Lost access to your Root CA in your 2-Tier PKI? Don’t worry, Use Cross Signing to Recover!
Hello, this is Byron from the Microsoft Directory Services Support team. Today, I’d like to share information about an alternative recovery approach for Public Key Infrastructure (PKI) environments. Consider a scenario where the Root Certification Authority (CA) is permanently lost—for example, due to accidental deletion of the Root CA virtual machine, or the system entering an unrecoverable no-boot state without a valid backup. In some cases, there may be no backup of the Root CA’s private key or database, leaving no traditional path to restoration. In such cases, the supported and recommended recovery method is to rebuild the PKI hierarchy from scratch. However, for extremely rare and critical situations where this is not feasible, there is an alternate recovery method that may allow partial restoration and continuity. This approach is intended only as a last resort, and should only be considered when no supported recovery options remain. While not a replacement for proper backup and disaster recovery practices, this fallback method may help reduce downtime and effort in extreme cases. How If it is a two-tier PKI, and the Intermediate CA Server is the one issuing certificates to the environment, and we still have access to the Intermediate CA Certificate with private key, we can build another Root CA, and “link” the Intermediate CA Server to the new Root CA. The steps are simple, just renew the Intermediate CA server with “Same Key pair” with this new Root CA. A high-level diagram below: In fact, this is like the Concept of “Cross Signing”; the newly issued Intermediate CA Certificate could be considered as a “Cross Certificate” as well. Cross Signing is typically used to make one PKI hierarchy chains up through another hierarchy, even when originally it was not configured that way. This technique is designed for a leaf certificate to build a different certificate verification chain, in case one of the chains fails, allowing certification verification to succeed. Cross signing is also used by public CA companies to start their business which we will discuss more in a later section. Here are detailed steps on how to do this: On the Intermediate CA Server. Open the Certification Authority Management Console. Right click on the CA Name node -> All Tasks -> Renew CA Certificate. Renew Intermediate CA server with “Same Key pair” to create the Certificate renewal request file: Submit the certificate request file (.req) on the newly Deployed Root CA Server, issue it, and go to: Issued Certificates node-> Right click on issued Certificate -> All Taks -> Export Binary Data… -> Save as .cer file. Copy this .cer file to the Intermediate CA. On the Intermediate CA Server. Install the newly issued Intermediate CA certificate issued by the new Root CA. The existing issued Leaf Certificates verification should continue to work and chain up to the new Root CA. We can use command Certutil -urlfetch -verify c:\certificate.cer > certificate.txt to export the certificate verification and chain build information. Previous Chain: Leaf certificate: 5700000006d907d38be599e05a000000000006 Issuer: CN=ContosoRootCA01,DC=contoso,DC=com (Previous corrupted Root CA) Current Chain: Leaf certificate: 5700000006d907d38be599e05a000000000006 Issuer: CN=ContosoRootCA02,DC=contoso,DC=com (New built Root CA) How and Why So exactly what is “Cross Signing”? Let’s see the following story (I found this story online): A long time ago (maybe not that long 😊), there was a Certification Authority company called LetsEncrypt. When they started their business, they generated their own 'ISRG' Root CA Certificate (ISRG Root X1). However, it takes time for the industry to accept new Root Certification Authorities. They could not wait that long to start their business, as this might have taken years. They deployed an Intermediate CAs first (LetsEncrypt X3 and LetsEncrypt X4) and used a popular public Certification Authority (DST Root X3) to sign those Intermediate CA Certificates. This allowed them to start their business immediately and issue Leaf Certificates to customers without waiting for the world to accept their own Root CA. So, the Certificate Chain back then was DST Root X3 --> LetsEncrypt