Understanding Certificate Chains in Palo Alto Networks Firewalls

Overview

In public key infrastructure (PKI), trust is not inherent; it must be established. Certificate chains are the mechanism used in SSL/TLS and other protocols to establish trust in an end-entity's certificate (like a web server's SSL certificate or a user's client certificate). Palo Alto Networks firewalls rely heavily on certificate chain validation for features like SSL Decryption, GlobalProtect, IPSec VPNs using certificates, administrative access, and more.

Understanding how the firewall builds and validates these chains is crucial for configuring these features correctly and troubleshooting related issues.

     A certificate chain links an end-entity certificate back to a trusted root Certificate Authority (CA) through a series of intermediate CA certificates. Each certificate in the chain vouches for the authenticity of the previous one.
    

Chain Concepts: What is a Certificate Chain?

A certificate chain (also known as a chain of trust or certification path) is an ordered sequence of digital certificates:

End-Entity Certificate (Leaf Certificate): This is the certificate being presented for validation (e.g., the web server's certificate, a user's client certificate). It identifies the subject (server FQDN, username). It is signed by an Intermediate CA or sometimes directly by a Root CA.
Intermediate CA Certificate(s): One or more certificates that act as links between the end-entity certificate and the Root CA. Each Intermediate CA certificate is signed by the *next* certificate higher up in the chain (either another Intermediate CA or the Root CA). They delegate signing authority from the Root CA.
Root CA Certificate: The top-level certificate in the chain. It is self-signed (meaning it signs itself) and acts as the ultimate trust anchor. The Root CA certificate must be explicitly trusted by the validating entity (in this case, the Palo Alto Networks firewall).

The validation process works by verifying the signature of each certificate in the chain using the public key contained in the *next* certificate up the chain, until the trusted Root CA is reached.

     Understand the components: End-Entity (Leaf), Intermediate(s), and Root CA. Know that each certificate signs the one below it in the chain, establishing trust back to the Root.
    

Visual representation of a typical certificate chain structure.

Chain Concepts: How Certificate Chains Work (Validation Process)

When a Palo Alto Networks firewall receives a certificate during a TLS handshake or for authentication (e.g., from a GlobalProtect client), it performs several validation steps to establish trust:

Chain Building: The firewall receives the end-entity certificate and typically receives the intermediate certificates from the presenting entity (e.g., the web server usually sends its cert plus intermediates). The firewall attempts to build a complete path from the end-entity certificate up to a Root CA certificate that exists in its own trusted CA certificate store.
Signature Verification: For each certificate in the constructed chain (starting from the end-entity cert), the firewall verifies its digital signature using the public key contained in the *issuer's* certificate (the next one up the chain). This process repeats until the Root CA is reached. The Root CA's signature is verified using its own public key (as it's self-signed).
Trust Anchor Check: The Root CA certificate at the top of the validated chain must be present in the firewall's list of trusted Root CAs (either the default list or manually imported trusted CAs).
Validity Period Check: The firewall checks the "Not Before" and "Not After" dates for *every* certificate in the chain (end-entity, intermediates, root) to ensure none have expired or are not yet valid.
Revocation Check: If configured in the applicable Certificate Profile, the firewall checks if any certificate in the chain (usually the end-entity and potentially intermediates) has been revoked using CRL or OCSP.
Other Checks: Additional checks like Key Usage, Basic Constraints (verifying CAs are marked as CAs), and Name Constraints might also be performed.

     For successful validation, the presenting entity (e.g., web server, client) MUST provide not only its own certificate but also all necessary Intermediate CA certificates required to link back to a Root CA trusted by the firewall. A missing intermediate certificate is a common cause of validation failure.
    

     Understand the steps: Chain Building -> Signature Verification (up the chain) -> Trust Anchor Check (Root CA trusted?) -> Validity Period Check -> Revocation Check. Know the critical importance of the server/client providing the complete chain including intermediates.
    

