Best Pattern for AllowUnsafeUpdates

Question

So far, in my research I have seen that it is unwise to set AllowUnsafeUpdates on GET request operation to avoid cross site scripting. But, if it is required to allow this, what is the proper way to handle the situation to mitigate any exposure?

Here is my best first guess on a reliable pattern if you absolutely need to allow web or site updates on a GET request.

Best Practice?

protected override void OnLoad(System.EventArgs e)
{
    if(Request.HttpMethod == "POST")
    {
        SPUtility.ValidateFormDigest();
        // will automatically set AllowSafeUpdates to true
    }

    // If not a POST then AllowUnsafeUpdates should be used only
    // at the point of update and reset immediately after finished

    // NOTE: Is this true? How is cross-site scripting used on GET
    // and what mitigates the vulnerability?
}

// Point of item update

    using(SPSite site = new SPSite(SPContext.Current.Site.Url, SPContext.Current.Site.SystemAccount.UserToken))
    {
        using (SPWeb web = site.RootWeb)
        {
            bool allowUpdates = web.AllowUnsafeUpdates; //store original value
            web.AllowUnsafeUpdates = true;

            //... Do something and call Update() ...

            web.AllowUnsafeUpdates = allowUpdates; //restore original value

        }
    }

Feedback on the best pattern is appreciated.

Answer 1

If you're performing any operations which modify something, then anyone that can convince the user to click on a link can perform that operation. For instance, let's assume that you have a GET request to a page which lets the user add an administrator to a site, and the user clicks a link to a page which does a Response.Redirect("http://yourserver/_layouts/admin.aspx?operation=addAdministrator&username=attackerNameHere").

While normally a POST does not offer much protection against this (nothing will stop someone from having a <form method="post" action="http://yourserver/_layouts/admin.aspx">), SharePoint has a concept of form digests, which contain information about the previous request that is generating the post back (including the user's name). This reduces the footprint for this kind of attack significantly.

The only time that it is not a security issue to AllowUnsafeUpdates on a GET is if you're not taking input from the user. For instance, if you have a web part which also logs visits to a list, then there's no security vulnerability exposed.

Edit: If you are going to use AllowUnsafeUpdates, there's no need to reset it to its previous value. It does not get persisted. It's just something you need to set on an SPWeb object before performing updates from a GET (or other cases)

Answer 2

Another clean way to implement would be to use a combination of extension methods and anonymous delegates as such:

public static void DoUnsafeUpdate(this SPWeb web, Action action)
{
    bool allowUnsafeUpdates = web.AllowUnsafeUpdates;
    web.AllowUnsafeUpdates = true;
    action();
    web.AllowUnsafeUpdates = allowUnsafeUpdates;
}

Using the above extension method, you can then perform your "unsafe update" action as follows:

var web = SPContext.Current.Web;
web.DoUnsafeUpdate(delegate()
{
    // Put your "unsafe update" code here
});

Answer 3

I would slightly modify Trent's delegate to accept the web to update:

public static void DoUnsafeUpdate(this SPWeb web, Action<SPWeb> action)
{
    try
    {
        web.AllowUnsafeUpdates = true;
        action(web);
    }
    finally
    {
        web.AllowUnsafeUpdates = false;
    }
}

And then extend HttpContext to encapsulate verification of the form digest, with an option to elevate using the technique described here:

public static void DoUnsafeUpdate(this HttpContext context, Action<SPWeb> action, bool elevated)
{
    SPWeb web = SPControl.GetContextWeb(context);
    if (!context.Request.HttpMethod.Equals("POST", StringComparison.Ordinal)
        || web.ValidateFormDigest())
        throw new SPException("Error validating postback digest");

    if (elevated)
        web.RunAsSystem(w => w.DoUnsafeUpdate(action));
    else
        web.DoUnsafeUpdate(action);
}

Usage:

protected override void OnLoad(System.EventArgs e)
{
    Context.DoUnsafeUpdate(web =>
    {
        // Update elevated web
    }, true);
}

Answer 4

I use a wrapper class for handling most manipulation of SPWeb objects. This helps me remember to close the web, and it eases the problems of unsafeupdates setting. It is a bit bloated, as I have patched on new constructors and members. but, then again; so is the SPWeb class.

