Attaching client certificates
Fiddler2 includes the ability to decrypt, view, and modify HTTPS-secured traffic for debugging purposes. This feature is disabled by default, but can be enabled in Fiddler's Tools > Fiddler Options dialog.
See Decrypting HTTPS traffic with Fiddler2 for more information on HTTPS decryption support in Fiddler.
Fiddler's HTTPS decryption feature also offers basic support for intercepting requests that require client certificates and responding with a client certificate from the machine running Fiddler.
Selection of Client Certificate
If a .CER file has been specified for a given session as follows:
oSession["https-Client-Certificate"] = "C:\\test\\someCert.cer";
...then Fiddler will simply use that certificate for the current session.
Otherwise, Fiddler will look for the file %USERPROFILE%\My Documents\Fiddler2\ClientCertificate.cer and attach that certificate.
CER files and Private Keys
A .CER file does not actually contain the private key for the certificate. The local CER merely acts as a pointer to a certificate in your Personal Certificates store-- that certificate, installed the Windows Personal Certificate store (certmgr.msc) contains the private key.
Where do I get the .CER File?
After your certificate is installed in the Current User's Personal Certificates store (see certmgr.msc), simply right-click the certificate, choose All Tasks > Export....
If the certificate isn't yet in the Current User's Personal Certificates store (e.g. you only have a .pfx file) then first import it into the certificate store, then export a .CER file.
Using the certificate from a SmartCard
If you export a .CER from the SmartCard certificate, Fiddler will use the matching certificate from the smartcard to authenticate to the server.