Tamper-Evident Configuration Files in ASP.NET

A couple weeks ago someone sent a message to one of our internal mailing lists. His message was pretty straightforward: how do you prevent modifications to a configuration file for an application [while the user has administrative rights on the machine]?

There were a couple responses including mine, which was to cryptographically sign the configuration file with an asymmetric key. For a primer on digital signing, take a look here. Asymmetric signing is one possible way of signing a file. By signing it this way the configuration file could be signed by an administrator before deploying the application, and all the application needed to validate the signature was the public key associated with the private key used to sign the file. This separated the private key from the application, preventing the configuration from being re-signed maliciously. It’s similar in theory to how code-signing works.

In the event that validation of the configuration file failed, the application would not load, or would gracefully fail and exit the next time the file was checked (or the application had an exclusive lock on the configuration file so it couldn’t be edited while running).

We are also saved the problem of figuring out the signature format because there is a well-respected XML signature schema: http://www.w3.org/2000/09/xmldsig#. WCF uses this format to sign messages. For a good code-walkthrough see Barry Dorrans’ Beginning ASP.NET Security. More on the code later here though.

Technically, this won’t prevent changes to the file, but it will prevent the application from accepting those changes. It’s kind of like those tamper-evident tags manufacturers stick on the enclosures of their equipment. It doesn’t prevent someone from opening the thing, but they will get caught if someone checks it. You’ll notice I didn’t call them “tamper-resistance” tags.

Given this problem, I went one step further and asked myself: how would I do this with a web application? A well-informed ASP.NET developer might suggest using aspnet_regiis to encrypt the configuration file. Encrypting the configuration does protect against certain things, like being able to read configuration data. However, there are a couple problems with this.

  • If I’m an administrator on that server I can easily decrypt the file by calling aspnet_regiis
  • If I’ve found a way to exploit the site, I can potentially overwrite the contents of the file and make the application behave differently
  • The encryption/decryption keys need to be shared in web farms

Consider our goal. We want to prevent a user with administrative privileges from modifying the configuration. Encryption does not help us in this case.  Signing the configuration will help though (As an aside, for more protection you encrypt the file then sign it, but that’s out of the scope of this) because the web application will stop working if a change is made that invalidates the signature.

Of course, there’s one little problem. You can’t stick the signature in the configuration file, because ASP.NET will b-itch complain about the foreign XML tag. The original application in question was assumed to have a custom XML file for it’s configuration, but in reality it doesn’t, so this problem applies there too.

There are three possible solutions to this:

  • Create a custom ConfigurationSection class for the signature
  • Create a custom configuration file and handler, and intercept all calls to web.config
  • Stick the signature of the configuration file into a different file

The first option isn’t a bad idea, but I really didn’t want to muck about with the configuration classes. The second option is, well, pretty much a stupid idea in almost all cases, and I’m not entirely sure you can even intercept all calls to the configuration classes.

I went with option three.

The other file has two important parts: the signature of the web.config file, and a signature for itself. This second signature prevents someone from modifying the signature for the web.config file. Our code becomes a bit more complicated because now we need to validate both signatures.

This makes us ask the question, where is the validation handled? It needs to happen early enough in the request lifecycle, so I decided to stick it into a HTTP Module, for the sake of modularity.

Hold it, you say. If the code is in a HTTP Module, then it needs to be added to the web.config. If you are adding it to the web.config, and protecting the web.config by this module, then removing said module from the web.config will prevent the validation from occurring.


There are two ways around this:

  • Add the validation call into Global.asax
  • Hard code the addition of the HTTP Module

It’s very rare that I take the easy approach, so I’ve decided to hard code the addition of the HTTP Module, because sticking the code into a module is cleaner.

In older versions of ASP.NET you had to make some pretty ugly hacks to get the module in because it needs to happen very early in startup of the web application. With ASP.NET 4.0, an assembly attribute was added that allowed you to call code almost immediately after startup:

[assembly: PreApplicationStartMethod(typeof(Syfuhs.Security.Web.Startup), "Go")]


Within the Startup class there is a public static method called Go(). This method calls the Register() within an instance of my HttpModule. This module inherits from an abstract class called DynamicallyLoadedHttpModule, which inherits from IHttpModule. This class looks like:

public abstract class DynamicallyLoadedHttpModule : IHttpModule
    public void Register()
        DynamicHttpApplication.RegisterModule(delegate(HttpApplication app) { return this; });

    public abstract void Init(HttpApplication context);

    public abstract void Dispose();


