Lemur: Sub-CA creation never checks `AuthorityPermission` on the parent authority

Summary

Repo under test: https://github.com/Netflix/lemur

When ADMIN_ONLY_AUTHORITY_CREATION=False (an explicitly supported and documented configuration), POST /api/1/authorities with type=subca never verifies that the caller holds AuthorityPermission on the supplied parent authority. The parent field is resolved by AssociatedAuthoritySchema via a raw fetch_objects(Authority, data) lookup, then passed straight through service.create → mint → cryptography-issuer.create_authority, which loads options["parent"].authority_certificate.private_key and signs a brand-new intermediate CA on the caller's behalf.

Any authenticated non-read-only user can therefore mint a sub-CA chained to any internal root whose private key Lemur holds — including roots they hold no role on — attach a role they already belong to, and immediately issue or offline-sign trusted leaf certificates for arbitrary names.

Affected route

POST /api/1/authorities (with type=subca)

Affected code

• lemur/authorities/views.py:231 — gates only on AuthorityCreatorPermission() + StrictRolePermission(); no AuthorityPermission on data['parent']
• lemur/authorities/schemas.py:58 — parent = fields.Nested(AssociatedAuthoritySchema); validate_subca only checks presence
• lemur/schemas.py:107 — AssociatedAuthoritySchema.get_object → fetch_objects(Authority, data) resolves any authority by id/name with no permission check
• lemur/plugins/lemur_cryptography/plugin.py:40 — private_key = options["authority"].authority_certificate.private_key (set from options["parent"]) signs the new intermediate
• lemur/auth/permissions.py:62 — AuthorityCreatorPermission becomes always-allow when ADMIN_ONLY_AUTHORITY_CREATION=False
• lemur/certificates/views.py:538 — is_private_authority (true for cryptography-issuer) skips USER_DOMAIN_AUTHORIZATION_PROVIDER for subsequent leaf issuance

Impact

In deployments that set ADMIN_ONLY_AUTHORITY_CREATION=False to enable self-service CA creation, any authenticated non-read-only user — with zero permission on a given internal root CA — can obtain a working intermediate CA chained to that root. They can then:

• Issue TLS certificates for arbitrary names trusted by every relying party that trusts the internal root, bypassing LEMUR_ALLOWED_DOMAINS, sensitive-domain flags, and the per-user domain-authorization plugin.
• Export the sub-CA private key and sign end-entity certificates entirely outside Lemur, defeating all in-product issuance controls.

This converts "can create a self-contained test CA" into "can mint trusted certs under any internal PKI root in the organisation". The ADMIN_ONLY_AUTHORITY_CREATION documentation does not warn operators of this consequence.

Root cause

AuthoritiesList.post evaluates AuthorityCreatorPermission (a global "may create authorities" flag) and StrictRolePermission, but never evaluates AuthorityPermission(parent.id, parent.roles) against the caller-supplied parent. AssociatedAuthoritySchema is a pure lookup schema with no authz hook, and neither authorities.service.create nor mint re-check before invoking issuer_plugin.create_authority(options). The bundled cryptography-issuer then uses the parent's stored private key directly.

Validated evidence

Static trace, confirmed by code inspection (validation status: CONFIRMED):

• parent is loaded via AssociatedAuthoritySchema (raw fetch_objects), passed unchecked through views.post → service.create → mint → plugin.create_authority → issue_certificate, where the parent authority's stored private key is read and used to sign the new intermediate.
• No call to AuthorityPermission(parent.id, ...) exists anywhere on this path.
• Precondition ADMIN_ONLY_AUTHORITY_CREATION=False is an explicitly supported config (docs/administration.rst:517).

Proof of concept / reproducer

Status: reconstructed from source report (static control-flow trace; not executed against a live CA).

Preconditions: ADMIN_ONLY_AUTHORITY_CREATION=False; attacker is an authenticated Lemur user holding any role other than read-only; <PARENT_AUTHORITY_ID> is any internal cryptography-issuer root CA the attacker holds no role on; <ATTACKER_ROLE> is any role the attacker already belongs to.

curl -sS -X POST "<TARGET_BASE_URL>/api/1/authorities" \
  -H "Authorization: Bearer <AUTH_TOKEN>" \
  -H "Content-Type: application/json" \
  -d '{
        "name": "attacker-subca",
        "owner": "attacker@example.com",
        "description": "poc",
        "type": "subca",
        "parent": {"id": <PARENT_AUTHORITY_ID>},
        "plugin": {"slug": "cryptography-issuer"},
        "roles": [{"name": "<ATTACKER_ROLE>"}],
        "commonName": "attacker-intermediate",
        "validityYears": 1
      }'

The response contains a new authority whose authority_certificate is signed by <PARENT_AUTHORITY_ID>'s private key. The caller is recorded as creator and holds <ATTACKER_ROLE> on it, so POST /api/1/certificates against the new authority succeeds (and skips allowed_issuance_for_domain because is_private_authority is true).

Static-trace validation command from the source report:

grep -n 'parent' lemur/authorities/schemas.py lemur/authorities/views.py lemur/authorities/service.py \
  && sed -n '37,55p' lemur/plugins/lemur_cryptography/plugin.py

Source artifact: audit/harnesses/public-repo-threat-model-harness/results/netflix-lemur-100run-mythos-20260627T051129Z/findings.jsonl (run_022, finding cluster lemur-subca-parent-authz, 5/100 runs).

Suggested fix

In AuthoritiesList.post (or authorities.service.create), when data.get('parent') is present, enforce AuthorityPermission(parent.id, [r.name for r in parent.roles]).can() before invoking the issuer plugin, regardless of ADMIN_ONLY_AUTHORITY_CREATION. Additionally, update the ADMIN_ONLY_AUTHORITY_CREATION documentation to state that disabling it currently grants every authenticated user the ability to chain sub-CAs off any internal root whose private key Lemur holds. Consider also requiring admin (or an explicit per-parent capability) for any type=subca creation independent of the global flag.