Usage:

using (WebWrapper wrapper = new WebWrapper("http://localhost"))
            {
                wrapper.AllowUnsafeUpdates();

                //Do work on wrapper.
            }

The class definition:

using System;
using System.Collections.Specialized;
using System.Data;
using System.Diagnostics;
using System.Globalization;
using System.Runtime.Serialization;
using Microsoft.SharePoint;
using Microsoft.SharePoint.Administration;

namespace Skaar.SharePoint.Customization
{
    /// <summary>
    /// A wrapper for a <see cref="SPWeb"/> object.
    /// <remarks>Closes web object on Dispose if applicable.</remarks>
    /// </summary>
    [Serializable]
    [DebuggerDisplay("{Uri} Unsafe:{AllowUnsafeUpdatesSetting} Update:{UpdatePending}")]
    public sealed class WebWrapper : IDisposable, IDeserializationCallback, IEquatable<WebWrapper>
    {
        [NonSerialized] private bool unsafeUpdatesSetting;

        [NonSerialized] private SPWeb web;

        /// <summary>
        /// Determines if the inner web object should be closed.
        /// </summary>
        [NonSerialized] private bool webShouldBeClosed;

        /// <summary>
        /// This value is used in serialization to restore <see cref="Web"/>.
        /// </summary>
        private string webUrl;

        /// <summary>
        /// Creates a new wrapper object.
        /// </summary>
        /// <param name="web">A web that should be closed/disposed when done.</param>
        public WebWrapper(SPWeb web) : this(web, true)
        {
        }

        /// <summary>
        /// Creates a new wrapper object.
        /// </summary>
        /// <param name="web">An inner web object</param>
        /// <param name="webShouldBeClosed">If true, the web object is closed in the <see cref="Dispose()"/> method.</param>
        public WebWrapper(SPWeb web, bool webShouldBeClosed)
        {
            setWeb(web, webShouldBeClosed);
        }

        /// <summary>
        /// Creates a new wrapper object.
        /// </summary>
        /// <param name="webAddress">The address to a web.</param>
        public WebWrapper(Uri webAddress)
        {
            using (SPSite site = new SPSite(webAddress.ToString()))
            {
                string relativeUrl = renderWebRootRelativeUrl(webAddress);
                if (relativeUrl == null)
                {
                    setWeb(site.OpenWeb(), true);
                }
                else
                {
                    setWeb(site.OpenWeb(relativeUrl), true);
                }
            }
        }

        private string renderWebRootRelativeUrl(Uri address)
        {
            for (int i = 0; i < address.Segments.Length; i++)
            {
                string segment = address.Segments[i];
                if (string.Equals(segment, "_layouts/"))
                {
                    string newUrl=string.Join(null, address.Segments, 0, i).Trim('/');
                    return newUrl;
                }
            }
            return null;
        }

        /// <summary>
        /// If true, <see cref="SPWeb.Update"/> will be called in <see cref="Dispose()"/>.
        /// </summary>
        public bool UpdatePending { get; private set; }

        /// <summary>
        /// The setting of the inner web (<see cref="SPWeb.AllowUnsafeUpdates"/>)
        /// </summary>
        public bool AllowUnsafeUpdatesSetting
        {
            get { return Web.AllowUnsafeUpdates; }
        }

        /// <summary>
        /// The inner object.
        /// </summary>
        /// <exception cref="ObjectDisposedException">Exception is thrown if <see cref="IsDisposed"/> is true.</exception>
        public SPWeb Web
        {
            get
            {
                if(IsDisposed)
                {
                    throw new ObjectDisposedException("Web wrapper is disposed.");
                }
                return web;
            }
        }

