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:

# 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:

# 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:

# 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:

# 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 other authenticated handler derives the account from request.environ['issuer']; credentials.py is the only one that trusts the header. Additionally, core.credential.get_signed_url never checks that the supplied url's host/bucket belongs to the target RSE, so even a legitimate root caller gets an arbitrary-object signing oracle. In multi-VO the VO is also read from the header, allowing cross-tenant targeting.