X3 / LetsEncrypt X4 --> Leaf Certificate. After 5 years of hard work, the market accepted their Root CA (DST Root X3) and added it to all kinds of products’ Trusted Root. They signed their Intermediate CA certificates with their own Root CA. Now, another chain is available: ISRG Root X1 --> LetsEncrypt X3 / LetsEncrypt X4 --> Leaf Certificate. How come the same Intermediate CA can be chained up to different Root CAs? The magic trick is the Intermediate CA Certificate kept the “Same Asymmetric Key Pair” when it got signed by both DST Root X3 and ISRG Root X1. As you can see from the above screenshots, the issuer is different, and they are two different intermediate CA certificates, but the trick here is the Same Key Pair, even though they were signed by different Root CA’s, aka “Cross Signing”. Another key point here is not all Certificate Chains rely on the AIA path. Another common Certificate chain build method is Key match using AKI/SKI and PKCS#7 which means the server side sends both Leaf Certificate along with Intermediate CA to the client for verification, Client does not need to build chain to Intermediate using AIA. You can refer to this document for more information about this story: https://scotthelme.co.uk/cross-signing-alternate-trust-paths-how-they-work/ There is another “Cross Sign” method targeting the Root CA Certificate itself. The concept is a little bit different: Using one Root CA Certificate to sign another Root CA Certificate. What is the usage scenario? Why do we need this? Scenario 1 Here is a story: In a large corporation, deploying Root CA certificate to all devices might be challenging. It could be time-consuming and might take a lot of administrative effort. Therefore, during the time periods when large corporations renew their Root CAs, they must find a way for newly issued Certificates under the renewed Root CA start to work as soon as possible. The idea is to sign the new Root CA certificate using the old Root CA Certificate, so the chain could be: leaf Certificate --> Intermediate CA -> New Root CA (Signed by Old Root CA)--> Old Root CA. Because devices should trust Old Root CA already, the new Leaf Certificate works immediately after renewal without waiting for new Root CA Deployed to all devices. Scenario 2 Another scenario is once the New Root CA is deployed to the environment, the company wants to remove the old Root CA certificate from the Devices’ trusted Store for company policy reasons. How can the existing Certificate continue to work if they are not yet expired and issued by old Root CA Certificate? The solution is also Cross Certificate: Create a CA Certificate with old CA key pair but signed by the new Root CA Certificate. The chain build would be below: Existing Issued Leaf Certificate --> Intermediate CA --> Old CA Certificate (cross signed by new Root CA) --> New Root CA. In fact, Active Directory Certificate Service supports this and will generate Cross Certificates by default when renewing a Root CA with a new key. Remember that when we renew the Root CA, there will be two additional CRT files called XXXXX (0-1).crt and XXXXX(1-0).crt. These certificates(.crt) are Cross CA Certificate’s. (0-1) is the New Root CA Certificate signed by Old Root CA Certificate. (1-0) is the old Root CA Certificate signed by new Root CA Certificate. They are used for the above Scenario 1 and Scenario 2 As you can see, the Root CA Certificate has an AKI (Authority Key Identifier), which means it was signed by a CA and an SKI (Subject Key Identifier) that matches it. Of course, in Active Directory, we rarely see the deployment of the Cross Certificate, because for Windows devices, the Active Directory is sufficient to quickly deploy new Root CA Certificate to Windows domain members. Summary The common method to perform PKI disaster recovery when the CA private key and database cannot be restored involves rebuilding the entire PKI Hierarchy PKI from scratch, and replacing every single certificate used by applications and servers. While the common method mentioned above is still valid, the “Cross Signing” method illustrated in this blog offers an alternative quick method to recover our PKI Hierarchy. This could potentially save us from spending a lot of disaster recovery time & administrative effort 😊.
