GHSA-5j9r-fx4x-rpvg - rucio has an Authorization Bypass on REST API endpoint for signing URLs

## Summary The `/credentials/signurl` endpoint decides authorization from the client-supplied `X-Rucio-Account` HTTP header instead of the authenticated token's account. Any client holding **any** valid auth token (even a low-privilege account they legitimately own) can send `X-Rucio-Account: root` to pass the root-only permission check and have the Rucio server mint a presigned **read / write / delete** URL for an **arbitrary** object, signed with the RSE's stored S3 / GCS / Swift credentials. ## Details `SignURL.get()` takes the authorization principal from a request header and never uses the token-authenticated identity: ```python # lib/rucio/web/rest/flaskapi/v1/credentials.py account = request.headers.get('X-Rucio-Account', default=None) # L165 (attacker-controlled) ... result = get_signed_url(account, rse=rse, service=service, # L194 operation=operation, url=url, lifetime=lifetime, vo=vo) ``` The gateway then uses that header value as the `issuer` for the permission check: ```python # lib/rucio/gateway/credential.py auth_result = permission.has_permission(issuer=account, vo=vo, # L57 action='get_signed_url', kwargs=kwargs, ...) ``` and the permission function is root-only: ```python # lib/rucio/core/permission/generic.py def perm_get_signed_url(issuer, kwargs, session): return _is_root(issuer) # _is_root(x) -> x.external == 'root' ``` `InternalAccount('root', vo).external == 'root'` unconditionally, so the gate passes whenever the header is `root`. The endpoint is on `AuthenticatedBlueprint`, whose `before_request` hook validates the token and sets `request.environ['issuer']` **from the token** — but nothing ever compares `X-Rucio-Account` to it: ```python # lib/rucio/web/rest/flaskapi/v1/common.py (request_auth_env) flask.request.environ['issuer'] = auth.get('account') # L168 (the real account; unused by SignURL) ``` This is anomalous: every othe