Simplified sequence of TLS handshake focusing on certificate chain validation by the client (firewall).

Trust Management: Default Trusted Certificate Authorities

Palo Alto Networks firewalls ship with a pre-installed list of commonly trusted public Root CA certificates. This list forms the default trust basis for validating certificates presented by external entities, such as public web servers during SSL/TLS decryption (Forward Proxy) or external VPN peers.

Managing Default Trusted CAs:

Location: Device > Certificate Management > Certificates > Default Trusted Certificate Authorities .
Actions:
- View: Browse the list of included Root CAs.
- Enable/Disable: While generally not recommended unless you have a specific reason, you can technically disable trust for a specific default CA.
- Export: Export a default CA certificate if needed elsewhere.
Updates: This default trust list is updated periodically via PAN-OS content updates. Keeping content current helps maintain trust in newly established public CAs and potentially removes distrusted ones.

     The default trusted CA list simplifies validation for common public websites and services, as administrators don't typically need to manually import certificates for major public CAs like DigiCert, Let's Encrypt, GoDaddy, etc.
    

     The default list contains
     
      Root CAs only
     
     . It does *not* contain intermediate CA certificates. The firewall relies on the server presenting its certificate to also provide the necessary intermediate certificates in the chain.

     Know where to view the Default Trusted CAs list. Understand that it contains Root CAs and is updated via content updates. Recognize its importance for validating external server certificates (like in SSL Forward Proxy decryption).
    

Trust Management: Adding a Certificate That Is Not Trusted

If the firewall encounters a certificate issued by a CA that is *not* in its default trusted list (e.g., an internal enterprise CA, a less common public CA, or a self-signed certificate), validation will fail unless trust is manually established.

Process for Adding Trust:

Obtain the CA Certificate(s): You need the certificate file(s) for the Root CA and any Intermediate CAs in the chain that issued the end-entity certificate. These are usually obtained from the CA administrator or downloaded from the CA's website. Common formats are PEM (.crt, .cer, .pem) or DER (.der, .cer).
Import into Firewall Certificate Store:
- Navigate to Device > Certificate Management > Certificates .
- Click Import .
- Provide a unique Certificate Name .
- Browse to and select the CA certificate file.
- Select the correct File Format (PEM or DER).
- Crucially, check the box labeled "Certificate Authority (CA)". This marks the imported certificate as a potential CA.
- Click OK .
- Repeat the import process for all required Intermediate CAs and the Root CA in the chain.
Forgetting to check the "Certificate Authority (CA)" box during import is a common mistake. If unchecked, the firewall will treat it as an end-entity certificate and won't be able to use it to validate other certificates.
Add CA to Certificate Profile (or rely on global trust):
- For specific use cases (Recommended): Edit the relevant Certificate Profile ( Device > Certificate Management > Certificate Profile ) used by the feature (e.g., GlobalProtect, IPSec) and add the newly imported Root CA (and potentially intermediates if needed for specific checks) to the profile's 'CA Certificates' list. This limits trust for that specific CA to only the features using that profile.
- For global trust (Use with caution): While possible to mark an imported CA as globally trusted (similar to the default list), it's generally better practice to manage trust granularly via Certificate Profiles unless the CA needs to be trusted for nearly all functions (like an internal enterprise Root CA often is).
Commit Changes.

     Know the steps to import an external CA certificate and the importance of checking the "Certificate Authority (CA)" box during import. Understand that imported CAs need to be added to relevant Certificate Profiles to be used for validation by specific features.
    

