rapidly becomes an extremely high-priority situation that must be investigated because the implication can be that an administrator account has been compromised. Connection Was Torn Down
The termination of connections is a relatively routine process that is a part of normal communications. Where this event is particularly important, however, is in listing the reason why the connection was torn down. For example, the connection may have been torn down as a result of SYN timeout, which can be an indicator that someone is attempting to cause a denial of service, especially if there are a lot of events of that nature. In determining the cause of the connection tear down, it is important to review the firewall documentation for the teardown causes. For example, Cisco Secure PIX Firewall version 7.0 message ID 302014 lists the potential reasons for a TCP connection being torn down as shown in Table 12-3.
As you can see, reasons such as "Unauth Deny" or "Flow closed by inspection" can be indicators of malicious traffic and warrant more concern and investigation than a reason such as "TCP ResetI" (which is a normal method of applications terminating their communications session). Authentication Failed
Authentication-failed events can be indicators of everything from users making a typo when they enter their password to malicious users making a brute-force attack in an attempt to determine the password. Authentication-failed events should be examined in particular detail when the account in question is a privileged or administrator-level account. Traffic Dropped (Addressed to the Firewall)
These events are similar to the traffic dropped that is not addressed to the firewall, with the obvious difference being that in this case the traffic is addressed to the firewall. As a general rule, the firewall should not have any traffic addressed directly to it on the external interface; instead, all traffic should be destined for the resources being protected by the firewall. These events can be indicators of malicious users attempting to gain access to the firewall or a misconfiguration of things such as ICMP, IPsec, or management or routing protocols and therefore should be investigated in more detail to determine the exact nature of why the traffic was dropped. Administrator Session Ended
Similar to administrator access being granted, administrator sessions ending should be monitored to ensure that the administrator who had access was supposed to have access. This type of event can also be used as a time benchmark because only administrators should be able to make changes to the firewall, and therefore the logs should be investigated in more detail for the time preceding the administrator session ending to see exactly what commands may have been run. Cisco Secure PIX Firewall Syslog Event Baseline
The following syslog events constitute a good baseline of events that should be monitored and paid careful attention to in most environments. In essence, this list is here to answer this question: What specific kinds of events should I look for? It is not meant to be an exhaustive list of all syslog message IDs or the only syslog message IDs that you should be filtering for. You can use this information to help build filtering rules for your particular logging softwarefor example, to identify the messages that administrators should get a page or e-mail notification over (for instance, message %PIX-3-201008) versus messages that can just be logged without any special notification occurring. This can be done by using the message ID (for example, %PIX-3-201008) in your logging software's filtering syntax/search strings. In general, every time Cisco releases a new version of software, syslog events are added/deleted from the list of events. Therefore, your particular version of software may or may not include all of these events, or it may have events that are not listed here. Obviously, not all events are relevant for all environments, but this list provides a sound starting point of events to be on the look out for, from which you can further customize to meet the logging requirements in your environment. This list can be easily modified to cover both the Cisco Adaptive Security Appliance (ASA) and Cisco Firewall Services Module (FWSM) by just replacing the %PIX syntax with either a %ASA or %FWSM, respectively (in fact, the log messages use %PIX|ASA to mean that either %PIX or %ASA can be used):
Note For an exhaustive list of all Cisco PIX/ASA/FWSM syslog messages, see http://www.cisco.com/en/US/products/sw/secursw/ps2120/products_system_message_guides_list.html. |
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