IIS 7+ Kerberos authentication failure: KRB_AP_ERR_MODIFIED

Artikel
10/25/2011

KRB_AP_ERR_MODIFIED is a common Kerberos failure message. This means some encrypted Kerberos authentication data sent by the client did not decrypt properly at the server.

When a Kerberos client requests a ticket for a specific service, the service is actually identified by its SPN. The KDC grants the client a service ticket that is encrypted using service’s secret key. Basically, the AD account password that that matches the SPN requested.

Under some scenarios, KDC may generate a service ticket that encrypted with password of a wrong account (or not expected one). Then, when client provide that ticket to the service for authentication, the service can’t decrypt it and authentication failed with KRB_AP_ERR_MODIFED.

In short, this happens because KDC issued a ticket encrypted using password of account A, but on the service side, it tries to decrypt this using the password of account B.

Common cause for this are duplicated SPN, wrong DNS settings, two computers in different domains have the same name, client requests wrong SPN. And from IIS 7, it may due to the wrong setting of IIS (kernel/user mode authentication).

Collect data and identify the cause of Kerberos failure

Tools Used to collect data

1. Registry Editor(build in tool)
2. KList(build in for Windows 2008+)

https://technet.microsoft.com/en-us/library/hh134826(WS.10).aspx

3. Ipconfig (build in tool).

https://technet.microsoft.com/en-us/library/dd197434(WS.10).aspx

4. Network Monitor

https://www.microsoft.com/download/en/details.aspx?displaylang=en&id=4865

Steps to collect data

1. Enable Kerberos log on both client machine.

262177 How to enable Kerberos event logging

https://support.microsoft.com/default.aspx?scid=kb;EN-US;262177

2. Open a command console with elevated privilege, and run “klist purge” to clear cached Kerberos tickets.
3. Run “ipconfig /flushdns” to clear DNS cache.
4. Run Network monitor on both client and web server.
5. Reproduce the problem.
1. Network Monitor Trace

Identify the Kerberos error

By expanding the authenticate field in the HTTP response header returned by IIS, we could locate the reason for Kerberos authentication error.

- Http: Response, HTTP/1.1, Status: Unauthorized, URL: / , Using GSS-API Authentication

ProtocolVersion: HTTP/1.1

StatusCode: 401, Unauthorized

Reason: Unauthorized

…

- WWWAuthenticate: Negotiate …

- Authenticate: Negotiate oWwwaqADCgEBomMEYWBfBgkqhkiG9xIBAgIDAH5QME6abcdIBBaEDAgEepBEYDzIwMTExMabcd0MDUxMDE0WqUabcd2mAwIBKakKGwhURVNULkNPTaoXMBWgAwIBAaEOMAwbCmNvbnRvc29zdmM=

WhiteSpace:

- NegotiateAuthorization:

Scheme: Negotiate

- GssAPI: 0x1

- NegotiationToken:

- ChoiceTag:

- NegTokenResp:

- ResponseToken: 0x1

- KerberosToken: 0x1

- KerberosInitToken:

…

- InnerContextToken: 0x1

- KerberosToken: 0x1

TokId: Krb5Error (0x300)

- Error: KRB_ERROR (30)

….

+ ErrorCode: KRB_AP_ERR_MODIFIED (41)

+ Realm: TEST.COM

+ Sname: contososvc

Date: Fri, 14 Oct 2011 05:10:14 GMT

ContentLength: 341

It would be more straightforward using Wireshark.

2. From the system event log of client side, follow event will be logged.

Log Name: System

Source: Microsoft-Windows-Security-Kerberos

Date: 10/13/2011 10:10:05 PM

Event ID: 4

Task Category: None

Level: Error

Keywords: Classic

User: N/A

Computer: IIS02.test.com

Description:

The Kerberos client received a KRB_AP_ERR_MODIFIED error from the server contososvc. The target name used was HTTP/iis01.test.com. This indicates that the target server failed to decrypt the ticket provided by the client. This can occur when the target server principal name (SPN) is registered on an account other than the account the target service is using. Please ensure that the target SPN is registered on, and only registered on, the account used by the server. This error can also happen when the target service is using a different password for the target service account than what the Kerberos Key Distribution Center (KDC) has for the target service account. Please ensure that the service on the server and the KDC are both updated to use the current password. If the server name is not fully qualified, and the target domain (TEST.COM) is different from the client domain (TEST.COM), check if there are identically named server accounts in these two domains, or use the fully-qualified name to identify the server.

