Using Fiddler With iOS 10 and Android 7

If you’ve tried to use Fiddler with iOS10 beta or Android 7 Nougat, you have probably found that HTTPS decryption isn’t working, even if you use the latest Fiddler and the Fiddler Certificate Maker add-on. Unfortunately, at the moment both platforms are broken, but for different reasons. In both cases, the client will fail to receive responses for HTTPS requests, and Fiddler will only show a CONNECT tunnel.

iOS 10 Change

After installing the FiddlerRoot certificate, one also needs to go to Settings -> General -> About -> Certificate Trust Settings and manually enable full trust for the FiddlerRoot root certificate, including accepting a dialog that says that this will allow a third-party to eavesdrop on all your communications.

iOS 10 Beta Bug (Fixed for final version)

The beta of iOS 10 had a bug whereby, if the response to a HTTP CONNECT tunnel request contains a Connection: close response header, the client will close the connection instead of doing as it should and waiting until the TCP/IP connection closes. A few minor platforms have had the same bug over the years, but iOS is definitely the first important platform with this issue. At least two bugs have been filed with the Apple “Radar” bug reporter.

Working around this limitation is simple. In Fiddler, click Rules > Customize Rules. Scroll to the OnBeforeResponse function. Just inside that function, add the following lines:

  if (oSession.HTTPMethodIs("CONNECT")) {
oSession["ui-backcolor"] = "red";
oSession.ResponseHeaders.Remove("Connection");
}


Save the file and try connecting again.

Android 7 Feature

In contrast to the iOS regression, the change in Android 7 was intentional. The Android team has decided that, by default, HTTPS certificate validation for apps targeting API Level 24 and later will ignore all user-installed root certificates, meaning that your efforts to manually trust Fiddler’s root certificate will be fruitless. Individually application developers can temporarily override this change while debugging by updating the application’s configuration:

<network-security-config>  
      <debug-overrides>  
           <trust-anchors>  
                <!-- Trust user added CAs while debuggable only -->
                <certificates src="user" />  
           </trust-anchors>  
      </debug-overrides>  
 </network-security-config>

…or at all times…

<network-security-config>  
      <base-config>  
            <trust-anchors>  
                <!-- Trust preinstalled CAs -->  
                <certificates src="system" />  
                <!-- Additionally trust user added CAs -->  
                <certificates src="user" />  
           </trust-anchors>  
      </base-config>  
 </network-security-config>

…Unfortunately, these changes can only be undertaken by application developers and not end-users. End-users will probably need to jailbreak their devices, akin to what is required to circumvent certificate pinning.

Certificate Validity Length

By default, Fiddler-generated certificates are valid for five years (and backdated one year). However, this can cause an ERR_CERT_VALIDITY_TOO_LONG error in Chrome on Android.  To fix this for Fiddler’s default (CertEnroll) certificate generator,  run about:config in QuickExec to edit preferences. Set fiddler.certmaker.ValidDays to 820 and, if needed, reset your certificates in Fiddler using the Tools > Fiddler Options > HTTPS > Actions button.

-Eric