bucket-antivirus-function

Scan new objects added to any s3 bucket using AWS Lambda. more details in this post

Features

• Easy to install
• Send events from an unlimited number of S3 buckets
• Prevent reading of infected files using S3 bucket policies
• Accesses the end-user’s separate installation of open source antivirus engine ClamAV

How It Works

• Each time a new object is added to a bucket, S3 invokes the Lambda function to scan the object
• The function package will download (if needed) current antivirus definitions from a S3 bucket. Transfer speeds between a S3 bucket and Lambda are typically faster and more reliable than another source
• The object is scanned for viruses and malware. Archive files are extracted and the files inside scanned also
• The objects tags are updated to reflect the result of the scan, CLEAN or INFECTED, along with the date and time of the scan.
• Object metadata is updated to reflect the result of the scan (optional)
• Metrics are sent to DataDog (optional)
• Scan results are published to a SNS topic (optional) (Optionally choose to only publish INFECTED results)
• Files found to be INFECTED are automatically deleted (optional)

Installation

Build from Source

To build the archive to upload to AWS Lambda, run make. The build process is completed using the amazonlinux Docker image. The resulting archive will be built at build/lambda.zip. This file will be uploaded to AWS for both Lambda functions below.

AV Definition Bucket

Create an s3 bucket to store current antivirus definitions. This provides the fastest download speeds for the scanner. This bucket can be kept as private.

To allow public access, useful for other accounts, add the following policy to the bucket.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowPublic",
            "Effect": "Allow",
            "Principal": "*",
            "Action": [
                "s3:GetObject",
                "s3:GetObjectTagging"
            ],
            "Resource": "arn:aws:s3:::<bucket-name>/*"
        }
    ]
}

Definition Update Lambda

This function accesses the user’s ClamAV instance to download updated definitions using freshclam. It is recommended to run this every 3 hours to stay protected from the latest threats.

1. Create the archive using the method in the Build from Source section.

2. From the AWS Lambda Dashboard, click Create function

3. Choose Author from scratch on the Create function page

4. Name your function bucket-antivirus-update when prompted on the Configure function step.

5. Set Runtime to Python 2.7

6. Create a new role name bucket-antivirus-update that uses the following policy document

   {
      "Version":"2012-10-17",
      "Statement":[
         {
            "Effect":"Allow",
            "Action":[
               "logs:CreateLogGroup",
               "logs:CreateLogStream",
               "logs:PutLogEvents"
            ],
            "Resource":"*"
         },
         {
            "Action":[
               "s3:GetObject",
               "s3:GetObjectTagging",
               "s3:PutObject",
               "s3:PutObjectTagging",
               "s3:PutObjectVersionTagging"
            ],
            "Effect":"Allow",
            "Resource":"arn:aws:s3:::<bucket-name>/*"
         }
      ]
   }

7. Click next to go to the Configuration page

8. Add a trigger from the left of CloudWatch Event using rate(3 hours) for the Schedule expression. Be sure to check Enable trigger

9. Choose Upload a ZIP file for Code entry type and select the archive downloaded in step 1.

10. Add a single environment variable named AV_DEFINITION_S3_BUCKET and set its value to the name of the bucket created to store your AV definitions.

11. Set Lambda handler to update.lambda_handler

12. Under Basic Settings, set Timeout to 5 minutes and Memory to 512

13. Save and test your function. If prompted for test data, just use the default provided.

AV Scanner Lambda

1. Create the archive using the method in the Build from Source section.

2. From the AWS Lambda Dashboard, click Create function

3. Choose Author from scratch on the Create function page

4. Name your function bucket-antivirus-function

5. Set Runtime to Python 2.7

6. Create a new role name bucket-antivirus-function that uses the following policy document

   {
      "Version":"2012-10-17",
      "Statement":[
         {
            "Effect":"Allow",
            "Action":[
               "logs:CreateLogGroup",
               "logs:CreateLogStream",
               "logs:PutLogEvents"
            ],
            "Resource":"*"
         },
         {
            "Action":[
               "s3:GetObject",
               "s3:GetObjectTagging",
               "s3:PutObjectTagging",
               "s3:PutObjectVersionTagging",
            ],
            "Effect":"Allow",
            "Resource": [
              "arn:aws:s3:::<bucket-name-1>/*",
              "arn:aws:s3:::<bucket-name-2>/*"
            ]
         },
         {
            "Action":[
               "s3:GetObject",
               "s3:GetObjectTagging",
            ],
            "Effect":"Allow",
            "Resource": [
              "arn:aws:s3:::<av-definition-s3-bucket>/*"
            ]
         },
         {
            "Action":[
               "kms:Decrypt",
            ],
            "Effect":"Allow",
            "Resource": [
              "arn:aws:s3:::<bucket-name-1>/*",
              "arn:aws:s3:::<bucket-name-2>/*"
            ]
         },
         {
            "Action":[
               "sns:Publish",
            ],
            "Effect":"Allow",
            "Resource": [
              "arn:aws:sns:::<av-scan-start>",
              "arn:aws:sns:::<av-status>"
            ]
         }
      ]
   }

7. Click next to head to the Configuration page

8. Add a new trigger of type S3 Event using ObjectCreate(all).

9. Choose Upload a ZIP file for Code entry type and select the archive created in step 1.

10. Set Lambda handler to scan.lambda_handler

11. Add a single environment variable named AV_DEFINITION_S3_BUCKET and set its value to the name of the bucket created to store your AV definitions. If your bucket is s3://my-bucket, the value should be my-bucket.

