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[Openstack-security] [Bug 1840288] Re: Trusts GET API leaks existence information to unauthorized users


I think Adam's correct. Doesn't appear user can obtain a trust token if
he's not a trustee. i.e.

{
    "auth": {
        "identity": {
            "methods": [
                "token"
            ],
            "token": {
                "id": "trustee_token"
            }
        },
        "scope": {
            "OS-TRUST:trust": {
                "id": "254e874294354f7288160b6a23c174d2"
            }
        }
    }
}

If the token is not a trustee token, Keystone will reject the request
with 403.

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https://bugs.launchpad.net/bugs/1840288

Title:
  Trusts GET API leaks existence information to unauthorized users

Status in OpenStack Identity (keystone):
  In Progress
Status in OpenStack Security Advisory:
  Won't Fix

Bug description:
  The current implementation of the GET /v3/OS-TRUST/trusts/{trust_id}
  API leaks information about the existence of a trust to unauthorized
  users.

  If an authenticated user requests a trust that either does not exist
  or has no remaining uses, the returned response is a 404 regardless of
  whether the user is an admin or a trustor/trustee of the hypothetical
  (e.g. soft-deleted or used-up) trust. If the trust does exist but the
  user has no access to it, the returned response is a 403. If an
  attacker had some reasonable way of guessing or brute-forcing the UUID
  of a trust, they could use this leak to confirm its existence. A valid
  trust ID can then be used as part of a token request in combination
  with the trustee's credentials.

  The issue is here:

  https://opendev.org/openstack/keystone/src/commit/5beddfaddbb4c59d7a24fa1d7ff534da4c69ddc5/keystone/api/trusts.py#L149-L150

  The current "identity:get_trust" default policy rule is "" which is
  all-permissive, and authorization is hardcoded in the trust controller
  code. To enforce the "only the trustor or trustee can GET this" rule,
  it does a lookup of the trust and doesn't catch a NotFound, thereby
  leaking it directly back to the requester.

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