First Issuance manual, with automated renewals
Hey all Rob Greene again. Seems like I have been on this PKI kick lately, and today is not going to be any different. Occasionally, I will get a customer who must get certificates issued for things like Web sites, and they must have custom Subject Alternative Name (SAN) DNS values on the issued certificate. They hate that their web server admins must submit the request, and then as Certificate Authority Managers, they must look in the Certification Authorities database and review the Pending Requests table. Some background The first question that they ask is if there is a way to not require them (the CA Manager) to approve the request as this slows down the process of getting the certificate issued. From a technical perspective most of you would know that the answer is YES, you can configure the certificate template to automatically issue. However, this is NOT a good thing to do, and is seen as a Microsoft CA vulnerability; if you have a CA audit happen you might fail the audit because you allowed this. This specific vulnerability is considered the ESC1 vulnerability. ESC1 wants either an Enrollment Agent to cross sign the certificate request, a CA Manager to review the request, or both to validate the information in the Subject Alternative Name (SAN), when the certificate template’s Subject Name tab is set to “Supply in the request”, before allowing the certificate to be issued. This is recommended since the submitter is allowed to enter unvalidated information in the SAN, unlike when the certificate templates Subject Name tab is configured for “Build from this Active Directory information”. How to protect the template against ESC1? Given this background information, you should already be thinking to yourself, “Rob is not going to tell me to break anything security wise.”, and you would be correct. Rather, I am going to tell you that any certificate template that you have configured for “Supply in the request”, should at a minimum be configured to require CA Manager approval. Then for something like the certificate template that Network Device Enrollment Services (NDES) uses for its certificate, I would advise you to use the Enrollment Agent configuration. The Exchange Enrollment Agent (Offline request) template has the Enhanced Key Usage (EKU) of “Certificate Request Agent”, and this is the certificate NDES uses to sign the Certificate Service Request (CSR) that the SCEP client sends to the NDES Server. Of course, I am just giving some good examples of real-world certificate templates that you are going to be using within the environment. The last part is that the security permissions SHOULD be locked down as to who can enroll for each of these certificate templates. Specifically, enrollment for the two Certificate templates needed by the NDES services, it should be the CA Manager NDES Admin and the NDES Server. The Exchange Enrollment Agent (Offline request) and CEP Encryption certificate templates should be locked down as to who can request enrollments. Just like the custom Adatam Web Server certificate template, only IIS (Internet Information Services) Admins, and IIS Web Servers should have access to this template to be able to enroll with it. So what can you do for me here? Let’s first discuss the scenario that I am going to cover in the blog. Although I would love to cover all the possible uses that all you lovely internetz users might have for certificate enrollment, this blog will have to end at some point. The Scenario IIS Admins are tired of remembering to go to all their web servers and request new certificates for them before they expire. The CA Manager is also tired of the IIS Admins always sending emails nagging that they need their certificate enrollment requests issued. Then always having to go to the Certification Authority snap-in to approve/issue the same certificates over and over each year because of rules that web server certificates can only be valid for 398 days. See (https://thehackernews.com/2020/09/ssl-tls-certificate-validity-398.html) What is the solution? Side bar: If you want to modify the settings of any default certificate template, it is best to duplicate the original certificate template and then make changes to it. This is recommended so that you always have the default templates available in case you must create a new template in the future that is based off the original template. If the original template is modified two to three years down the line, you may not remember what was changed so that it could be changed back to the original template configuration. As far as naming goes, we would recommend that you put your company name or initials in the template name and keep the original template name after that. In this blog you will see the template used is called Adatam Web Server. This is a customized template based off the Web Server template. Keep in mind that this can be done for any certificate template that has the Subject Name tab configured for “Supply in the request”, this not limited to just web server certificates. I am in, so what’s next As you can imagine, there are several things that need to be configured to get this working: A Security group needs to be created for Users who are going to request the certificate via the Local Computer Certificate enrollment wizard based on the certificate template. A security group needs to be created for the computer accounts that need the certificate issued based on the security template. Certificate template needs to be created. Certificate template