12. Under Basic settings, set Timeout to 5 minutes and Memory to 1024

13. Save the function. Testing is easiest performed by uploading a file to the bucket configured as the trigger in step 4.

S3 Events

Configure scanning of additional buckets by adding a new S3 event to invoke the Lambda function. This is done from the properties of any bucket in the AWS console.

Note: If configured to update object metadata, events must only be configured for PUT and POST. Metadata is immutable, which requires the function to copy the object over itself with updated metadata. This can cause a continuous loop of scanning if improperly configured.

Configuration

Runtime configuration is accomplished using environment variables. See the table below for reference.

Variable	Description	Default	Required
AV_DEFINITION_S3_BUCKET	Bucket containing antivirus definition files		Yes
AV_DEFINITION_S3_PREFIX	Prefix for antivirus definition files	clamav_defs	No
AV_DEFINITION_PATH	Path containing files at runtime	/tmp/clamav_defs	No
AV_SCAN_START_SNS_ARN	SNS topic ARN to publish notification about start of scan		No
AV_SCAN_START_METADATA	The tag/metadata indicating the start of the scan	av-scan-start	No
AV_STATUS_CLEAN	The value assigned to clean items inside of tags/metadata	CLEAN	No
AV_STATUS_INFECTED	The value assigned to clean items inside of tags/metadata	INFECTED	No
AV_STATUS_METADATA	The tag/metadata name representing file's AV status	av-status	No
AV_STATUS_SNS_ARN	SNS topic ARN to publish scan results (optional)		No
AV_STATUS_SNS_PUBLISH_CLEAN	Publish AV_STATUS_CLEAN results to AV_STATUS_SNS_ARN	True	No
AV_STATUS_SNS_PUBLISH_INFECTED	Publish AV_STATUS_INFECTED results to AV_STATUS_SNS_ARN	True	No
AV_TIMESTAMP_METADATA	The tag/metadata name representing file's scan time	av-timestamp	No
CLAMAVLIB_PATH	Path to ClamAV library files	./bin	No
CLAMSCAN_PATH	Path to ClamAV clamscan binary	./bin/clamscan	No
FRESHCLAM_PATH	Path to ClamAV freshclam binary	./bin/freshclam	No
DATADOG_API_KEY	API Key for pushing metrics to DataDog (optional)		No
AV_PROCESS_ORIGINAL_VERSION_ONLY	Controls that only original version of an S3 key is processed (if bucket versioning is enabled)	False	No
AV_DELETE_INFECTED_FILES	Controls whether infected files should be automatically deleted	False	No
EVENT_SOURCE	The source of antivirus scan event "S3" or "SNS" (optional)	S3	No

S3 Bucket Policy Examples

Deny to download the object if not "CLEAN"

This policy doesn't allow to download the object until:

1. The lambda that run Clam-AV is finished (so the object has a tag)
2. The file is not CLEAN

Please make sure to check cloudtrail for the arn:aws:sts, just find the event open it and copy the sts. It should be in the format provided below:

 {
    "Effect": "Deny",
    "NotPrincipal": {
        "AWS": [
            "arn:aws:iam::<<aws-account-number>>:role/<<bucket-antivirus-role>>",
            "arn:aws:sts::<<aws-account-number>>:assumed-role/<<bucket-antivirus-role>>/<<bucket-antivirus-role>>",
            "arn:aws:iam::<<aws-account-number>>:root"
        ]
    },
    "Action": "s3:GetObject",
    "Resource": "arn:aws:s3:::<<bucket-name>>/*",
    "Condition": {
        "StringNotEquals": {
            "s3:ExistingObjectTag/av-status": "CLEAN"
        }
    }
}

Deny to download and re-tag "INFECTED" object

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Deny",
      "Action": ["s3:GetObject", "s3:PutObjectTagging"],
      "Principal": "*",
      "Resource": ["arn:aws:s3:::<<bucket-name>>/*"],
      "Condition": {
        "StringEquals": {
          "s3:ExistingObjectTag/av-status": "INFECTED"
        }
      }
    }
  ]
}

Manually Scanning Buckets

You may want to scan all the objects in a bucket that have not previously been scanned or were created prior to setting up your lambda functions. To do this you can use the scan_bucket.py utility.

pip install boto3
scan_bucket.py --lambda-function-name=<lambda_function_name> --s3-bucket-name=<s3-bucket-to-scan>

This tool will scan all objects that have not been previously scanned in the bucket and invoke the lambda function asynchronously. As such you'll have to go to your cloudwatch logs to see the scan results or failures. Additionally, the script uses the same environment variables you'd use in your lambda so you can configure them similarly.

Testing

There are two types of tests in this repository. The first is pre-commit tests and the second are python tests. All of these tests are run by CircleCI.

pre-commit Tests

The pre-commit tests ensure that code submitted to this repository meet the standards of the repository. To get started with these tests run make pre_commit_install. This will install the pre-commit tool and then install it in this repository. Then the github pre-commit hook will run these tests before you commit your code.

To run the tests manually run make pre_commit_tests or pre-commit run -a.

Python Tests

The python tests in this repository use unittest and are run via the nose utility. To run them you will need to install the developer resources and then run the tests:

make test

License

This code is open source software licensed under the Apache 2.0 License.

ClamAV is released under the GPL Version 2 License and all source for ClamAV is available for download on Github.