The DynamicHttpApplication class inherits from HttpApplication and allows you to load HTTP modules in code. This code was not written by me. It was originally written by Nikhil Kothari:

using HttpModuleFactory = System.Func<System.Web.HttpApplication, System.Web.IHttpModule>;

public abstract class DynamicHttpApplication : HttpApplication
    private static readonly Collection<HttpModuleFactory> Factories 
= new Collection<HttpModuleFactory>(); private static object _sync = new object(); private static bool IsInitialized = false; private List<IHttpModule> modules; public override void Init() { base.Init(); if (Factories.Count == 0) return; List<IHttpModule> dynamicModules = new List<IHttpModule>(); lock (_sync) { if (Factories.Count == 0) return; foreach (HttpModuleFactory factory in Factories) { IHttpModule m = factory(this); if (m != null) { m.Init(this); dynamicModules.Add(m); } } } if (dynamicModules.Count != 0) modules = dynamicModules; IsInitialized = true; } public static void RegisterModule(HttpModuleFactory factory) { if (IsInitialized) throw new InvalidOperationException(Exceptions.CannotRegisterModuleLate); if (factory == null) throw new ArgumentNullException("factory"); Factories.Add(factory); } public override void Dispose() { if (modules != null) modules.ForEach(m => m.Dispose()); modules = null; base.Dispose(); GC.SuppressFinalize(this); }


Finally, to get this all wired up we modify the Global.asax to inherit from DynamicHttpApplication:

public class Global : DynamicHttpApplication

{ … }

Like I said, you could just add the validation code into Global (but where’s the fun in that?)…

So, now that we’ve made it possible to add the HTTP Module, lets actually look at the module:

public sealed class SignedConfigurationHttpModule : DynamicallyLoadedHttpModule
    public override void Init(HttpApplication context)
        if (context == null)
            throw new ArgumentNullException("context");

        context.BeginRequest += new EventHandler(context_BeginRequest);
        context.Error += new EventHandler(context_Error);

    private void context_BeginRequest(object sender, EventArgs e)
        HttpApplication app = (HttpApplication)sender;

        SignatureValidator validator 
= new SignatureValidator(app.Request.PhysicalApplicationPath); validator.ValidateConfigurationSignatures(
CertificateLocator.LocateSigningCertificate()); } private void context_Error(object sender, EventArgs e) { HttpApplication app = (HttpApplication)sender; foreach (var exception in app.Context.AllErrors) { if (exception is XmlSignatureValidationFailedException) { // Maybe do something // Or don't... break; } } } public override void Dispose() { } }


Nothing special here. Just hooking into the context.BeginRequest event so validation occurs on each request. There would be some performance impact as a result.

The core validation is contained within the SignatureValidator class, and there is a public method that we call to validate the signature file, ValidateConfigurationSignatures(…). This method accepts an X509Certificate2 to compare the signature against.

The specification for the schema we are using for the signature will actually encode the public key of the private key into the signature element, however we want to go one step further and make sure it’s signed by a particular certificate. This will prevent someone from modifying the configuration file, and re-signing it with a different private key. Validation of the signature is not enough; we need to make sure it’s signed by someone we trust.

The validator first validates the schema of the signature file. Is the XML well formed? Does the signature file conform to a schema we defined (the schema is defined in a Constants class)? Following that is validates the signature of the file itself. Has the file been tampered with? Following that it validates the signature of the web.config file. Has the web.config file been tampered with?

Before it can do all of this though, it needs to check to see if the signature file exists. The variable passed into the constructor is the physical path of the web application. The validator knows that the signature file should be in the App_Data folder within the root. This file needs to be here because the folder by default will not let you access anything in it, and we don’t want anyone downloading the file. The path is also hardcoded specifically so changes to the configuration cannot bypass the signature file validation.

Here is the validator:

