How to use a client certificate to authenticate and authorize in a Web API

Question

I am trying to use a client certificate to authenticate and authorize devices using a Web API and developed a simple proof of concept to work through issues with the potential solution   I am running into an issue where the client certificate is not being received by the web application   A number of people of reported this issue  including in this Q amp A  but none of them have an answer   My hope is to provide more detail to revive this issue and hopefully get an answer for my issue   I am open to other solutions   The main requirement is that a standalone process written in C  can call a Web API and be authenticated using a client certificate    The Web API in this POC is very simple and just returns a single value   It uses an attribute to validate that HTTPS is used and that a client certificate is present     public class SecureController   ApiController        RequireHttps      public string Get int id                return  value              Here is the code for the RequireHttpsAttribute   public class RequireHttpsAttribute   AuthorizationFilterAttribute         public override void OnAuthorization HttpActionContext actionContext                  if  actionContext Request RequestUri Scheme    Uri UriSchemeHttps                          actionContext Response   new HttpResponseMessage System Net HttpStatusCode Forbidden                                  ReasonPhrase    HTTPS Required                                      else                        var cert   actionContext Request GetClientCertificate                if  cert    null                                actionContext Response   new HttpResponseMessage System Net HttpStatusCode Forbidden                                        ReasonPhrase    Client Certificate Required                                                 base OnAuthorization actionContext                          In this POC I am just checking for the availability of the client certificate   Once this is working I can add checks for information in the certificate to validate against a list of certificates   Here are the setting in IIS for SSL for this web application     Here is the code for the client that sends the request with a client certificate   This is a console application       private static async Task SendRequestUsingHttpClient                 WebRequestHandler handler   new WebRequestHandler            X509Certificate certificate   GetCert  ClientCertificate cer            handler ClientCertificates Add certificate           handler ServerCertificateValidationCallback   new RemoteCertificateValidationCallback ValidateServerCertificate           handler ClientCertificateOptions   ClientCertificateOption Manual          using  var client   new HttpClient handler                         client BaseAddress   new Uri  https   localhost 44398                 client DefaultRequestHeaders Accept Clear                client DefaultRequestHeaders Accept Add new MediaTypeWithQualityHeaderValue  application json                  HttpResponseMessage response   await client GetAsync  api Secure 1                if  response IsSuccessStatusCode                                string content   await response Content ReadAsStringAsync                    Console WriteLine  Received response   0   content                             else                               Console WriteLine  Error  received status code  0    1    response StatusCode  response ReasonPhrase                                      public static bool ValidateServerCertificate        object sender        X509Certificate certificate        X509Chain chain        SslPolicyErrors sslPolicyErrors                Console WriteLine  Validating certificate  0    certificate Issuer           if  sslPolicyErrors    SslPolicyErrors None              return true           Console WriteLine  Certificate error   0    sslPolicyErrors               Do not allow this client to communicate with unauthenticated servers          return false          When I run this test app I get back a status code of 403 Forbidden with a reason phrase of    Client Certificate Required    indicating that it is getting into my RequireHttpsAttribute and it is not finding any client certificates   Running this through a debugger I have verified that the certificate is getting loaded and added to the WebRequestHandler   The certificate is exported into a CER file that is being loaded   The full certificate with the private key is located on the Local Machine   s Personal and Trusted Root stores for the web application server   For this test the client and web application are being run on the same machine   I can call this Web API method using Fiddler  attaching the same client certificate  and it works fine   When using Fiddler it passes the tests in RequireHttpsAttribute and returns a successful status code of 200 and returns the expected value   Has anybody run into the same issue where HttpClient does not send a client certificate in the request and found a solution   Update 1   I also tried getting the certificate from the certificate store that includes the private key   Here is how I retrieved it       private static X509Certificate2 GetCert2 string hostname                X509Store myX509Store   new X509Store StoreName My  StoreLocation LocalMachine           myX509Store Open OpenFlags ReadWrite           X509Certificate2 myCertificate   myX509Store Certificates OfType lt X509Certificate2 gt    FirstOrDefault cert   gt  cert GetNameInfo X509NameType SimpleName  false     hostname           return myCertificate          I verified that this certificate was getting retrieved correctly and it was being added to the client certificate collection   But I got the same results where the server code does not retrieve any client certificates   For completeness here is the code used to retrieve the certificate from a file       private static X509Certificate GetCert string filename                X509Certificate Cert   X509Certificate CreateFromCertFile filename           return Cert           You will notice that when you get the certificate from a file it returns an object of type X509Certificate and when you retrieve it from the certificate store it is of type X509Certificate2   The X509CertificateCollection Add Method is expecting a type of X509Certificate   Update 2  I am still trying to figure this out and have tried many different options but to no avail      I changed the web application to run on a host name instead of local host  I set the web application to require SSL I verified that the certificate was set for Client Authentication and that it is in the trusted root Besides testing the client certificate in Fiddler I also validated it in Chrome    At one point during trying these options it started working   Then I started backing out changes to see what caused it to work  It continued to work   Then I tried removing the certificate from the trusted root to validate that this was required and it stopped working and now I cannot get it back to working even though I put the certificate back in the trusted root   Now Chrome will not even prompt me for a certificate like it used too and it fails in Chrome  but still works in Fiddler   There must be some magic configuration I am missing   I also tried enabling  Negotiate Client Certificate  in the binding but Chrome still will not prompt me for a client certificate   Here is the settings using  netsh http show sslcert    IP port                   0 0 0 0 44398  Certificate Hash          429e090db21e14344aa5d75d25074712f120f65f  Application ID             4dc3e181-e14b-4a21-b022-59fc669b0914   Certificate Store Name    MY  Verify Client Certificate Revocation      Disabled  Verify Revocation Using Cached Client Certificate Only      Disabled  Usage Check      Enabled  Revocation Freshness Time   0  URL Retrieval Timeout     0  Ctl Identifier             null   Ctl Store Name             null   DS Mapper Usage      Disabled  Negotiate Client Certificate      Enabled   Here is the client certificate I am using         I am baffled as to what the issue is   I am adding a bounty for anyone that can help me figure this out

