Resolving certmonger SCEP CA_UNREACHABLE errors on Ubuntu 22.04

by

Summary

During a routine security hardening task, an automated certificate enrollment process using certmonger on Ubuntu 22.04 failed despite successful interaction with the FortiAuthenticator (FAC). While the administrator could manually approve the SCEP (Simple Certificate Enrollment Protocol) request within the FortiAuthenticator GUI, the client-side utility returned a CA_UNREACHABLE status. The failure manifests as low-level ASN.1 encoding errors during the signature verification phase of the SCEP response.

Root Cause

The failure is not a network connectivity issue, despite the CA_UNREACHABLE error string. Instead, it is a cryptographic payload mismatch caused by malformed or unexpected ASN.1 structures in the SCEP response.

  • ASN.1 Parsing Error: The error error:0688010A:asn1 encoding routines::nested asn1 error indicates that the OpenSSL library used by scep-submit encountered data that violates the expected structure of a PKCS#7 or CMS (Cryptographic Message Syntax) container.
  • Payload Mismatch: When FortiAuthenticator signs the SCEP response, it wraps the certificate in a signed structure. If the FAC version (v8.0) produces a specific encoding variant (e.g., unexpected DER encoding or a non-standard padding) that the local Ubuntu OpenSSL implementation does not expect, the parser fails.
  • Response Corruption: The specific error too long and bad object header suggests that the bytes returned by the server are being misinterpreted as a different length or type, often due to extra whitespace, incorrect line endings, or improperly wrapped PKCS#7 envelopes being sent over the wire.

Why This Happens in Real Systems

In distributed security architectures, “Protocol Implementation Divergence” is a common phenomenon.

  • Library Discrepancies: Different vendors implement RFCs (Request for Comments) with slight variations. A SCEP implementation might strictly follow the RFC, while another might include extra metadata in the response envelope.
  • Strictness of Modern Tooling: Newer versions of OpenSSL (shipped with Ubuntu 22.04) are significantly stricter regarding ASN.1 validation than older versions. What was considered “sloppy but acceptable” in older environments is now treated as a critical parsing error.
  • Middlebox Interference: Load balancers, WAFs, or transparent proxies between the client and the CA can inadvertently alter the payload (e.g., stripping headers or modifying line endings), breaking the binary integrity of the signature.

Real-World Impact

  • Automation Breakdown: Security teams cannot rely on Zero Touch Provisioning (ZTP), forcing manual certificate management which is prone to human error.
  • Service Outages: If certificates expire and the automated renewal mechanism (certmonger) fails due to this parsing error, services will go offline unexpectedly.
  • Operational Overhead: Highly skilled engineers are pulled away from feature work to debug low-level cryptographic byte-streams.

Example or Code (if necessary and relevant)

To diagnose the exact structure of the failing response, engineers should capture the raw HTTP response using tcpdump or curl to see if the payload is indeed valid DER/ASN.1.

# Capture the SCEP response to a file for manual inspection
curl -v --request GET "http:///scep_endpoint" \
     --data "operation=GetCACert" \
     --output scep_response.bin

# Attempt to parse the captured response using OpenSSL to check for encoding errors
openssl asn1parse -inform DER -in scep_response.bin

How Senior Engineers Fix It

A senior engineer moves past the “connection” error and looks at the data integrity.

  • Payload Inspection: Use asn1parse to identify exactly which offset in the byte stream causes the nested asn1 error.
  • Compatibility Shims: If the CA is sending non-standard encoding, the fix might involve using a custom script or a different SCEP client that is less pedantic about the ASN.1 structure.
  • Vendor Escalation: Provide the raw hex dump of the failing response to Fortinet support to demonstrate that the FAC is producing an invalid PKCS#7 structure.
  • MTU/Fragmentation Check: Verify that the larger signed response isn’t being fragmented or truncated by network MTU settings, which would lead to “unexpected end of file” or “too long” errors.

Why Juniors Miss It

  • Surface-Level Interpretation: Juniors often see CA_UNREACHABLE and immediately begin troubleshooting firewalls, routing, and ICMP, ignoring the actual cryptographic error provided in the log.
  • Ignoring Log Depth: They may see the certmonger error but fail to look at the underlying scep-submit or openssl error codes, which contain the actual truth about the failure.
  • Assumption of “Broken Hardware”: There is a tendency to assume the network or the server is “down” rather than realizing the mathematical representation of the data is what’s broken.

Leave a Comment