internal sealed class SignatureValidator
    public SignatureValidator(string physicalApplicationPath)
        this.physicalApplicationPath = physicalApplicationPath;
        this.signatureFilePath = Path.Combine(this.physicalApplicationPath, 
"App_Data\\Signature.xml"); } private string physicalApplicationPath; private string signatureFilePath; public void ValidateConfigurationSignatures(X509Certificate2 cert) { Permissions.DemandFilePermission(FileIOPermissionAccess.Read, this.signatureFilePath); if (cert == null) throw new ArgumentNullException("cert"); if (cert.HasPrivateKey) throw new SecurityException(Exceptions.ValidationCertificateHasPrivateKey); if (!File.Exists(signatureFilePath)) throw new SecurityException(Exceptions.CouldNotLoadSignatureFile); XmlDocument doc = new XmlDocument() { PreserveWhitespace = true }; doc.Load(signatureFilePath); ValidateXmlSchema(doc); CheckForUnsignedConfig(doc); if (!X509CertificateCompare.Compare(cert, ValidateSignature(doc))) throw new XmlSignatureValidationFailedException(
Exceptions.SignatureFileNotSignedByExpectedCertificate); List<XmlSignature> signatures = ParseSignatures(doc); ValidateSignatures(signatures, cert); } private void CheckForUnsignedConfig(XmlDocument doc) { List<string> signedFiles = new List<string>(); foreach (XmlElement file in doc.GetElementsByTagName("File")) { string fileName = Path.Combine(this.physicalApplicationPath,
file["FileName"].InnerText); signedFiles.Add(fileName.ToUpperInvariant()); } CheckConfigFiles(signedFiles); } private void CheckConfigFiles(List<string> signedFiles) { foreach (string file in Directory.EnumerateFiles(
this.physicalApplicationPath, "*.config", SearchOption.AllDirectories)) { string path = Path.Combine(this.physicalApplicationPath, file); if (!signedFiles.Contains(path.ToUpperInvariant())) throw new XmlSignatureValidationFailedException(
string.Format(CultureInfo.CurrentCulture, Exceptions.ConfigurationFileWithoutSignature, path)); } } private void ValidateXmlSchema(XmlDocument doc) { using (StringReader fileReader = new StringReader(Constants.SignatureFileSchema)) using (StringReader signatureReader = new StringReader(Constants.SignatureSchema)) { XmlSchema fileSchema = XmlSchema.Read(fileReader, null); XmlSchema signatureSchema = XmlSchema.Read(signatureReader, null); doc.Schemas.Add(fileSchema); doc.Schemas.Add(signatureSchema); doc.Validate(Schemas_ValidationEventHandler); } } void Schemas_ValidationEventHandler(object sender, ValidationEventArgs e) { throw new XmlSignatureValidationFailedException(Exceptions.InvalidSchema, e.Exception); } public static X509Certificate2 ValidateSignature(XmlDocument xml) { if (xml == null) throw new ArgumentNullException("xml"); XmlElement signature = ExtractSignature(xml.DocumentElement); return ValidateSignature(xml, signature); } public static X509Certificate2 ValidateSignature(XmlDocument doc, XmlElement signature) { if (doc == null) throw new ArgumentNullException("doc"); if (signature == null) throw new ArgumentNullException("signature"); X509Certificate2 signingCert = null; SignedXml signed = new SignedXml(doc); signed.LoadXml(signature); foreach (KeyInfoClause clause in signed.KeyInfo) { KeyInfoX509Data key = clause as KeyInfoX509Data; if (key == null || key.Certificates.Count != 1) continue; signingCert = (X509Certificate2)key.Certificates[0]; } if (signingCert == null) throw new CryptographicException(Exceptions.SigningKeyNotFound); if (!signed.CheckSignature()) throw new CryptographicException(Exceptions.SignatureValidationFailed); return signingCert; } private static void ValidateSignatures(List<XmlSignature> signatures, X509Certificate2 cert) { foreach (XmlSignature signature in signatures) { X509Certificate2 signingCert
= ValidateSignature(signature.Document, signature.Signature); if (!X509CertificateCompare.Compare(cert, signingCert)) throw new XmlSignatureValidationFailedException( string.Format(CultureInfo.CurrentCulture,
Exceptions.SignatureForFileNotSignedByExpectedCertificate, signature.FileName)); } } private List<XmlSignature> ParseSignatures(XmlDocument doc) { List<XmlSignature> signatures = new List<XmlSignature>(); foreach (XmlElement file in doc.GetElementsByTagName("File")) { string fileName
= Path.Combine(this.physicalApplicationPath, file["FileName"].InnerText); Permissions.DemandFilePermission(FileIOPermissionAccess.Read, fileName); if (!File.Exists(fileName)) throw new FileNotFoundException(
string.Format(CultureInfo.CurrentCulture, Exceptions.FileNotFound, fileName)); XmlDocument fileDoc = new XmlDocument() { PreserveWhitespace = true }; fileDoc.Load(fileName); XmlElement sig = file["FileSignature"] as XmlElement; signatures.Add(new XmlSignature() { FileName = fileName, Document = fileDoc, Signature = ExtractSignature(sig) }); } return signatures; } private static XmlElement ExtractSignature(XmlElement xml) { XmlNodeList xmlSignatureNode = xml.GetElementsByTagName("Signature"); if (xmlSignatureNode.Count <= 0) throw new CryptographicException(Exceptions.SignatureNotFound); return xmlSignatureNode[xmlSignatureNode.Count - 1] as XmlElement; } }


You’ll notice there is a bit of functionality I didn’t mention. Checking that the web.config file hasn’t been modified isn’t enough. We also need to check if any *other* configuration file has been modified. It’s no good if you leave the root configuration file alone, but modify the <authorization> tag within the administration folder to allow anonymous access, right?

