Une faille pour Internet Explorer de plus (on en est à combien ?) avec les comptes Hotmail et Yahoo

Bof, une de plus ou une de moins ça ne changera pas grand chose ! Cette faille découverte par une firme de sécurité israélienne permettrait à une personne de mal intentionné de passer outre la confidentialité des comptes et pendant le chargement de Hotmail ou Yahoo, récupérer les mots de passes et d'accéder aux différents comptes.

La faille exploite le module "HTML+TIME". Après la découverte, GreyMagic a avertit Microsoft le 11 mars dernier et celui-ci a corrigé les serveurs en 48H. Ainsi les utilisateurs de Hotmail sont hors de danger mais quand est il pour Yahoo ? Personne ne sait si la situation a évolué !

Voici la description fournit par GreyMagic (en Anglais)

GreyMagic Security Advisory GM#005-MC

- By GreyMagic Software, Israel.

- 23 Mar 2004. Topic: Remotely Exploitable Cross-Site Scripting in Hotmail and Yahoo.

Discovery date: 06 Mar 2004.

Affected applications: Hotmail web-based email service (when used with IE).

Yahoo web-based email service (when used with IE).

Note that many other web-based services may be vulnerable to this method of exploitation, as it is a completely new way to embed script.

Introduction: Both Hotmail and Yahoo make tremendous efforts to sanitize incoming emails from potentially unsafe HTML content. Flawed filtering of such unsafe content may result in severe consequences that would occur as soon as a user opens an email for reading, including:

-* Theft of login and password.
-* Content disclosure of any email in the mailbox.
-* Automatically send emails from the mailbox.
-* Exploitation of known vulnerabilities in the browser to access the user's file system and eventually take over the machine.
-* Distribution of a web-based email worm.
-* Disclosure of all contacts within the address book.

Discussion: GreyMagic devised a method to inject such arbitrary (potentially malicious) content to a Yahoo or Hotmail email message. The method is not limited to Hotmail and Yahoo alone though, it may apply to other web-based services that attempt to filter HTML input.

The vulnerability makes use of an Internet Explorer technology called HTML+TIME (based on SMIL), which is meant to add timing and media synchronization support to HTML pages.

One of the features included in HTML+TIME is the ability to manipulate any attribute on an element via special control elements. For example, the element exposes the attributes "attributeName" and "to", which make it possible to inject ANY HTML content to the document when "attributeName" is set to "innerHTML" and "to" is set to any HTML the attacker would like to execute, including script.

Exploit: For the HTML+TIME module to be activated, the document must fulfill two requirements. It must declare the designated namespace and it must bind the namespace to the HTML+TIME behavior implementation.

In order to fulfill the first requirement it is usually necessary to be able to access the element, with the syntax . However, Hotmail completely filters out that element, so another method of namespace declaration is needed. It so happens that Internet Explorer provides one other mechanism to declare a namespace, via the non-standard processing instruction, which may be used anywhere in the document and does not get filtered.
The second requirement usually involves the use of the CSS "behavior" property, with the syntax "behavior:url(#default#time)". However, Hotmail blocks all instances of "url(...)" in the incoming mail, so another way to bind the behavior must be used. It comes in the form of the
< ?import namespace="t" implementation="#default#time2">
Optional text here...
< div >

< /div >

Demonstation: We put together a proof of concept demonstration for this method of script execution.

Solution: GreyMagic started work on this issue with Microsoft on 11-Mar-2004. They have quickly confirmed our findings and were able to produce a fix less than two days later. As a result, Hotmail is no longer vulnerable to this method of exploitation.
All attempts to contact Yahoo unfortunately failed. Mail was sent to security and secure at yahoo.com and at yahoo-inc.com, no replies were received to date.
Tested on: Hotmail.

Disclaimer: The information in this advisory and any of its demonstrations is provided "as is" without warranty of any kind.

GreyMagic Software is not liable for any direct or indirect damages caused as a result of using the information or demonstrations provided in any part of this advisory.

démonstration : [http://www.greymagic.com/security/advisories/gm005-mc/inject.asp-> http://www.greymagic.com/security/advisories/gm005-mc/inject.asp]

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