Skip to content

AWS S3 bucket is publicly writable

Description

Unprotected S3 buckets are one of the major causes of data theft and intrusions. An S3 bucket that allows WRITE access to everyone allows attackers to create, overwrite and delete objects within the bucket, which can lead to: S3 data loss, unintended changes to applications using that bucket, and unexpected charges. The only S3 buckets that should be globally accessible for unauthenticated users or for Any AWS Authenticate Users are those used for hosting static websites. Bucket ACL helps manage access to S3 bucket data.

We recommend AWS S3 buckets are not publicly accessible for WRITE actions to protect S3 data from unauthorized users and exposing sensitive data to public access.

Fix - Runtime

Procedure

S3 buckets should be protected by using the bucket ACL and bucket policies. If you want to share data with other users via S3 buckets, you could create pre-signed URLs with a short expiration duration.

To generate a pre-signed URL for the file samplefile.zip, use the following command:

aws s3 presign --expires-in 36000 s3://sharedfolder/samplefile.zip

To generate pre-signed URLS for every object in an S3 bucket, use the following command:

aws s3 ls --recursive s3://sharedfolder | awk '{print $4}' | 
while read line; do aws s3 presign --expires-in 36000 s3://sharedfolder/$line; done

📘 Note

For all automation-related work use the bucket policy and grant access to the required roles.

Fix - Buildtime

CloudFormation

  • Resource: AWS::S3::Bucket
  • Argument: Properties.AccessControl
Type: AWS::S3::Bucket
    Properties:
        ...
-     AccessControl: PublicReadWrite

Fix - Buildtime

Terraform

  • Resource: aws_s3_bucket, aws_s3_bucket_acl
  • Argument: acl

go aws_s3_bucket.fail.tf resource "aws_s3_bucket_acl" "example" { bucket = aws_s3_bucket.example.id - acl = "public-read-write" + acl = "private" }

Replace the acl with "private" instead of "public-read-write", or remove entirely as the default is private.