        /// <summary>
        /// The address of the <see cref="Web"/> wrapped as a <see cref="Uri"/> object.
        /// </summary>
        public Uri Uri
        {
            get { return new Uri(Web.Url); }
        }

        /// <summary>
        /// The address of the <see cref="Web"/> wrapped as a <see cref="Uri"/> object.
        /// </summary>
        public Uri GetUri(SPUrlZone zone)
        {
            return Site.WebApplication.GetResponseUri(zone, Uri.AbsolutePath);
        }

        /// <summary>
        /// Creates a wrapper around the context web.
        /// <remarks>The web will not be closed when wrapper is disposed. Returns null if context is unavailable.</remarks>
        /// </summary>
        public static WebWrapper Context
        {
            get
            {
                return SPContext.Current==null?null:new WebWrapper(SPContext.Current.Web, false);
            }
        }

        /// <summary>
        /// This is a static property wrapping of
        /// the <see cref="CloneOf(SPWeb)"/> method, using
        /// the <see cref="SPContext"/> current web as
        /// parameter.
        /// <remarks>Returns null if context is unavailable.</remarks>
        /// </summary>
        public static WebWrapper CloneOfContext
        {
            get
            {
                if (SPContext.Current != null)
                {
                    SPWeb contextWeb = SPContext.Current.Web;
                    return CloneOf(contextWeb);
                }
                return null;
            }
        }

        /// <summary>
        /// Returns the <see cref="SPWeb.Exists"/> property of the <see cref="Web"/> object.
        /// </summary>
        public bool Exists
        {
            get { return Web != null && Web.Exists; }
        }

        /// <summary>
        /// Gets the <see cref="SPSite"/> object of <see cref="Web"/>.
        /// </summary>
        /// <remarks>This object should not be closed by user code.</remarks>
        public SPSite Site
        {
            get { return web.Site; }
        }

        /// <summary>
        /// Gets the owner defined in <see cref="SPSite.Owner"/>.
        /// </summary>
        public SPUser Owner
        {
            get
            {
                return Site.Owner;
            }
        }

        /// <summary>
        /// Returns a context of the inner <see cref="Web"/>.
        /// </summary>
        public SPContext ContextOfWeb
        {
            get { return SPContext.GetContext(web); }
        }

        /// <summary>
        /// Gets the language of <see cref="Web"/>.
        /// </summary>
        public CultureInfo Locale
        {
            get { return Web.Locale; }
        }

        /// <summary>
        /// Gets the language of the root web.
        /// </summary>
        public CultureInfo LocaleOfRoot
        {
            get
            {
                using (WebWrapper root = Root)
                {
                    return root.Locale;
                }
            }
        }

        /// <summary>
        /// Returns a new <see cref="WebWrapper"/> wrapping the root <see cref="SPWeb"/> of this.
        /// </summary>
        public WebWrapper Root
        {
            get
            {
                if (webShouldBeClosed)
                    using (SPSite site = Site)
                    {
                        return new WebWrapper(site.RootWeb);
                    }
                return new WebWrapper(Site.RootWeb);
            }
        }

        /// <summary>
        /// A wrapper for <see cref="SPWeb.Title"/>.
        /// </summary>
        public string Title
        {
            get { return Web.Title; }
            set { Web.Title = value; }
        }

        /// <summary>
        /// A wrapper for <see cref="SPWeb.ID"/>.
        /// </summary>
        public Guid ID
        {
            get { return Web.ID; }
        }

        #region Web Properties

        [NonSerialized] private bool updatePropertiesPending;

        /// <summary>
        /// A wrapper method to <see cref="Web"/> object's <see cref="SPWeb.Properties"/> indexer.
        /// </summary>
        /// <param name="key">The key to use when fetching property value.</param>
        /// <returns>A string containing the value.</returns>
        public string GetProperty(string key)
        {
            return Web.Properties[key];
        }

