Avoiding Holes in Your AWS S3 Buckets
April 24, 2018
Originally published in InfoSec Island on April 12, 2018.
Enterprises are moving to the cloud at a breathtaking pace, and they’re taking valuable data with them. Hackers are right behind them, hot on the trail of as much data as they can steal. The cloud upends traditional notions of networks and hosts, and it topples security practices that use them as a proxy to protect data access. In public clouds, networks and hosts are no longer the most adequate control options available for resources and data.
Amazon Web Services (AWS) S3 buckets are the destination for much of the data moving to the cloud. Given how important this sensitive data is, one would expect enterprises to pay close attention to their S3 security posture. Unfortunately, many news stories highlight how many S3 buckets have been mistakenly misconfigured and left open to public access. It’s one of the most common security weaknesses in the great migration to the cloud, leaving gigabytes of data for hackers to grab.
When investigating why cloud teams were making what seemed to be an obvious configuration mistake, two primary reasons surfaced:
1. Too Much Flexibility (Too Many Options) Turns into Easy Mistakes
S3 is the oldest AWS service and was available before EC2 or Identity and Access Management (IAM). Some access controls capabilities were built specifically for S3 before IAM existed. As it stands, there are five different ways to configure and manage access to S3 buckets.
- S3 Bucket Policies
- IAM Policies
- Access Control Lists
- Query string authentication/ static Web hosting
- API access to change the S3 policies
The more ways to configure implies more flexibility but also means that higher chances of making a mistake. The other challenge is that there are two separate policies one for buckets and one for the objects within the bucket which make things more complex.
2. A “User” in AWS is Different from a “User” in your Traditional Datacenter
Amazon allows great flexibility in making sure data sharing is simple and users can easily access data across accounts or from the Internet. For traditional enterprises, the concept of a “user” typically means a member of the enterprise. In AWS the definition of user is different. On an AWS account, the “Everyone” group includes all users(literally anyone on the internet) and “AWS Authenticated User” means any user with an AWS account. From a data protection perspective, that’s just as bad because anyone on the Internet can open an AWS account.
The customer moving from traditional enterprise – if not careful – can easily misread the meaning of these access groups and open S3 buckets to “Everyone” or “AWS authenticated User” – which means opening the buckets to the world.
S3 Security Checklist
If you are in AWS, and using S3, here is a checklist of things you should configure to ensure your critical data is secure.
Audit for Open Buckets Regularly: On regular intervals check for buckets which are open to the world. Malicious users can exploit these open buckets to find objects which have misconfigured ACL permissions and then can access these compromised objects.
Encrypt the Data: Enable server-side encryption on AWS as then it will encrypt the data at rest i.e. when objects are written and decrypt when data is read. Ideally, you should enable client-side.
Encrypt the Data in Transit: SSL in transport helps secure data in transit when it is accessed from S3 buckets. Enable Secure Transport in AWS to prevent man in middle attacks.
Enable Bucket Versioning: Ensure that your AWS S3 buckets have the versioning enabled. This will help preserve and recover changed and deleted S3 objects which can help with ransomware and accidental issues.
Enable MFA Delete: The “S3 Bucket” can be deleted by the user even if he/she does not login using MFA by default. It is highly recommended that only users authenticated using MFA have the ability to delete buckets. Using MFA to protect against accidental or intentional deletion of objects in S3 buckets will add an extra layer of security
Enable Logging: If the S3 buckets have Server Access Logging feature enabled you will be able to track every request made to access the bucket. This will allow the user to ability to monitor activity, detect anomalies and protect against unauthorized access
Monitor all S3 Policy Changes: AWS CloudTrail provides logs for all changes to S3 policy. The auditing of policies and checking for public buckets help – but instead of waiting for regular audits, any change to the policy of existing buckets should be monitored in real-time.
Track Applications Accessing S3: In one attack vector, hackers create an S3 bucket in their account and send data from your account to their bucket. This reveals a limitation of network-centric security in the cloud: traffic needs to be permitted to S3, which is classified as an essential service. To prevent that scenario, you should have IDS capabilities at the application layer and track all the applications in your environment accessing S3. The system should alert if a new application or user starts accessing your S3 buckets.
Limit Access to S3 Buckets: Ensure that your AWS S3 buckets are configured to allow access only to specific IP addresses and authorized accounts in order to protect against unauthorized access.
Close Buckets in Real-time: Even a few moments of public exposure of an S3 bucket can be risky as it can result in leakage. S3 supports tags which allow users to label buckets. Using these tags, administrators can label buckets which need to be public with a tag called “Public”. CloudTrail will alert when policy changes on a bucket and it becomes public which does not have the right tag. Users can use Lambda functions to change the permissions in real-time to correct the policies on anomalous or malicious activity.