User · Answer

I came upon a similar issue recently and following Fabian's advice actually led me to the solution. Turns out with client certs you have to ensure two things:

The private key is actually being exported as part of the cert.
The application pool identity running the app has access to said private key.

In our case I had to:

Import the pfx file into the local server store while checking the export checkbox to ensure the private key was sent out.
Using MMC console, grant the service account used access to the private key for the cert.

The trusted root issue explained in other answers is a valid one, it was just not the issue in our case.

User · Answer

Update   Example from Microsoft   https   docs microsoft com en-us azure app-service app-service-web-configure-tls-mutual-auth special-considerations-for-certificate-validation  Original  This is how I got client certification working and checking that a specific Root CA had issued it as well as it being a specific certificate   First I edited  lt src gt   vs config applicationhost config and made this change   lt section name  access  overrideModeDefault  Allow    gt   This allows me to edit  lt system webServer gt  in web config and add the following lines which will require a client certification in IIS Express  Note  I edited this for development purposes  do not allow overrides in production   For production follow a guide like this to set up the IIS   https   medium com  hafizmohammedg configuring-client-certificates-on-iis-95aef4174ddb  web config    lt security gt     lt access sslFlags  Ssl SslNegotiateCert SslRequireCert    gt   lt  security gt    API Controller    RequireSpecificCert  public class ValuesController   ApiController          GET api values     public IHttpActionResult Get                 return Ok  It works               Attribute   public class RequireSpecificCertAttribute   AuthorizationFilterAttribute       public override void OnAuthorization HttpActionContext actionContext                if  actionContext Request RequestUri Scheme    Uri UriSchemeHttps                        actionContext Response   new HttpResponseMessage System Net HttpStatusCode Forbidden                                ReasonPhrase    HTTPS Required                                   else                       X509Certificate2 cert   actionContext Request GetClientCertificate                if  cert    null                                actionContext Response   new HttpResponseMessage System Net HttpStatusCode Forbidden                                        ReasonPhrase    Client Certificate Required                                                else                               X509Chain chain   new X509Chain                       Needed because the error  The revocation function was unable to check revocation for the certificate  happened to me otherwise                 chain ChainPolicy   new X509ChainPolicy                                         RevocationMode   X509RevocationMode NoCheck                                     try                                       var chainBuilt   chain Build cert                       Debug WriteLine string Format  Chain building status   0    chainBuilt                         var validCert   CheckCertificate chain  cert                        if  chainBuilt    false    validCert    false                                                actionContext Response   new HttpResponseMessage System Net HttpStatusCode Forbidden                                                        ReasonPhrase    Client Certificate not valid                                                     foreach  X509ChainStatus chainStatus in chain ChainStatus                                                        Debug WriteLine string Format  Chain error   0   1    chainStatus Status  chainStatus StatusInformation                                                                                      catch  Exception ex                                        Debug WriteLine ex ToString                                                  base OnAuthorization actionContext                        private bool CheckCertificate X509Chain chain  X509Certificate2 cert                var rootThumbprint   WebConfigurationManager AppSettings  rootThumbprint   ToUpper   Replace      string Empty            var clientThumbprint   WebConfigurationManager AppSettings  clientThumbprint   ToUpper   Replace      string Empty              Check that the certificate have been issued by a specific Root Certificate         var validRoot   chain ChainElements Cast lt X509ChainElement gt    Any x   gt  x Certificate Thumbprint Equals rootThumbprint  StringComparison InvariantCultureIgnoreCase               Check that the certificate thumbprint matches our expected thumbprint         var validCert   cert Thumbprint Equals clientThumbprint  StringComparison InvariantCultureIgnoreCase            return validRoot  amp  amp  validCert            Can then call the API with client certification like this  tested from another web project    RoutePrefix  api certificatetest    public class CertificateTestController   ApiController        public IHttpActionResult Get                 var handler   new WebRequestHandler            handler ClientCertificateOptions   ClientCertificateOption Manual          handler ClientCertificates Add GetClientCert             handler UseProxy   false          var client   new HttpClient handler           var result   client GetAsync  https   localhost 44331 api values   GetAwaiter   GetResult            var resultString   result Content ReadAsStringAsync   GetAwaiter   GetResult            return Ok resultString              private static X509Certificate GetClientCert                 X509Store store   null          try                       store   new X509Store StoreName My  StoreLocation CurrentUser               store Open OpenFlags OpenExistingOnly   OpenFlags ReadOnly                var certificateSerialNumber    81 c6 62 0a 73 c7 b1 aa 41 06 a3 ce 62 83 ae 25  ToUpper   Replace      string Empty                  Does not work for some reason  could be culture related               var certs   store Certificates Find X509FindType FindBySerialNumber  certificateSerialNumber  true                  if  certs Count    1                                    var cert   certs 0                     return cert                               var cert   store Certificates Cast lt X509Certificate gt    FirstOrDefault x   gt  x GetSerialNumberString   Equals certificateSerialNumber  StringComparison InvariantCultureIgnoreCase                 return cert                    finally                       store  Close