        /// <summary>
        /// Sets the value in the <see cref="Web"/> object's <see cref="SPWeb.Properties"/>. Creates a new key, or updates an existing as needed.
        /// </summary>
        /// <param name="key">The key to use when storing the property value.</param>
        /// <param name="value">The value to set in the key.</param>
        /// <remarks>The property <see cref="UpdatePending"/> is set to true.</remarks>
        public void SetProperty(string key, string value)
        {
            if (!Web.Properties.ContainsKey(key))
            {
                Web.Properties.Add(key, value);
            }
            else
            {
                Web.Properties[key] = value;
            }
            updatePropertiesPending = true;
        }

        #endregion

        #region IDeserializationCallback Members

        ///<summary>
        ///Runs when the entire object graph has been deserialized.
        ///</summary>
        ///
        ///<param name="sender">The object that initiated the callback. The functionality for this parameter is not currently implemented. </param>
        public void OnDeserialization(object sender)
        {
            using (SPSite site = new SPSite(webUrl))
            {
                setWeb(site.OpenWeb(), true);
            }
        }

        #endregion

        #region IDisposable Members

        ///<summary>
        ///Closes inner web object if appropriate.
        ///</summary>
        ///<filterpriority>2</filterpriority>
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        public void Dispose(bool isDisposing)
        {
            if (IsDisposed) return;
            if (isDisposing)
            {
                doDisposeOfWeb();
                IsDisposed = true;
            }
        }

        #endregion

        /// <summary>
        /// Value is true if <see cref="Dispose()"/> method has been called. Object is not in a usable state.
        /// </summary>
        internal bool IsDisposed
        {
            get; private set;
        }

        #region IEquatable<WebWrapper> Members

        /// <summary>
        /// This tests whether the two objects wraps the same web. It may however be two different instances of the same web.
        /// </summary>
        /// <param name="other">Another wrapper object.</param>
        /// <returns>True if <see cref="Uri"/> equals, false otherwise.</returns>
        public bool Equals(WebWrapper other)
        {
            if (other == null)
            {
                return false;
            }
            return Uri.Equals(other.Uri);
        }

        #endregion

        /// <summary>
        /// Reopens the inner <see cref="SPWeb"/> object. May be used when web object needs to be rereferenced in a new security context.
        /// </summary>
        public void ReOpen()
        {
            bool unsafeSetting = AllowUnsafeUpdatesSetting;
            using (SPSite site = new SPSite(Web.Url))
            {
                SPWeb newWeb = site.OpenWeb();
                doDisposeOfWeb();
                web = newWeb;
                web.AllowUnsafeUpdates = unsafeSetting;
                unsafeUpdatesSetting = false;
                webShouldBeClosed = true;
            }
        }

        private void doDisposeOfWeb()
        {
            if (Web == null) return;
            Update(true);
            if (webShouldBeClosed)
            {
                Web.Close();
            }
            else if (Web.Exists)
            {
                Web.AllowUnsafeUpdates = unsafeUpdatesSetting;
            }
            web = null;
        }

        /// <summary>
        /// Calls <see cref="SPWeb.Update"/> on the <see cref="Web"/> object.
        /// </summary>
        public void Update()
        {
            Update(false);
        }

        /// <summary>
        /// Sets <see cref="UpdatePending"/> to <c>true</c>.
        /// </summary>
        public void SetUpdatePending()
        {
            UpdatePending = true;
        }

        /// <summary>
        /// Calls <see cref="SPWeb.Update"/> on the <see cref="Web"/> object.
        /// <param name="onlyIfPending">If true, update will depend on state of the <see cref="UpdatePending"/> property.</param>
        /// </summary>
        public void Update(bool onlyIfPending)
        {
            if (onlyIfPending)
            {
                if (updatePropertiesPending)
                {
                    Web.Properties.Update();
                    updatePropertiesPending = false;
                }
                if (UpdatePending)
                {
                    Web.Update();
                    UpdatePending = false;
                }
            }
            else
            {
                Web.Update();
                UpdatePending = false;
            }
        }

