April 16, 2019
AWS CloudTrail is a service that “enables governance, compliance, operational auditing, and risk auditing of your AWS account.” It continuously monitors accounts and it is one of the most valuable and probably the best data source for security analysis in AWS. Security and Operations teams love Cloudtrail because of the added visibility into user and resource activity across their AWS infrastructure.
Getting ahead of a potential compromises and being able to respond quickly is the name of the game when providing operational security. There are several industry-standard documents that outline how to best prepare for securing an AWS accounts. A very well-known one is the CIS Benchmark document which includes many best-practice security recommendations ranging from properly configuring IAM, to enabling Logging, to Monitoring etc. CloudTrail plays a central role here and its data can help reveal potential security gaps, assist in detecting early signs of compromise and help you stay compliant with your security policies and requirements. Therefore, extracting these insights as close to real-time as possible becomes a security necessity. This is where Cribl helps. With Cribl you can ingest CloudTrail logs natively using the Serverless Forwarding App and in real-time you can:
Let’s dive in and see how to collect, filter/route, categorize and check/score CloudTrail data with Cribl.
First, if you haven’t already done so, see our previous blog post: Serverless data forwarding to Cribl for AWS Services for a detailed walkthrough on how to collect CloudTrail data serverlessly.
Next, let’s de-noise our events so that we have a lean stream of data to work with. Everything is way more efficient and faster when working with clean data.
eventName that fit this criteria:
/(^Describe|^List|^Get)/.test(eventName). These voluminous, read-only events tend to be not that useful in security analysis. For example, here are top 10 eventNames from our sample/lab environment.
eventName count ------------------------------ DescribeTargetHealth 311198 DescribeInstances 263879 DescribeRouteTables 263072 AssumeRole 149884 DescribeInstanceStatus 113441 DescribeServices. 87955 DescribeTable 86896 GetBucketLocation 44775 DescribeVolumes 44067 GetBucketEncryption 43641
Note: Additional trimming happens below.
Yet another alternative to dropping or suppressing events is routing them to different locations. E.g. you can send full fidelity stream to archive while sending an important subset to an analytics platform.
Not all CloudTrail data is created equal. Even when filtering or suppressing read-only events, there is still a lot left to sift through. One way to address this is to introduce categories for important or notable events. Think of this step as prep work that will pay dividends in the future; it will make your analysis easier and response faster.
Here we will use a lookup table to categorize events – in real-time – with labels that best describe the context in which that activity belongs to. For instance, events that indicate security-relevant S3 bucket configuration changes are labelled with
S3BucketActivity. Sample association table:
eventName,securityEventCategory ConsoleLogin,LoginActivity PutBucketAcl,S3BucketActivity PutBucketPolicy,S3BucketActivity ... ReplaceNetworkAclAssociation,NetworkACLActivity CreateCustomerGateway,GatewayActivity ... StopLogging,CloudTrailActivity DeleteGroupPolicy,IAMPolicyActivity
The context and what’s important naturally depends on threat model and from one account to another. The lookup table approach accommodates this by allowing editing. You can add/remove/change the categories per your needs.
To label the data, we use a Lookup function that enriches each event with a
securityEventCategory field if a matching
eventName is found.
CloudTrail events may contain other indications of security-relevant activity that cannot be done with a Lookup function. This needs conditional checking across multiple fields. For instance, checking for console authentication failures requires to evaluate
eventName=='ConsoleLogin' && errorMessage=='Failed authentication'
To do this an Eval function is used to assign the
securityEventCategory field an appropriate value if a check returns true. The value can be a label (string) or, if a scoring model is introduced, a number. Numerical scoring is useful when further statistical analysis is done in downstream analytic platforms or when filtering is needed avoid alert fatigue.
With Cribl these checks can be articulated effortlessly given the expressibility of the language. In fact, most of the CIS Benchmark document recommendations that deal with CloudTrail events can be codified easily.
Some CloudTrail events contain fields with enormously sized payloads. For example, the
sessionToken value in
AssumeRole events is close to 1KB! With Cribl you can be very surgical and target these fields independently – we can either toss it out or trim it down. We can do this with the Parser function in reserialize mode; it has the capability to remove/keep any fields before converting the JSON object/event into a string and sending it downstream.
CloudTrail logs are critical for operating your AWS infrastructure. With added context and checks they can become invaluable for security operations. Cribl makes it super easy to bring in additional insights in real-time and enable you to get ahead of potential compromises, decrease response time and ultimately improve your security posture.
If you’d like to work with these configurations get them for free in GitHub.
Enjoy it! — The Cribl Team
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