So there is code looks through the site for any files that have the “config” extension, and if that file isn’t in the signature file, it throws an exception.

There is also a check done at the very beginning of the validation. If you pass an X509Certificate2 with a private key it will throw an exception. This is absolutely by design. You sign the file with the private key. You validate with the public key. If the private key is present during validation that means you are not separating the keys, and all of this has been a huge waste of time because the private key is not protected. Oops.

Finally, it’s important to know how to sign the files. I’m not a fan of generating XML properly, partially because I’m lazy and partially because it’s a pain to do, so mind the StringBuilder:

public sealed class XmlSigner
    public XmlSigner(string appPath)
        this.physicalApplicationPath = appPath;

    string physicalApplicationPath;

    public XmlDocument SignFiles(string[] paths, X509Certificate2 cert)
        if (paths == null || paths.Length == 0)
            throw new ArgumentNullException("paths");

        if (cert == null || !cert.HasPrivateKey)
            throw new ArgumentNullException("cert");

        XmlDocument doc = new XmlDocument() { PreserveWhitespace = true };
        StringBuilder sb = new StringBuilder();


        foreach (string p in paths)

p.Replace(this.physicalApplicationPath, "")); sb.AppendFormat("<FileSignature><Signature xmlns=\"http://www.w3.org/2000/09/xmldsig#\">{0}</Signature></FileSignature>",
SignFile(p, cert).InnerXml); sb.Append("</File>"); } sb.Append("</Files>"); sb.Append("</Configuration>"); doc.LoadXml(sb.ToString()); doc.DocumentElement.AppendChild(doc.ImportNode(SignXmlDocument(doc, cert), true)); return doc; } public static XmlElement SignFile(string path, X509Certificate2 cert) { if (string.IsNullOrWhiteSpace(path)) throw new ArgumentNullException("path"); if (cert == null || !cert.HasPrivateKey) throw new ArgumentException(Exceptions.CertificateDoesNotContainPrivateKey); Permissions.DemandFilePermission(FileIOPermissionAccess.Read, path); XmlDocument doc = new XmlDocument(); doc.PreserveWhitespace = true; doc.Load(path); return SignXmlDocument(doc, cert); } public static XmlElement SignXmlDocument(XmlDocument doc, X509Certificate2 cert) { if (doc == null) throw new ArgumentNullException("doc"); if (cert == null || !cert.HasPrivateKey) throw new ArgumentException(Exceptions.CertificateDoesNotContainPrivateKey); SignedXml signed = new SignedXml(doc) { SigningKey = cert.PrivateKey }; Reference reference = new Reference() { Uri = "" }; XmlDsigC14NTransform transform = new XmlDsigC14NTransform(); reference.AddTransform(transform); XmlDsigEnvelopedSignatureTransform envelope = new XmlDsigEnvelopedSignatureTransform(); reference.AddTransform(envelope); signed.AddReference(reference); KeyInfo keyInfo = new KeyInfo(); keyInfo.AddClause(new KeyInfoX509Data(cert)); signed.KeyInfo = keyInfo; signed.ComputeSignature(); XmlElement xmlSignature = signed.GetXml(); return xmlSignature; } }


To write this to a file you can call it like this:

XmlWriter writer = XmlWriter.Create(
@"C:\Dev\Projects\Syfuhs.Security.Web\Syfuhs.Security.Web.WebTest\App_Data\Signature.xml"); XmlSigner signer = new XmlSigner(Request.PhysicalApplicationPath); XmlDocument xml = signer.SignFiles(new string[] { @"C:\Dev\Projects\Syfuhs.Security.Web\Syfuhs.Security.Web.WebTest\Web.config", @"C:\Dev\Projects\Syfuhs.Security.Web\Syfuhs.Security.Web.WebTest\Web.debug.config", @"C:\Dev\Projects\Syfuhs.Security.Web\Syfuhs.Security.Web.WebTest\Web.release.config", @"C:\Dev\Projects\Syfuhs.Security.Web\Syfuhs.Security.Web.WebTest\Account\Web.config", @"C:\Dev\Projects\Syfuhs.Security.Web\Syfuhs.Security.Web.WebTest\test.config" }, new X509Certificate2(
@"C:\Dev\Projects\Syfuhs.Security.Web\Syfuhs.Security.Web.WebTest\cert.pfx", "1")); xml.WriteTo(writer); writer.Flush();


Now within this code, you have to pass in a X509Certificate2 with a private key, otherwise you can’t sign the files.

