Lemur: Any user can revoke arbitrary certificates at the CA by uploading a duplicate record and revoking it

Summary

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

PUT /api/1/certificates/<id>/revoke authorizes the caller against the Lemur database row (creator == current user, or CertificatePermission over the row's roles) rather than the underlying CA-side certificate identity. Separately, POST /api/1/certificates/upload lets any user passing StrictRolePermission create a new Certificate row while freely supplying body, authority (resolved by id/name with no AuthorityPermission check) and external_id; there is no uniqueness constraint on body, serial, or external_id.

An attacker can therefore read a target certificate's public body, authority.id, and external_id via GET /certificates/<id>, upload a duplicate row, and revoke that duplicate. The creator-bypass skips CertificatePermission, the empty-endpoints check passes because the duplicate has none, and service.revoke() then revokes at the CA using the attacker-supplied body (ACME) or external_id (DigiCert/Entrust/Google CA/CFSSL) under the authority's stored CA credentials — revoking the real production certificate.

Affected route

POST /api/1/certificates/upload → PUT /api/1/certificates/<dup_id>/revoke

Affected code

• lemur/certificates/views.py:651 — only StrictRolePermission().can() gates upload; no AuthorityPermission check
• lemur/certificates/schemas.py:391 — CertificateUploadInputSchema accepts caller-supplied authority and external_id
• lemur/schemas.py:107 — AssociatedAuthoritySchema resolves any authority by id/name with no permission check
• lemur/certificates/service.py:489 — upload() binds the caller-supplied authority onto the new row
• lemur/certificates/models.py:119 — only name is unique; body/serial/external_id are not, and get_or_increase_name() auto-suffixes on collision
• lemur/certificates/views.py:1677 — if g.current_user != cert.user: ... CertificatePermission ... — creator of the duplicate row bypasses the owner check
• lemur/certificates/views.py:1687 — if cert.endpoints: ... — duplicate row has no endpoints, so the deployed-cert safeguard is bypassed
• lemur/certificates/service.py:1120 — plugin = plugins.get(certificate.authority.plugin_name); plugin.revoke_certificate(certificate, reason)
• lemur/plugins/lemur_acme/acme_handlers.py:268 — ACME revokes by certificate.body
• lemur/plugins/lemur_digicert/plugin.py:477, lemur/plugins/lemur_entrust/plugin.py:321 — commercial CAs revoke by certificate.external_id
• lemur/certificates/schemas.py:290 — CertificateOutputSchema exposes external_id, body, authority to any authenticated user

Impact

A low-privileged authenticated insider (or holder of a stolen non-admin token/API key) can revoke any certificate managed by Lemur — including high-value certificates they do not own and certificates currently attached to live endpoints — directly at the issuing CA. Iterating over GET /certificates yields fleet-wide revocation (mass DoS of TLS endpoints) without ever passing an AuthorityPermission or CertificatePermission check on the victim certificate. This is exactly the "Revocation as DoS vector" scenario flagged in the threat model and additionally defeats the built-in "cannot revoke while attached to endpoint" safeguard.

Root cause

Revocation authority is bound to ownership of the Lemur DB row, not to the CA-side certificate identity. Because upload allows creating a second row that aliases the same CA-side certificate (same body / external_id / authority) without any uniqueness constraint or AuthorityPermission check, the attacker can manufacture a row they own and then exercise the creator-bypass on revoke. The endpoint-attached guard inspects only the duplicate row's cert.endpoints, which is empty.

Validated evidence

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

• Upload is gated only by StrictRolePermission (default-open to any non-read-only user) and accepts attacker-chosen authority + external_id + body without an AuthorityPermission check.
• Certificate.body / external_id have no uniqueness constraint, so a duplicate row is created.
• The revoke endpoint short-circuits the owner check when caller is the row creator, and the endpoint-attached guard inspects only the duplicate row.
• Issuer plugins revoke at the CA using body / external_id taken from the duplicate row under the authority's stored CA credentials.

Proof of concept / reproducer

Status: reconstructed from source report (static control-flow trace; not executed against a live CA — revoking at a real CA is destructive).

Preconditions: attacker is an authenticated Lemur user holding any role other than read-only. <VICTIM_CERT_ID> is any certificate id readable via GET /api/1/certificates.

# 1. Read the victim's body / authority / external_id (exposed to any authenticated user)
curl -sS "<TARGET_BASE_URL>/api/1/certificates/<VICTIM_CERT_ID>" \
  -H "Authorization: Bearer <AUTH_TOKEN>" \
  | jq '{body, external_id, authority: .authority.id}'

# 2. Upload a duplicate row aliasing the same CA-side certificate
curl -sS -X POST "<TARGET_BASE_URL>/api/1/certificates/upload" \
  -H "Authorization: Bearer <AUTH_TOKEN>" \
  -H "Content-Type: application/json" \
  -d '{
        "name": "victim-dup",
        "owner": "[email protected]",
        "body": "<VICTIM_BODY_PEM>",
        "authority": {"id": <VICTIM_AUTHORITY_ID>},
        "externalId": "<VICTIM_EXTERNAL_ID>"
      }'
# → returns {"id": <DUP_ID>, ...}; attacker is now cert.user of <DUP_ID>

# 3. Revoke the duplicate — issuer plugin revokes at the CA by body/external_id
curl -sS -X PUT "<TARGET_BASE_URL>/api/1/certificates/<DUP_ID>/revoke" \
  -H "Authorization: Bearer <AUTH_TOKEN>" \
  -H "Content-Type: application/json" \
  -d '{"crlReason": "unspecified"}'

Static-trace validation command from the source report:

grep -n 'StrictRolePermission' lemur/certificates/views.py | grep -v Authority
grep -n 'cert.authority = kwargs.get' lemur/certificates/service.py
grep -n 'g.current_user != cert.user' lemur/certificates/views.py
grep -n 'certificate.external_id\|certificate.body' \
  lemur/plugins/lemur_acme/acme_handlers.py \
  lemur/plugins/lemur_digicert/plugin.py \
  lemur/plugins/lemur_entrust/plugin.py

Source artifact: audit/harnesses/public-repo-threat-model-harness/results/netflix-lemur-100run-mythos-20260627T051129Z/findings.jsonl (run_050, finding cluster lemur-revoke-via-duplicate, 2/100 runs).

Suggested fix

Decouple CA-side revocation authority from Lemur row ownership:

1. On POST /certificates/upload, if authority is supplied enforce AuthorityPermission for that authority, and reject/ignore caller-supplied external_id.
2. Before calling plugin.revoke_certificate, look up all Certificate rows sharing the same (authority_id, serial) or body and require CertificatePermission on every match (and run the endpoint-attached check against all matches).
3. Consider a DB uniqueness constraint or dedup on (authority_id, serial) so a second row for the same CA-issued certificate cannot be created.
4. Stop exposing external_id in CertificateOutputSchema to non-owners.