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


  • 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)


Build from Source

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

Create Relevant AWS Infra via CloudFormation

Use CloudFormation with the cloudformation.yaml located in the deploy/ directory to quickly spin up the AWS infra needed to run this project. CloudFormation will create:

  • An S3 bucket that will store AntiVirus definitions.
  • A Lambda Function called avUpdateDefinitions that will update the AV Definitions in the S3 Bucket every 3 hours. This function accesses the user’s above S3 Bucket to download updated definitions using freshclam.
  • A Lambda Function called avScanner that is triggered on each new S3 object creation which scans the object and tags it appropriately. It is created with 1600mb of memory which should be enough, however if you start to see function timeouts, this memory may have to be bumped up. In the past, we recommended using 1024mb, but that has started causing Lambda timeouts and bumping this memory has resolved it.

Running CloudFormation, it will ask for 2 inputs for this stack:

  1. BucketType: private (default) or public. This is applied to the S3 bucket that stores the AntiVirus definitions. We recommend to only use public when other AWS accounts need access to this bucket.
  2. SourceBucket: [a non-empty string]. The name (do not include s3://) of the S3 bucket that will have its objects scanned. Note - this is just used to create the IAM Policy, you can add/change source buckets later via the IAM Policy that CloudFormation outputs

After the Stack has successfully created, there are 3 manual processes that still have to be done:

  1. Upload the build/ file that was created by running make all to the avUpdateDefinitions and avScanner Lambda functions via the Lambda Console.
  2. To trigger the Scanner function on new S3 objects, go to the avScanner Lambda function console, navigate to Configuration -> Trigger -> Add Trigger -> Search for S3, and choose your bucket(s) and select All object create events, then click Add. Note - if you chose more than 1 bucket as the source, or chose a different bucket than the Source Bucket in the CloudFormation parameter, you will have to also edit the IAM Role to reflect these new buckets (see "Adding or Changing Source Buckets")
  3. Navigate to the avUpdateDefinitions Lambda function and manually trigger the function to get the initial Clam definitions in the bucket (instead of waiting for the 3 hour trigger to happen). Do this by clicking the Test section, and then clicking the orange test button. The function should take a few seconds to execute, and when finished you should see the clam_defs in the av-definitions S3 bucket.

Adding or Changing Source Buckets

Changing or adding Source Buckets is done by editing the AVScannerLambdaRole IAM Role. More specifically, the S3AVScan and KmsDecrypt parts of that IAM Role's policy.

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.


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_SIGNATURE_METADATA The tag/metadata name representing file's AV type av-signature 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_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_ENDPOINT The Endpoint to use when interacting wth S3 None No
SNS_ENDPOINT The Endpoint to use when interacting wth SNS None No
LAMBDA_ENDPOINT The Endpoint to use when interacting wth Lambda None 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": [
    "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 utility.

pip install boto3 --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.


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:

pip install -r requirements.txt
pip install -r requirements-dev.txt
make test

Local lambdas

You can run the lambdas locally to test out what they are doing without deploying to AWS. This is accomplished by using docker containers that act similarly to lambda. You will need to have set up some local variables in your .envrc.local file and modify them appropriately first before running direnv allow. If you do not have direnv it can be installed with brew install direnv.

For the Scan lambda you will need a test file uploaded to S3 and the variables TEST_BUCKET and TEST_KEY set in your .envrc.local file. Then you can run:

direnv allow
make archive scan

If you want a file that will be recognized as a virus you can download a test file from the EICAR website and uploaded to your bucket.

For the Update lambda you can run:

direnv allow
make archive update


Upside Travel, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.

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