We kick off our discussion of additional monitoring technologies with a high-level overview of file integrity monitoring. As the name implies, file integrity monitoring detects changes to files – whether text, configuration data, programs, code libraries, critical system files, or even Windows registries. Files are a common medium for delivering viruses and malware, and detecting changes to key files can provide an indication of machine compromise.

File integrity monitoring works by analyzing changes to individual files. Any time a file is changed, added, or deleted, it’s compared against a set of policies that govern file use, as well as signatures that indicate intrusion. Policies are as simple as a list of operations on a specific file that are not allowed, or could include more specific comparisons of the contents and the user who made the change. When a policy is violated an alert is generated.

Changes are detected by examining file attributes: specifically name, date of creation, time last modified, ownership, byte count, a hash to detect tampering, permissions, and type. Most file integrity monitors can also ‘diff’ the contents of the file, comparing before and after contents to identify exactly what changed (for text-based files, anyway). All these comparisons are against a stored reference set of attributes that designates what state the file should be in. Optionally the file contents can be stored for comparison, and what to do in case a change is detected as a baseline.

File integrity monitoring can be periodic – at intervals from minutes to every few days. Some solutions offer real-time threat detection that performs the inspection as the files are accessed. The monitoring can be performed remotely – accessing the system with user credentials and running instructing the operating system to periodically collect relevant information – or an agent can be installed on the target system that performs the data collection locally, and returns data upstream to the monitoring server.

As you can imagine, even a small company changes files a lot, so there is a lot to look at. And there are lots of files on lots of machines – as in tens of thousands. Vendors of integrity monitoring products provide the basic list of critical files and policies, but you need to configure the monitoring service to protect the rest of your environment. Keep in mind that some attacks are not fully defined by a policy, and verification/investigation of suspicious activity must be performed manually. Administrators need to balance performance against coverage, and policy precision against adaptability. Specify too many policies and track too many files, and the monitoring software consumes tremendous resources. File modification policies designed for maximum coverage generate many ‘false-positive’ alerts that must be manually reviewed. Rules must balance between catching specific attacks and detecting broader classes of threats.

These challenges are mitigated in several ways. First, monitoring is limited to just those files that contain sensitive information or are critical to the operation of the system or application. Second, the policies have different criticality, so that changes to key infrastructure or matches against known attack signatures get the highest priority. The vendor supplies rules for known threats and to cover compliance mandates such as PCI-DSS. Suspicious events that indicate an attack policy violation are the next priority. Finally, permitted changes to critical files are logged for manual review at a lower priority to help reduce the administrative burden.

File integrity monitoring has been around since the mid-90s, and has proven very effective for detection of malware and system compromise. Changes to Windows registry files and open source libraries are common hacks, and very difficult to detect manually. While file monitoring does not help with many of the web and browser attacks that use injection or alter programs in memory, it does detect many types of persistant threats, and therefore is a very logical extension of existing monitoring infrastructure.