These processes should occur on different machines. The private key should never be on the server hosting the site. The basic steps for deployment would go something like:

1. Compile web application.
2. Configure site and configuration files on staging server.
3. Run application that signs the configuration and generates the signature file.
4. Drop the signature.xml file into the App_Data folder.
5. Deploy configured and signed application to production.

There is one final note (I think I’ve made that note a few times by now…) and that is the CertificateLocator class. At the moment it just returns a X509Certificate2 from a particular path on my file system. This isn’t necessarily the best approach because it may be possible to overwrite that file. You should store that certificate in a safe place, and make a secure call to get it. For instance a web service call might make sense. If you have a Hardware Security Module (HSM) to store secret bits in, even better.

Concluding Bits

What have we accomplished by signing our configuration files? We add a degree of trust that our application hasn’t been compromised. In the event that the configuration has been modified, the application stops working. This could be from malicious intent, or careless administrators. This is a great way to prevent one-off changes to configuration files in web farms. It is also a great way to prevent customers from mucking up the configuration file you’ve deployed with your application.

This solution was designed in a way mitigate quite a few attacks. An attacker cannot modify configuration files. An attacker cannot modify the signature file. An attacker cannot view the signature file. An attacker cannot remove the signature file. An attacker cannot remove the HTTP Module that validates the signature without changing the underlying code. An attacker cannot change the underlying code because it’s been compiled before being deployed.

Is it necessary to use on every deployment? No, probably not.

Does it go a little overboard with regard to complexity? Yeah, a little.

Does it protect against a real problem? Absolutely.

Unfortunately it also requires full trust.

Overall it’s a fairly robust solution and shows how you can mitigate certain types of risks seen in the real world.

And of course, it works with both WebForms and MVC.

You can download the full source here.

Find my Windows Phone 7

For the last month and a half I’ve been playing around with my new Windows Phone 7.  Needless to say, I really like it.  There are a few things that are still a little rough – side-loading application is a good example, but overall I’m really impressed with this platform.  It may be version 7 technically, but realistically its a v1 product.  I say that in a good way though – Microsoft reinvented the product.

Part of this reinvention is a cloud-oriented platform.  Today’s Dilbert cartoon was a perfect tongue-in-cheek explanation of the evolution of computing, and the mobile market makes no exception.  Actually, when you think about it, mobile phones and the cloud go together like peanut butter and chocolate.  If you have to ask, they go together really well.  Also, if you have to ask, are you living under a rock?

This whole cloud/phone comingling is central to the Windows Phone 7, and you can realize the potential immediately.

When you start syncing your phone via the Zune software, you will eventually get to the sync page for the phone.  The first thing I noticed was the link “do more with windows live”.


What does that do?

Well, once you have set up your phone with your Live ID, a new application is added to your Windows Live home.  This app is for all devices, and when you click on the above link in Zune, it will take you to the section for the particular phone you are syncing.


The first thing that caught my attention was the “Find my Phone” feature.  It brings up a list of actions for when you have lost your phone.


Each action is progressively bolder than the previous – and each action is very straightforward.

Map it

If the device is on, use the Location services on the phone to find it and display on a Bing Map.

Ring it

If you have a basic idea of where the phone is and the phone is on, ringing it will make the phone ring with a distinct tone even if you have it set to silent or vibrate.  Use this wisely. Smile

Lock it

Now it gets a little more complicated.  When you lock the phone you are given an option to provide a message on the lock screen:


If someone comes across your phone, you can set a message telling them what they can do with it.  Word of advice though: if you leave a phone number, don’t leave your mobile number. Winking smile

Erase it

Finally we have the last option.  The nuclear option if you will.  Once you set the phone to be erased, the next time the phone is turned on and tries to connect to the Live Network, the phone will be wiped and set to factory defaults.

A side effect of wiping your phone is that the next time you set it up and sync with the same Live ID, most settings will remain intact.  You will have to add your email and Facebook accounts, and set all the device settings, but once you sync with Zune, all of your apps will be reinstalled.  Now that is a useful little feature.


Overall I’m really happy with how the phone turned out.  It’s a strong platform and it’s growing quickly.  The Find my Phone feature is a relatively small thing, but it showcases the potential of a phone/cloud mash up and adds so much value to consumers for when the lose their phone.

In a previous post I talked about the security of the Windows Phone 7.  This post was all about how consumers can quickly mitigate any risks from losing their phone.  For more information on using this phone in the enterprise, check out the Windows Phone 7 Guides for IT Professionals.