        /// <summary>
        /// Returns the list from <see cref="Web"/> with <see cref="SPList.Title"/> equal to <see cref="title"/>.
        /// </summary>
        /// <param name="title">The <see cref="SPList.Title"/> of an existing list.</param>
        /// <returns>The first list found with the given title, or null, if no list is found.</returns>
        public SPList GetList(string title)
        {
            foreach (SPList list in Web.Lists)
            {
                if (list.Title == title)
                {
                    return list;
                }
            }
            return null;
        }
        /// <summary>
        /// A wrapper method to the <see cref="Web"/> object's <see cref="SPWeb.Lists"/> indexer. 
        /// </summary>
        /// <param name="id">The id of the list to return.</param>
        /// <returns>The list with the supplied id.</returns>
        public SPList GetList(Guid id)
        {
            return Web.Lists[id];
        }

        private void setWeb(SPWeb innerWeb, bool shouldBeClosed)
        {
            if (innerWeb == null || !innerWeb.Exists)
            {
                throw new ArgumentException("Web does not exist", "innerWeb");
            }
            web = innerWeb;
            webShouldBeClosed = shouldBeClosed;
            unsafeUpdatesSetting = innerWeb.AllowUnsafeUpdates;
            AllowUnsafeUpdates();
            webUrl = web.Url;
        }

        /// <summary>
        /// Creates a new <see cref="SPWeb"/> object using the
        /// url of the <see cref="web"/> parameter and wraps it
        /// in a new wrapper object. The web will be
        /// closed when the wrapper is disposed.
        /// The cloning is done using the <see cref="SPWeb.Url"/>, thus no security context is transferred to the new web.
        /// </summary>
        /// <remarks>Use this to create a clone of the context web.</remarks>
        /// <param name="web">The web to clone.</param>
        /// <returns>A new wrapper object.</returns>
        public static WebWrapper CloneOf(SPWeb web)
        {
            using (SPSite site = new SPSite(web.Url))
            {
                return new WebWrapper(site.OpenWeb());
            }
        }


        /// <summary>
        /// Creates a new <see cref="SPWeb"/> object using the
        /// <see cref="Web"/> of the <see cref="web"/> parameter and wraps it
        /// in a new wrapper object. The web will be
        /// closed when the wrapper is disposed.
        /// </summary>
        /// <remarks>Use this to create a clone of the context web.</remarks>
        /// <param name="web">The wrapper to clone.</param>
        /// <returns>A new wrapper object.</returns>
        public static WebWrapper CloneOf(WebWrapper web)
        {
            return CloneOf(web.Web);
        }

        /// <summary>
        /// Sets the AllowUnsafeUpdates property to true on the
        /// wrapped web object.
        /// <remarks>
        /// The setting is resat back in the dispose method, unless the
        /// web itself is closed.
        /// </remarks>
        /// </summary>
        public void AllowUnsafeUpdates()
        {
            Web.AllowUnsafeUpdates = true;
        }

        /// <summary>
        /// Returns the url of the inner web.
        /// </summary>
        /// <returns>A value that equals <see cref="Web"/> <see cref="SPWeb.Url"/> property.</returns>
        public override string ToString()
        {
            return webUrl;
        }

        /// <summary>
        /// Returns a new <see cref="WebWrapper"/> object wrapping a new copy of the inner <see cref="Web"/> object.
        /// The cloning is done using the <see cref="SPWeb.Url"/>, thus no security context is transferred to the new web.
        /// </summary>
        /// <remarks>The static method <see cref="CloneOf(SPWeb)"/> is used on the <see cref="Web"/> property.</remarks>
        /// <returns>A new wrapper.</returns>
        public WebWrapper Clone()
        {
            return CloneOf(Web);
        }

        /// <summary>
        /// Implicitly wraps the web object in a <see cref="WebWrapper"/> object.
        /// </summary>
        /// <param name="web">The web to wrap.</param>
        /// <returns>A new wrapper object. The original web may be accessed through the <see cref="Web"/> property.</returns>
        public static implicit operator WebWrapper(SPWeb web)
        {
            return new WebWrapper(web, false);
        }