changes need to be made to support this enrollment method. Certificate template Permissions need to be configured. Group Policy setting for Computer Certificate Autoenrollment Security groups need to be created. The first two steps should be self-explanatory as to how to create groups and set up the group scopes. We are not going to cover group creation in this blog. I do want to point out one thing that happens often: Most Administrators seem to remember that if you add a user to a group, the user needs to log off and back on before the group can be added to their tokens. When adding a computer account to a new group it also needs to log off and back on before the group membership can be added to its token. Typically, the log off and back on for a computer will be a computer reboot; that needs to happen after the new group membership is added to its token. Certificate Template creation and changes The base template that you use to duplicate can of course be anything you would like, but in our example, we are just going to duplicate the good’ol faithful Web Server certificate template. Launch the Certificate Template snapin: CertTmpl.msc Find the certificate template named: Web Server Right click on the Web Server template and select Duplicate Template. The Compatibility tab, set the values as you wish. NOTE: Stay away from using anything higher than Windows Server 2012 R2/Windows 8.1 for your templates if you are using CEP and CES as these two web services have problems seeing templates with compatibility higher than this. The General tab, type in a descriptive name for the new certificate name you are configuring. The Request Handling tab, set the values that make sense for your template. NOTE: It might make sense to set the checkbox “Renew with the same key”. The Cryptography tab, you may want to modify the Minimum key size value and select the Provider Category and Providers that makes sense for the certificate templates use. NOTE: I would strongly suggest NOT setting Use alternate signature format on the template either. This is a proprietary format that many applications will not understand. This check box will only be available when you are using a Key Storage Provider (KSP) provider type. The Issuance Requirements tab is where the heavy lifting is happening. Check the box “CA certificate manager approval”. Select the radio option “Valid existing certificate” under the Require the following for reenrollment section toward the bottom of the dialog box. The Security tab is where the security groups that were created for the server admins and the servers themselves need to be configured on the template. You would add both security groups, and they only need the Allow Enroll permissions. Neither group should have the Autoenroll permission defined on it. When done setting the other options for the template, click the OK button. Setup the computer certificate autoenrollment group policy Next, we are not going to cover how to create a Group Policy Object, and things like GPO ordering, etc. We are going to discuss just the policy settings that are important for the discussion of this blog. Launch Group Policy Management Console: GPMC.msc Either create a new Group Policy or modify an existing one. Navigate to: Computer Configuration\Policies\Windows Settings\Security Settings\Local Policies\Public Key Policies. Double click on the policy setting “Certificate Services Client - Auto-Enrollment”. Enable the policy, and check the two boxes: Renew expired certificates, update pending certificates, and remove revoked certificates. Update certificates that use certificate templates. NOTE: To learn more about these two settings I would recommend going over to Vadims blog. He does a great job of explaining what each of these do: https://www.sysadmins.lv/blog-en/certificate-autoenrollment-in-windows-server-2016-part-3.aspx Click the OK button on the dialog box. Close the Group Policy Management Editor. Link the Group Policy object to what seems appropriate within your Active Directory structure. Trying to test this feature, but it seems like the renewal does not work. Of course, with any of these things, you should always test things before relying on the solution in a production environment. I do want to caution you about setting up a test environment and then setting up the certificate for a short issuance period as this tends not to work out well for most customers, and they eventually end up calling stating that autoenrollment is not working. There are few things to keep in mind about certificate automatic enrollment / automatic renewal process. The first thing to remember is that certificate auto enrollment / renewal only happens on the following triggers: At User logon, or computer boot, for the corresponding security context. Every 8 hours after user logon, or computer boot. This means that after the user or computer has been logged in or turned on for 8 hours then the auto enrollment code will happen again for the security context in question. To trigger auto-enrollment to run manually you have two options. Run GPUpdate /Force. Part of the function that this does is it causes an autoenrollment cycle to happen. This can be a bit much especially if you are applying a lot of user and computer group policy. This will trigger both computer and user autoenrollment to run. Use one of the two CertUtil command lines based on which security context you want enrollment to run against. For computer: CertUtil -Pulse For User: CertUtil -User -Pulse Certificate renewal WILL NOT happen until 90% of the