Troubleshooting: Common Causes of Certificate Validation Failure

When certificate validation fails, connections are typically dropped or rejected. Understanding common causes helps in diagnosing the issue:

Missing Intermediate Certificates: (Very Common) The server or client presenting its certificate fails to include the necessary Intermediate CA certificate(s) needed to link its certificate back to a Root CA trusted by the firewall. The firewall cannot complete the chain.
Untrusted Root CA: The Root CA at the top of the presented chain is not present in the firewall's trusted CA list (either default or manually added).
Expired Certificate: Any certificate in the chain (end-entity, intermediate, or root) has passed its "Not After" validity date. Firewall system time must be accurate (use NTP).
Certificate Not Yet Valid: Any certificate in the chain has a "Not Before" date that is in the future relative to the firewall's system time.
Revoked Certificate: A certificate in the chain has been revoked by its issuing CA, and the firewall successfully confirmed this via OCSP or CRL check (requires revocation checking to be enabled in the Certificate Profile).
Revocation Check Unavailable / Unknown Status: The firewall cannot reach the CRL DP or OCSP Responder, or the check times out, AND the Certificate Profile is configured to block sessions when status is unknown.
Incorrect Certificate Order: While less common with modern systems, sometimes certificates in the chain are presented in the wrong order, confusing the validation process.
Signature Verification Failure: The digital signature of a certificate in the chain does not validate correctly using the public key of its stated issuer, indicating potential tampering or corruption.
Name Mismatch: For server certificates (e.g., in SSL Decryption), the hostname the client is trying to reach does not match the Common Name (CN) or any Subject Alternative Name (SAN) listed in the server's certificate.

     Missing intermediate certificates and untrusted root CAs are among the most frequent causes of validation failures. Expired certificates and failed revocation checks are also common.
    

Troubleshooting: Tips and Techniques

When facing certificate validation errors:

Check Firewall Logs:
- System Log: Look for PKI-related errors, especially around certificate expiration or loading issues.
- Decryption Log (for SSL Decryption): Provides specific error messages related to certificate validation failures during decryption (e.g., "Untrusted issuer", "Unable to get local issuer certificate", "Certificate has expired", "Certificate revoked").
- GlobalProtect Logs (System Log / GP Logs): Check for certificate-related errors during client connection attempts.
- IPSec System Logs: Look for errors related to peer certificate validation in Site-to-Site VPNs.
- PKI Debug Log (CLI): Use `debug pki validation enable yes level 9` and `tail follow yes mp-log pki.log` for highly detailed, real-time validation steps and errors. (Use cautiously, very verbose).
Verify the Chain Externally:
- Use online SSL checker tools (for public web servers) to view the chain presented by the server and check for completeness and trust issues against public CAs.
- Use OpenSSL commands from a separate machine to connect and view the presented chain:
  openssl s_client -connect <server_ip_or_fqdn>:<port> -showcerts
Check Server/Client Configuration: Ensure the server or client system is configured to send the complete certificate chain , including all necessary intermediate certificates. This is a common server-side misconfiguration.
Verify Firewall Trust Store:
- Confirm the Root CA certificate for the chain exists under Device > Certificate Management > Certificates .
- Ensure the Root CA is marked as a "Certificate Authority (CA)".
- Verify the Root CA is included in the correct Certificate Profile's 'CA Certificates' list if not relying on default trust.
- Import any necessary Intermediate CA certificates and ensure they are also marked as "Certificate Authority (CA)". While intermediates don't strictly *need* to be in the profile's CA list (trust flows from the root), importing them can sometimes help the firewall build the chain if the server fails to send them.
Check Certificate Validity: Manually inspect the validity dates ("Not Before", "Not After") of all certificates in the chain (End-entity, Intermediates, Root). Ensure the firewall's system time is accurate ( Device > Setup > Services > NTP ).
Test Revocation Connectivity: Use firewall CLI commands to test connectivity to CRL/OCSP URLs identified from the certificates:
test certificate-status crl url <url>

test certificate-status ocsp url <url> certificate <name_of_cert_on_fw>

Also use basic `ping` and `traceroute` (potentially specifying source interface/IP). Check firewall policies allow traffic to these destinations.

Diagrams: Certificate Chain Validation

Diagram: Certificate Chain Structure

Visual representation of a typical certificate chain structure.