User · Answer

Looking at the source code I also think there must be some issue with the private key   What it is doing is actually to check if the certificate that is passed is of type X509Certificate2 and if it has the private key    If it doesn t find the private key it tries to find the certificate in the CurrentUser store and then in the LocalMachine store  If it finds the certificate it checks if the private key is present    see source code from class SecureChannnel  method EnsurePrivateKey   So depending on which file you imported   cer - without private key or  pfx - with private key  and on which store it might not find the right one and Request ClientCertificate won t be populated   You can activate Network Tracing to try to debug this  It will give you output like this    Trying to find a matching certificate in the certificate store Cannot find the certificate in either the LocalMachine store or the CurrentUser store

User · Answer

Make sure HttpClient has access to the full client certificate  including the private key     You are calling GetCert with a file  ClientCertificate cer  which leads to the assumption that there is no private key contained - should rather be a pfx file within windows   It may be even better to access the certificate from the windows cert store and search it using the fingerprint   Be careful when copying the fingerprint  There are some non-printable characters when viewing in cert management  copy the string over to notepad   and check the length of the displayed string

User · Answer

I actually had a similar issue  where we had to many trusted root certificates  Our fresh installed webserver had over a hunded  Our root started with the letter Z so it ended up at the end of the list    The problem was that the IIS sent only the first twenty-something trusted roots to the client and truncated the rest  including ours  It was a few years ago  can t remember the name of the tool    it was part of the IIS admin suite  but Fiddler should do as well  After realizing the error  we removed a lot trusted roots that we don t need  This was done trial and error  so be careful what you delete   After the cleanup everything worked like a charm

User · Answer

Tracing helped me find what the problem was  Thank you Fabian for that suggestion    I found with further testing that I could get the client certificate to work on another server  Windows Server 2012    I was testing this on my development machine  Window 7  so I could debug this process   So by comparing the trace to an IIS Server that worked and one that did not I was able to pinpoint the relevant lines in the trace log   Here is a portion of a log where the client certificate worked  This is the setup right before the send  System Net Information  0    17444  InitializeSecurityContext In-Buffers count 2  Out-Buffer length 0  returned code CredentialsNeeded   System Net Information  0    17444  SecureChannel 54718731 - We have user-provided certificates  The server has not specified any issuers  so try all the certificates  System Net Information  0    17444  SecureChannel 54718731 - Selected certificate    Here is what the trace log looked like on the machine where the client certificate failed   System Net Information  0    19616  InitializeSecurityContext In-Buffers count 2  Out-Buffer length 0  returned code CredentialsNeeded   System Net Information  0    19616  SecureChannel 54718731 - We have user-provided certificates  The server has specified 137 issuer s   Looking for certificates that match any of the issuers  System Net Information  0    19616  SecureChannel 54718731 - Left with 0 client certificates to choose from  System Net Information  0    19616  Using the cached credential handle    Focusing on the line that indicated the server specified 137 issuers I found this Q amp A that seemed similar to my issue   The solution for me was not the one marked as an answer since my certificate was in the trusted root   The answer is the one under it where you update the registry   I just added the value to the registry key   HKEY LOCAL MACHINE SYSTEM CurrentControlSet Control SecurityProviders SCHANNEL  Value name  SendTrustedIssuerList Value type  REG DWORD Value data  0  False   After adding this value to the registry it started to work on my Windows 7 machine   This appears to be a Windows 7 issue

[c#] How to use a client certificate to authenticate and authorize in a Web API

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