In this event, the SPN used is HTTP/iis01.test.com, and the account used to decrypt the ticket is contososvc. This happens because the account account used to encrypt the ticket is not contososvc.

Scenario 1: Duplicated SPN

In the case of a duplicated SPN, the same SPN was registered on at least two accounts. For example, a SPN was registered on two accounts: A and B. What happens is that KDC will generate a service ticket that may be encrypted with password of account A. Then, when the client sends that ticket to the service during authentication, the service may try to decrypt this using account B.

Detect duplicated SPN using setspn

On Windows 2008 and later, detect duplicated SPN is easier. Since Windows Server 2008, the setspn itself includes a feature to search SPNs.

Besides HTTP/ SPN, please remember to check HOST/ SPN as well. HOST/ SPN will be used as a failover/alternative if HTTP/ SPN does not exist. In case of this scenario, wrong HOST/ SPN will result in Kerberos failure as well.

Here is a sample output of setspn on Windows Server 2008 SP2. For more details about this tool, please reference this document.

SetSPN

https://technet.microsoft.com/en-us/library/cc731241(WS.10).aspx

Find duplicated SPN using ldifde

For Windows 2003 and XP, we can use another tool named ldifde to search duplicated SPN. Here is a sample query for HTTP/contoso. Please remember, don’t forget HOST/ SPN as well.

Here is an example output of ldifde, for more details about this tool, please reference follow document.

https://technet.microsoft.com/en-us/library/cc731033(WS.10).aspx

The SPN is forest-wide object, it has to be unique inside the whole domain. For a complex environment, using follow command to search the entire forest, like this:

Ldifde -s GCName -t 3268 -f d:\spn.ldf -d “dc=test, dc=com” –l ServicePrincipleName –r “(ServicePrincipalName=HTTP/contoso)”

In addition, we can use a wild card search like this:

Ldifde -s GCName -t 3268 –f d:\spn.ldf -d “dc=test, dc=com” -l servicePrincipalName -r (servicePrincipalName=*contoso*)

Scenario 2: Internet Explorer (or other client) requested ticket for wrong SPN

This is a specific scenario which most related to the behavior of client. This problem occurs if the Web site uses a CNAME resource record in the Domain Name System (DNS). For example, the DNS setting looks like this:

Contoso CNAME iis01.test.com

iis01.test.com A 10.0.5.2

When you use Internet Explorer to access the Web site, Internet Explorer uses the host name of the server ((IIS01)) instead of the CNAME resource record(Contoso) to contact the server. The authentication may fail with KRB_AP_ERR_MODIFIED.

HTTP/Contoso.test.com Registered on test\contososvc

HOST/IIS01.test.com Registered on test\iis01(machine account)

Identify this scenario from Network Monitor trace.

1. IE sends request to https://contoso, and a DNS query for contoso was sent.

+ Ipv4: Src = 10.0.5.3, Dest = 10.0.5.1, Next Protocol = UDP, Packet ID = 9717, Total IP Length = 62

+ Udp: SrcPort = 64506, DstPort = DNS(53), Length = 42

- Dns: QueryId = 0x4BB1, QUERY (Standard query), Query for contoso.test.com of type Host Addr on class Internet

…..

2. DNS response for contoso

+ Ipv4: Src = 10.0.5.1, Dest = 10.0.5.3, Next Protocol = UDP, Packet ID = 6526, Total IP Length = 98

+ Udp: SrcPort = DNS(53), DstPort = 64506, Length = 78

- Dns: QueryId = 0x4BB1, QUERY (Standard query), Response - Success, 49, 0

QueryIdentifier: 19377 (0x4BB1)

…..

- ARecord: contoso.test.com of type CNAME on class Internet : iis01.test.com

- ARecord: iis01.test.com of type Host Addr on class Internet: 10.0.5.2

….