Certificate lifetime has expired. Trying to use the template setting “Renewal period” does not change this fact. Auto enrollment / renewal attempt MUST happen while the certificate is still valid, and after the start of the 10% certificate lifetime left and the expiration time of the certificate. (MUST NOT BE EXPIRED). Like being unable to re-animate something that is already dead, you cannot renew a certificate that has already expired. The math that needs to be done is: (CertLifeTimeInDays * 24) * .10 The value from the above MUST be larger than 8 hours. Let's do some math Here is an example: Set up a certificate that is valid for two days for testing and we will run through the exercise here: Find out how many hours the certificate is valid for: 2*24 = 48 Hrs. Figure out what 10% of certificate lifetime in hours is: 48 *.10 = 4.8 Hrs. This shows us that for one of these certificates to be renewed, the process that runs only every 8 hours would have to run within this 4.8 Hrs. window. This is going to tell us that sometimes you would get lucky with a certificate renewal attempt being done, but more than likely MOST of the time you would fail to renew the certificate because it would expire before the auto enrollment cycle happens. Another example is a certificate that is valid for 4 days. Let us run through the exercise here. Find out how many hours the certificate is valid for: 4*24 = 96 Hrs. Figure out what 90% of certificate lifetime in hours is: 96 *.10 = 9.6 Hrs. This shows us that for at a 10% lifetime left it is still valid for over 8 hours. This certificate would always get at least one auto enrollment cycle to be run to renew the certificate. Assuming you don’t have anything else wrong, the renewal would happen successfully. This is the minimum lifetime that any certificate testing should be set to. Test, test, and more testing! Well, now you got your test lab, issuing short lived certificates and getting them automatically renewed, but… Now you are noticing that your application is not using the new certificate. Unfortunately, there are going to be some applications for which the automatic renewal might not work out well in your organization. However, the certificate could still be renewed manually by your IIS admins, and it would still not require the CA manager to be involved in the renewal process. The key thing is your IIS Admins would need to go through the manual certificate renewal process BEFORE the current certificate expires, as it is used to cross sign the renewal request. UPDATE 5/30/2024: After release of the blog it was brought to my attention of two different things to consider. First unknown to me (as I am not an IIS support engineer) apparently there is a setting within the IIS Management console to help with updating the site binding configuration so that this automatic renewal behavior would work. This feature is called Certificate Rebind and was new in IIS 8.5. https://learn.microsoft.com/en-us/iis/get-started/whats-new-in-iis-85/certificate-rebind-in-iis85 Second, we recently has a case were the customer had several websites running on an IIS Web Server. These websites all used unique certificates and were all generated manually using the same certificate template. When one of the certificates were ready to be renewed, they found that all website certificates were archived, and only one of the website certificates were automatically renewed. This would be by design behavior. The only thing that we could really recommend in this situation is to use one certificate for all websites and make sure that all the websites DNS names were added to the certificates Subject Alternative Name (SAN) field. Failure to do this will result in going through the entire manual issuance of the certificate all over again. And, going through the CA Manager approval process as well, since this would be a new certificate request. Hopefully, you found some good nuggets of information about how to lockdown certificate templates to protect against ESC1 vulnerabilities and what some of those less often used tabs do. And of course, how the certificate renewal process works, especially around shot lived certificates. Rob “I coulda been a Welder” Greene Extra credit for those who can figure out where the saying came from.Renew Certificate Authority Certificates on Windows Server Core. No Problem!
Hi there! Rob and Jim are here from the Directory Services team. Today’s blog strives to clearly elucidate an administrative procedure that comes along more frequently with PKI Hierarchies being deployed to Windows Server Core operating systems. Installing the Certificate Services Role on Windows Server Core will not be covered in this blog, but this is good reference for this endeavor - https://learn.microsoft.com/en-us/powershell/module/adcsdeployment/install-adcscertificationauthority?view=windowsserver2022-ps In our scenario we already have an OFFLINE ROOT and an Enterprise Subordinate CA certificate that needs to be renewed. Both of these PKI roles are installed on the Windows Server Core operating system. To start the renewal process, validate if the customer has the following registry value in place so we know if / where the Certificate Signing Request (CSR) file is going to be written to. HKLM\System\CurrentControlSet\Services\CertSvc\Configuration\CA Name Value Name: RequestFileName Value Name: ParentCAMachine Value Name: ParentCAName These registry settings control where the CSR file and name will be located if they are specified. Make sure that the folder path already exists before