Policy Module - Uwe Gradenegger

How securing printers can turn into a security disaster - and how the TameMyCerts Policy Module for Active Directory Certificate Services (ADCS) can prevent it

Nowadays, it is essential to protect the authentication of devices on the company network and administrative interfaces. As a rule, digital certificates are used for this purpose.

Printers therefore also generally require digital certificates in order to be operated securely. From a certain number of devices, there is no getting around automatic certificate distribution.

Some printer manufacturers offer centralized management solutions for certificate distribution.

Unfortunately, it has been shown time and again that the secure handling of digital certificates requires a great deal of knowledge, experience and care, which is often not the case.

New ESC15 vulnerability discovered in Active Directory Certificate Services - easy-to-implement countermeasures

The purposes for which a digital certificate may be used are controlled via the "Key Usage" and "Extended Key Usage" certificate extensions. In the "Extended Key Usage" certificate extension, the extended key uses for which the certificate may be used.

However, there is another certificate extension called "Application Policies" for certificates issued by a Microsoft Certification Authority, which also contains a list very similar to the Extended Key Usages extension.

Justin Bollinger from TrustedSec has found outthat there are offline certificate requests against Schema version 1 certificate templates is possible (similar to the Security identifier extension), any Application Policies in the certificate request, which are transferred unchanged to the issued certificate and can then be used for an attack on the overall Active Directory structure. The attack was christened ESC15.

How the TameMyCerts Policy Module for Active Directory Certificate Services (ADCS) can prevent attacks against the ESC1 attack vector

Attacks on Microsoft certification authorities can be aimed at exploiting authorizations on certificate templates. In many cases, certificate templates must be configured to grant the applicant the right to apply for any identities. This can lead to the attacker taking over the identities of Active Directory accounts and subsequently to the elevation of rights. Attacks of this type are known in the security scene as "ESC1" is labeled.

How the TameMyCerts Policy Module for Active Directory Certificate Services (ADCS) can detect and prevent attacks against the ESC6 and ESC7 attack vectors

With the supposedly good intention of making it possible to issue such certificate requirements with a SAN, guess unfortunately much at many Instructions to set the flag on the certification authority EDITF_ATTRIBUTESUBJECTALTNAME2 to activate.

If this flag is activated, a very large attack surface is offered, as any applicant can now instruct the certification authority to issue certificates with any content. This type of attack is known in the security scene as ESC6 and ESC7 known.

A policy module to tame them all: Introducing the TameMyCerts Policy Module for the Microsoft Certification Authority.

As a Certification Authority operator, you are (among other things) responsible for the identification of the enrollees and the confirmation of the requested identities. The fact that this task is carried out conscientiously and without error is the central pillar of the trust placed in the certification body. Well-known companies are already failed in this task, even had to file for insolvency as a result of misrepresentations and / or were taken over by the big players in the market sensitive punished.

In many cases, we as (Microsoft) PKI operators in companies (regardless of the associated quality) are able to delegate our task of uniquely identifying an applicant to the Active Directory. In many cases, however, we unfortunately also have to instruct our certification authority(ies) to simply issue everything that is requested.

Changes to Certificate Issuance and Certificate-Based Logon to Active Directory with the May 10, 2022 Patch for Windows Server (KB5014754)

With the May 10, 2022 patch, Microsoft is attempting to patch a vulnerability in the Active Directory in which the certificate-based enrollment (commonly known as PKINIT or also Smartcard Logon) to close.

The update changes both the behavior of the Certification Authority as well as the behavior of Active Directory when processing certificate-based logins.

The "Application Policies" certificate extension

The purposes for which a digital certificate may be used are controlled via the certificate extensions "Key Usage" and "Extended Key Usage".

In the "Extended Key Usage" certificate extension, the extended key uses for which the certificate may be used.

New certificates are regularly requested via Autoenrollment

Assume the following scenario:

A certificate template is configured for automatic request and issuance (AutoEnrollment).
Users or computers apply for new certificates at regular intervals and long before the defined renewal period.

The key algorithm of certificate requests is not checked by the certification authority's policy module

Assume the following scenario:

A certificate template is configured to use elliptic curve based keys (e.g. ECDSA_P256).
As a result, a minimum key length of 256 bits is configured.
Nevertheless, certificate requests that use other ECC curves or RSA-based keys are also signed.

Active Directory forest compromised by EDITF_ATTRIBUTESUBJECTALTNAME2 flag

In net circulate unfortunately much at many Instructions (also the big players are not excluded from this, not even Microsoft itself or the Grand Master Komar), which fatally recommends that the EDITF_ATTRIBUTESUBJECTALTNAME2 flag should be set on the certification authority - supposedly to be able to issue Subject Alternative Name (SAN) extension certificates for manually submitted certificate requests.

Unfortunately, this approach is not only unnecessary, it also has some unpleasant side effects, which in the worst case can help an attacker to take over the entire Active Directory forest.