Diagram: TLS Handshake & Chain Validation Sequence

Sequence diagram showing TLS certificate exchange and the validation steps performed by the firewall.

Flowchart: Certificate Validation Decision

Decision flowchart for the firewall's certificate chain validation process.

State Diagram: Certificate Trust/Validity State

Simplified state diagram showing possible trust and validity states for a certificate from the firewall's perspective.

PCNSE Exam Focus Points

Key concepts related to Certificate Chains for the PCNSE exam:

Chain Structure: Understand the hierarchy: End-Entity (Leaf) -> Intermediate CA(s) -> Root CA. Know that each signs the one below it.
Trust Anchor: Recognize that the validation process must chain back to a Root CA certificate that is explicitly trusted by the firewall (either default or manually imported).
Intermediate Certificates: Understand their role in linking the end-entity cert to the root. Know that the server/client presenting the certificate is typically responsible for sending the necessary intermediate certificates along with the end-entity certificate. A common failure is when intermediates are missing.
Validation Steps: Be familiar with the sequence: Signature Checks -> Trust Anchor Check -> Validity Period Check -> Revocation Check (if enabled).
Default Trusted CAs: Know where to find this list ( Device > Certificate Management > Certificates ) and that it contains common public Root CAs, updated via content updates, but generally not intermediates.
Importing CAs: Understand the process of importing Root/Intermediate CAs and the critical importance of checking the "Certificate Authority (CA)" box during import.
Certificate Profiles: Know that Certificate Profiles are where you reference the trusted CAs and configure revocation checking (CRL/OCSP) settings for specific features (like GlobalProtect, IPSec VPNs).
Revocation Checking: Understand the basics of CRL (list) vs. OCSP (real-time query) and the firewall's check order (OCSP first, then CRL fallback). Know the importance of the "Block on Unknown" setting.
Troubleshooting: Recognize common failure reasons (missing intermediates, untrusted root, expired certs, revocation failures). Know that decryption logs are useful for SSL/TLS issues, and general system/feature logs (GP, IPSec) may indicate certificate problems. Basic connectivity tests (ping, traceroute) to CDP/OCSP URLs are relevant.

     Focus on the concept of the trust chain, the role of intermediate certificates (and who provides them), the validation steps, the default trust store, and how Certificate Profiles link trust + revocation checking to features. Troubleshooting common failures is also key.
    

Certificate Chains Knowledge Check (PCNSE Style)

Test your understanding of certificate chains and validation.

1. What is the primary purpose of a certificate chain?

To encrypt the communication between client and server. To establish a path of trust from an end-entity certificate back to a trusted root CA. To store the private key associated with the end-entity certificate. To check if a certificate has been revoked using OCSP.

2. Which component in a certificate chain is typically self-signed?

End-Entity (Leaf) Certificate Intermediate CA Certificate Root CA Certificate OCSP Responder Certificate

3. Where can you view the list of default public Root CAs trusted by the Palo Alto Networks firewall?

Objects > Security Profiles > Decryption Device > Authentication Profile Network > Network Profiles > SSL/TLS Service Profile Device > Certificate Management > Certificates > Default Trusted Certificate Authorities

4. What is typically responsible for providing the necessary Intermediate CA certificates during an SSL/TLS handshake?

The server presenting its end-entity certificate. The client initiating the connection. The Root CA queried via OCSP. The Palo Alto Networks firewall's default trust store.

5. If the firewall needs to validate a certificate issued by an internal Enterprise CA, what must an administrator typically do first?

Enable OCSP checking in the default Certificate Profile. Import the Enterprise Root CA (and any relevant Intermediate CAs) into the firewall and mark them as trusted CAs. Add the Enterprise CA's IP address to the firewall's management settings. Generate a new self-signed certificate on the firewall.