        /// <summary>
        /// Explicitly extracts the <see cref="Web"/> value from the <see cref="wrapper"/>.
        /// </summary>
        /// <param name="wrapper">The object wrapping the <see cref="SPWeb"/> to extract.</param>
        /// <returns>The inner <see cref="Web"/> of <see cref="wrapper"/>.</returns>
        /// <remarks>The returned <see cref="SPWeb"/> object should be properly disposed after use.</remarks>
        public static explicit operator SPWeb(WebWrapper wrapper)
        {
            wrapper.DoNotDisposeInnerWeb();
            return wrapper.Web;
        }

        /// <summary>
        /// Wrapper method for <see cref="SPWeb.GetList"/> on <see cref="Web"/> object.
        /// </summary>
        /// <param name="uri">A site relative uri to the list.</param>
        /// <returns>A list if found.</returns>
        public SPList GetList(Uri uri)
        {
            return web.GetList(uri.ToString());
        }

        /// <summary>
        /// Wrapper method for <see cref="SPWeb.GetSiteData"/> on <see cref="Web"/> object.
        /// </summary>
        /// <returns>The results of the query,</returns>
        public DataTable GetSiteData(SPSiteDataQuery query)
        {
            return Web.GetSiteData(query);
        }

        /// <summary>
        /// Creates a new <see cref="SPWeb"/> as a sub web to this.
        /// </summary>
        /// <param name="url">The proposed local url of the new web. The nearest available is selected.</param>
        /// <param name="name">The title of the new web.</param>
        /// <param name="description">The description of the new web.</param>
        /// <param name="language">The language of the new web. <remarks>If the language is not supported, the language of this is used.</remarks></param>
        /// <param name="template">The site template to use.</param>
        /// <returns>The new web wrapped in a new <see cref="WebWrapper"/> object.</returns>
        [DebuggerStepThrough]
        //debugger step through is to prevent this method to break when debugging, as it throws exceptions by [poor] design.
        public WebWrapper CreateSubWeb(string url, string name, string description, uint language,
                                       string template)
        {
            SPWeb newWeb;
            try
            {
                newWeb = Web.Webs.Add(findSuitableWebUrl(url), name, description, language, template, true, false);
            }
            catch (SPException err)
            {
                if (err.ErrorCode == -2130575266)
                {
                    //language not supported. Fallback to parent web language
                    newWeb = Web.Webs.Add(findSuitableWebUrl(url), name, description, Web.Language, template, true,
                                          false);
                }
                else
                    throw;
            }
            return new WebWrapper(newWeb);
        }

        private string findSuitableWebUrl(string proposedName)
        {
            StringCollection names = new StringCollection();
            names.AddRange(Web.Webs.Names);
            int suffixIndex = 0;
            const int maxIterations = 100000;
            string name = proposedName;
            while (names.Contains(name) && suffixIndex < maxIterations)
            {
                name = string.Format("{0}_{1}", proposedName, suffixIndex++);
            }
            return name;
        }

        /// <summary>
        /// Calling this method will inhibit the default behaviour of closing the web on disposal.
        /// </summary>
        /// <remarks>Use with caution.</remarks>
        internal void DoNotDisposeInnerWeb()
        {
            webShouldBeClosed = false;
        }

    }
}

Answer 5

For AllowUnsafeUpdates, I follow this process:

if( HttpContext.Current is null )
{
  Do nothing, no need to set AllowUnsafeUpdates to true nor
  to call ValidateFormDigest() because update will be carried out
}
else // HttpContext.Current is NOT null
{
  if( SPContext.Current is null )
  {
    Need to set AllowUnsafeUpdates to true
  }
  else // SPContext.Current is NOT null
  {
    Call ValidateFormDigest()
  }
}

Answer 6

Not so sure it is worth remembering the previous value of allow unsafe updates.

I would want to wrap the call around the minimum possible amount of code, so that nested calls to it would not occur.

Then you can just turn it to false afterwards.