moving forward. GENERATING A CSR WILL NOT create a missing folder. If the Root CA is an Enterprise Root CA (domain joined) the CSR creation will use the two Parent registry values to submit the certificate request to this Root CA. However, these values will not work and will probably not be present when utilizing an Offline (Standalone) Root CA. 2. In an elevated command prompt on the subordinate Issuing CA run the following command after deciding if reuse of the CA’s existing private key is in order or if a new private key should be generated: CertUtil -RenewCert [ReuseKeys] If you want to generate a new private key for the Subordinate CA, then type: If you want to generate a new private key for the Subordinate CA, then type: CertUtil -RenewCert If you want to use the existing private key for the Subordinate CA, then type: CertUtil -RenewCert ReuseKeys Additional information on CA certificate renewal options can be found here - Certification Authority Renewal - Win32 apps | Microsoft Learn Copy the resultant CSR .req File over to the Root CA. Now we can submit the request that we just copied to The Root CA which is also running on Windows Core OS. We are going to use the Certreq.exe command to submit this request to the Standalone Root CA. CertReq -Submit -Config "RootCAComputerDNSName\RootCAName" SubCACSRFIleName.req Example of the command line. Root CA Computer name: FourthCoffeeCA01.FourthCoffee.com Root CA Name: FourthCoffee Root CA 01 Sub CA CSR File Name: FourthCoffeeSubCARenewal01.req CertReq -Submit -Config "FourthCoffeeCA01.FourthCoffee.com\FourthCoffee Root CA 01" FourthCoffeeSubCARenewal01.req IMPORTANT: You should get a Request ID as part of the output. You will need to make a note of this for forthcoming steps. 5. If the CA Manager needs to approve the CSR, this can be accomplished via the certificate services management UI (cetsrv.msc) if possible as it is easier. However, since the Root is also running the Windows Core OS, we must run the following command: CertUtil -Resubmit RequestIDNumber Example of the command line: Request ID from step 4: Request ID = 3 CertUtil -Config "FourthCoffeeCA01.FourthCoffee.com\FourthCoffee Root CA 01" -resubmit 3 6. We next need to retrieve the CER/CRT file from the Root CA so that we can install the certificate on the Subordinate CA to complete the renewal. CertUtil -View -Restrict "RequestID=RequestIDNumber" -out RawCertificate > C:\FourthCoffeeSubCACert.cer 7. Assuming the Root CA's certificate has not been renewed, we just need to copy the resultant FourthCoffeeSubCACert.cer file back to the subordinate CA that is being renewed. 8. Back on the subordinate CA in an elevated command prompt we then need to install the subordinate CA's certificate. Using the following command: CertUtil -InstallCert CACertFileName Example: Certutil -InstallCert FourthCoffeeSubCACert.cer When this command is run the Certificate Service Service on the subordinate CA will start. We hope this blog will take some of the mystery and challenge out of interacting with Microsoft PKI running on Windows Server Core. Robert “what were you thinking” Greene and Jim “that’s for the birds” Tierney.Moving Your Organization from a Single Microsoft CA to a Microsoft Recommended PKI
First published on TechNet on Aug 23, 2010 Hi, folks! Jonathan here again, and today I want to talk about what appears to be an increasingly common topic: migrating from a single Windows Certification Authority (CA) to a multi-tier hierarchy.Firewall Rules for Active Directory Certificate Services
First published on TECHNET on Jun 25, 2010 Below is a list of ports that need to be opened on Active Directory Certificate Services servers to enable HTTP and DCOM based enrollment The information was developed by Microsoft Consultant Services during one of our customer engagementsProtocolPortFromToActionCommentsKerberos464Certificate Enrollment Web Services Domain Controllers (DC)AllowSource Certificate Enrollment Web ServicesDestination: DCService: Kerberos (network port tcp/464)LDAP389Certificate Enrollment Web Services Domain Controllers (DC)AllowSource Certificate Enrollment Web ServicesDestination: DCService: LDAP (network port tcp/389)LDAP636Certificate Enrollment Web Services Domain Controllers (DC)AllowSource Certificate Enrollment Web ServicesDestination: DCService: LDAP (network port tcp/636)DCOM/RPCRandom port above port 1023· Certificate Enrollment Web Services· All XP clients requesting certs CAAllowPlease see for details on RPC/DCOM configuration: http://support.Designing and Implementing a PKI: Part V Disaster Recovery
First published on TechNet on Apr 07, 2011 The series: Designing and Implementing a PKI: Part I Design and Planning Designing and Implementing a PKI: Part II Implementation Phases and Certificate Authority Installation Designing and Implementing a PKI: Part III Certificate Templates Designing and Implementing a PKI: Part IV Configuring SSL for Web Enrollment and Enabling Key Archival Designing and Implementing a PKI: Part V Disaster Recovery Chris here again.Designing and Implementing a PKI: Part I Design and Planning
First published on TechNet on Sep 01, 2009 The series: Designing and Implementing a PKI: Part I Design and Planning Designing and Implementing a PKI: Part II Implementation Phases and Certificate Authority Installation Designing and Implementing a PKI: Part III Certificate Templates Designing and Implementing a PKI: Part IV Configuring SSL for Web Enrollment and Enabling Key Archival Designing and Implementing a PKI: Part V Disaster Recovery Chris here again.