3. TGS ticket request, IE requests SPN for : HTTP/iis01.test.com instead of expected HTTP/contoso.test.com

+ Ipv4: Src = 10.0.5.3, Dest = 10.0.5.1, Next Protocol = TCP, Packet ID = 9728, Total IP Length = 0

+ Tcp: Flags=...AP..., SrcPort=50044, DstPort=Kerberos(88), PayloadLen=1488, Seq=4106960882 - 4106962370, Ack=354586390, Win=513 (scale factor 0x8) = 131328

- Kerberos: TGS Request Realm: TEST.COM Sname: HTTP/iis01.test.com

…

Solutions for CName

1. If the client is IE, KB 911149 described the solution for this problem.

Error message in Internet Explorer when you try to access a Web site that requires Kerberos authentication on a Windows XP-based computer: "HTTP Error 401 - Unauthorized: Access is denied due to invalid credentials"

https://support.microsoft.com/default.aspx?scid=kb;EN-US;911149

2. If the client is an application uses System.Net.HttpWebRequest, using CustomTargetNameDictionary.

AuthenticationManager.CustomTargetNameDictionary

https://msdn.microsoft.com/en-us/library/system.net.authenticationmanager.customtargetnamedictionary.aspx

3. On DNS server side, configure the IIS server to a host record (A) instead of Alias(CNAME).

Scenario 3: SPN set to unexpected account (Wrong IIS 7+ authentication settings)

Internet Information Services (IIS) 7.0 enables kernel mode authentication by default. Kernel mode authentication runs under the machine account no matter what account is used to run the application pool. The machine account is used to decrypt the Kerberos ticket.

However, there are some scenarios you need to use a domain service account for authentication process instead of machine account. For example, a web arm scenario.

For this scenario, Instead of disabling kernel mode authentication in IIS, you can configure IIS to use the Web application pool’s identity for authentication (by setting useAppPoolCredentials="true").

For IIS 7+, we have 3 Windows authentication configuration. Different scenario requires register SPN on different accounts. In the scenario of improper SPN and IIS 7 configuration, it may result in authentication failure with KRB_AP_ERR_MODIFIED if the SPN was set to unexpected account.

· Kernel mode authentication disabled
· Kernel mode authentication enabled, useAppPoolCredentials
· Kernel mode authentication enabled

NOTE: Machine account includes all accounts can represent the machine on network including Network Service, Local System, Local Service and ApplicationPoolIdentity for IIS 7+. Service accounts means a domain account used as the application pool identity.

Here, I listed couple of scenarios which can result in Kerberos authentication failed with KRB_AP_ERR_MODIFIED.

Scenarios result in KRB_AP_ERR_MODIFIED

Wrong Configuration Scenario 1

Kernel Mode Authentication	Enabled(default)
useAppPoolCredentials	False(default)
Application Pool Identity	Service Account like (domain\contosoService)
Web Site Binding To	IIS server’s NetBIOS Name. Access like this way: http(s)://IIS_Server_NetBIOS_Name http(s)://IIS_Server_FQDN
SPN	HTTP/ SPN registered on service account
Comments	For this scenario, the Kerberos ticket is encrypted by service account, and is decrypted by IIS server’s computer account.

Wrong Configuration Scenario 2

Kernel Mode Authentication	Enabled(default)
useAppPoolCredentials	False(default)
Application Pool Identity	Service Account like (domain\contosoService)
Web Site Binding To	A customized host header. Access like this way: http(s)://Contoso
SPN	HTTP/ SPN registered on service account
Comments	For this scenario, the Kerberos ticket is encrypted by service account, and decrypted by IIS server’s computer account.

Wrong Configuration Scenario 3

Kernel Mode Authentication	Enabled(default)
useAppPoolCredentials	True
Application Pool Identity	Service Account like (domain\contosoService)
Web Site Binding To	IIS server’s NetBIOS Name. Access like this way: http(s)://IIS_Server_NetBIOS_Name http(s)://IIS_Server_FQDN
SPN	HTTP/ IIS_Server_NetBIOS_Name doesn’t registered on any account Or, registered on IIS server’s computer account
Comments	For this scenario, the Kerberos ticket is encrypted by IIS server’s computer account, and decrypted by service account.

