Advisory: Insecure Redirect in Microsoft SharePoint

By Irene Abezgauz September 13th, 2011
This vulnerability was discovered by Seeker®


Overview
An Insecure Redirect vulnerability has been identified in Microsoft SharePoint shared infrastructure. This vulnerability allows an attacker to craft links that contain redirects to malicious sites in the source parameter used throughout SharePoint portal.

The exploitation technique detailed in this document bypasses the cross application redirection restriction which normally limits such redirects restricting access to external sites.

Details
Multiple pages and components in Microsoft Sharepoint use the source parameter to redirect users to a new location after accessing a certain page, such as:

POST 
  /Docs/Lists/Announcements/NewForm.aspx?Source=http%3a%2f%2f127.0.0.1%2fDocs%2fdefault.aspx
In order to avoid cross application redirects (which pose a threat to the system), Microsoft Sharepoint enforces checks on these redirects, and limits them to localhost or 127.0.0.1, or the SharePoint server IP (the IP redirect is only valid if the redirect is to an actual SharePoint page on the server, redirects to localhost or 127.0.0.1 will work regardless of existence of relevant page).

The implementation of this verification, however, is flawed, and can be circumvented by creating hostnames which begin with the string localhost, or 127.0.0.1 even if they are not localhost.

Due to domain naming restrictions the 127.0.0.1 prefix cannot be used in exploitation, as http://127.0.0.1.quotium.com is not a valid domain name – subdomain names cannot be digits only. However, redirects to http://localhost.quotium.com or http://localhostie.quotium.com are valid. The following prefixes can be provided into the Source parameter to exploit this vulnerability:
localhostaaa, localhost.quotium.com, etc.

An attacker can generate an attack by creating a site containing localhost in its name, and crafting a URL which embeds into the source parameter a link that lead to sites outside the current application. Once a victim follows the specially crafted link he indeed arrives at the selected page of the vulnerable SharePoint application. Once the page operation is completed, the user will be redirected to the URL in the source parameter.

Exploit
Sample exploitation of this vulnerability would be crafting the following link:

http://MySharePoint/Docs/Lists/Announcements/NewForm.aspx?Source=http%3a%2f%2flocalhost.quotium.com
It is important to note that in many situations, even if the application does not use the source parameter by default, this parameter can be added manually to the URL, leading to exploitation of this vulnerability.

Affected Systems
Microsoft Sharepoint 2007
Microsoft Sharepoint 2010

Solution
Microsoft has released a fix for this vulnerability, see http://technet.microsoft.com/security/bulletin/MS11-074 for further information.

Credit
The vulnerability was automatically discovered by Seeker® - New generation application security testing solution, utilising ground breaking BRITE™ technology (Behavioral Runtime Intelligent Testing Engine).

Further research and publication was performed by Irene Abezgauz, Product Manager, Seeker.
For more information about Seeker please visit www.quotium.com