6. Which firewall component defines how revocation checking (CRL/OCSP) is performed for certificate validation?

SSL/TLS Service Profile Authentication Profile Certificate Profile Decryption Profile

7. What is a common reason for a "certificate issuer is not trusted" error during validation?

The Root CA or a necessary Intermediate CA certificate is missing from the firewall's trust store or the relevant Certificate Profile. The certificate has been listed on a CRL. The firewall cannot reach the OCSP responder. The certificate's validity period has expired.

8. What potential problem can occur if a server sends its certificate chain in the wrong order?

Increased latency during the handshake. The certificate will be incorrectly marked as revoked. The firewall will trust the certificate without validation. The firewall might fail to build the correct trust path, leading to validation failure.

9. When importing a CA certificate (Root or Intermediate) that will be used to validate other certificates, which checkbox is critical to enable during the import process?

Sign Certificates Certificate Authority (CA) Private Key Encrypted Use as GlobalProtect Server Certificate

10. Which tool or method is most suitable for checking the certificate chain actually being presented by a public web server?

Firewall Traffic Logs Firewall System Logs Online SSL checker tools or OpenSSL command (`s_client -showcerts`) Firewall Certificate Profile configuration page

11. The firewall's list of Default Trusted Certificate Authorities typically contains:

Well-known public Root CA certificates only. Root CAs and common Intermediate CAs. Only certificates generated by the firewall itself. All certificates currently stored on the firewall.

12. If a Certificate Profile has both OCSP and CRL enabled, but the OCSP check times out, what will the firewall do next?

Immediately block the connection. Mark the certificate as valid and allow the connection. Retry the OCSP check indefinitely. Attempt to download and check the Certificate Revocation List (CRL).

13. Which firewall log is most likely to contain specific error messages about certificate validation failures during SSL decryption?

System Log Decryption Log Threat Log Traffic Log

14. What component links a trusted CA list and revocation settings to a feature like GlobalProtect or IPSec VPN?

The Root CA certificate itself. An SSL/TLS Service Profile. A Certificate Profile. An Authentication Profile.

15. A website's certificate chain is: Server Cert (Signed by Intermediate A) -> Intermediate A Cert (Signed by Root B). The firewall trusts Root B but does not have Intermediate A imported. The web server only sends the Server Cert during the TLS handshake. What will happen?

Validation will likely fail because the firewall cannot link the Server Cert to the trusted Root B without Intermediate A. Validation will succeed because the firewall trusts Root B. The firewall will automatically download Intermediate A from a public repository. The firewall will use OCSP to bypass the need for Intermediate A.

16. Checking the validity period applies to which certificates in the chain during validation?

Only the End-Entity certificate. Only the Root CA certificate. Only the End-Entity and the Root CA certificates. All certificates in the chain (End-Entity, Intermediates, and Root).

17. How are the Default Trusted Certificate Authorities on the firewall updated?

By manually importing updates from the support portal. Via PAN-OS dynamic content updates (like Applications and Threats). Only when upgrading the PAN-OS software version. Through synchronization with Panorama.

18. If a firewall cannot connect to the CRL Distribution Point (CDP) specified in a certificate, what is a likely consequence?

The firewall will automatically trust the certificate. The firewall will attempt an OCSP check instead, even if OCSP is disabled. The CRL check will fail, potentially leading to validation failure if "Block on Unknown" is enabled or if OCSP also fails. The firewall will generate a self-signed CRL.

19. Which certificate feature is most commonly used to carry the User Principal Name (UPN) for user identification?

Subject Alternative Name (SAN) Subject Common Name (CN) Basic Constraints Key Usage

20. You are troubleshooting an SSL Forward Proxy decryption failure with an "Untrusted issuer" error for a specific website. The website's chain appears valid in your browser. What is a likely cause on the firewall?

The website's certificate has expired. The firewall's Forward Trust certificate is incorrect. The firewall's Forward Untrust certificate is missing. The Root CA or an Intermediate CA for the website's certificate is not in the firewall's default or manually added trusted CA list.