SPN and IIS configuration reference

Scenario 1

Kernel Mode Authentication	Enabled(default)
useAppPoolCredentials	False(default)
Application Pool Identity	No Matter
URL used to access web site	http(s)://IIS_Server_NetBIOS_Name http(s)://IIS_Server_FQDN
SPN requirement	No HTTP/ SPN required. By default, the HOST/ IIS_Server_NetBIOS_Name will be used. If you want, you can register HTTP/ IIS_Server_NetBIOS_Name on the server name.
Comments	This is the default scenario for IIS 7+ when using IIS server’s computer name to access the web application.

Scenario 2

Kernel Mode Authentication	Enabled(default)
useAppPoolCredentials	False(default)
Application Pool Identity	No Matter
URL used to access web site	http(s)://Customer_Host_Name
SPN requirement	Need register SPN on IIS server’s computer account, like: SetSPN -a HTTP/Customer_Host_NAME IIS_SRV_NetBIOS
Comments	Some application requires this when they need special permission for application pool identity.

Scenario 3

Kernel Mode Authentication	Enabled(default)
useAppPoolCredentials	True
Application Pool Identity	Service Account like (domain\contosoService)
URL used to access web site	http(s)://Customer_Host_Name
SPN requirement	Need register SPN on service account, like: SetSPN -a HTTP/Customer_Host_NAME domain\contosoService
Comments	1. This is a typical requirement for NLB environment. 2. Some complex products consisted by couple services/applications like SharePoint. They require set the SPN on a domain account, and run the all the services/applications using this domain account.

Scenario 4

Kernel Mode Authentication	Enabled(default)
useAppPoolCredentials	True
Application Pool Identity	Service Account like (domain\contosoService)
URL used to access web site	http(s)://IIS_Server_NetBIOS_Name http(s)://IIS_Server_FQDN
SPN requirement	Need register SPN on service account, like: SetSPN -a HTTP/IIS_SERVER_FQDN domain\contosoService
Comments	You need select this scenario if you want web site binding to IIS server’s computer name and running the site with a domain account.

Scenario 5

Kernel Mode Authentication	Disabled
useAppPoolCredentials	No Matter
Application Pool Identity	Service Account like (domain\contosoService)
URL used to access web site	http(s)://Customer_Host_Name
SPN requirement	Need register SPN on service account, like: SetSPN -a HTTP/Customer_Host_NAME domain\contosoService
Comments	This is same for IIS 6 scenario.

Scenario 6

Kernel Mode Authentication	Disabled
useAppPoolCredentials	No Matter
Application Pool Identity	Service Account like (domain\contosoService)
URL used to access web site	http(s)://IIS_SERVER_NetBIOS_NAME
SPN requirement	Need register SPN on service account, like: SetSPN -a HTTP/ IIS_SERVER_NetBIOS_NAMEdomain\contosoService
Comments	This is same for IIS 6 scenario.

Scenario 7

Kernel Mode Authentication	Disabled
useAppPoolCredentials	No Matter
Application Pool Identity	Machine Account
URL used to access web site	http(s)://Customer_Host_Name
SPN requirement	Need register SPN on IIS server’s computer account, like: SetSPN -a HTTP/Customer_Host_NAME IIS_SRV_NetBIOS
Comments	This is same for IIS 6 scenario.

Scenario 8

Kernel Mode Authentication	Disabled
useAppPoolCredentials	No Matter
Application Pool Identity	Machine Account
URL used to access web site	http(s)://IIS_SERVER_NetBIOS_NAME
SPN requirement	No HTTP/ SPN required. By default, the HOST/ IIS_Server_NetBIOS_Name will be used. If you want, you can register HTTP/ IIS_Server_NetBIOS_Name on the server name.
Comments	This is similar to the default scenario of IIS 6.

See you Next Time,

Wei from APGC DSI Team

Comments

Anonymous
April 30, 2012
An insightful and extremely useful document. Your analysis of the problem and method of examining the possible causes shows an understanding of Kerberos that few other bloggers seem to possess. Thank you for your work - it has been extremely useful!Rob.

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