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''Extensible Stylesheet Language Transformation (XSLT)'' is a language for transforming XML documents into other documents, for example, XML, HTML, JSON or even PDF. The XML Signature standard allows the usage of ''XSLT'' by definition, and thus, ''XSLT'' can be used in [http://www.sso-attacks.org/index.php?title=SAML SAML]. ''XSLT'' is a Turing complete language. By this means, it is possible to use XSLT, for example, to read/write files on the local filesystem and send them over the Internet. Furthermore, the ''XSLT'' transformation will be executed before the digital signature is verified. Thus, an attacker can send a ''SAML'' token including a digital signature containing the ''XSLT Attack (XSLTA)'' vector, but it is not required that the signature is valid.
XSLTA allows accessing files within the context of the used web server.
There are no attack subtypes for this attack.
=Prerequisites for attack=
In order to start XSLT, the attacker has to create a valid XML message containing a [https://en.wikipedia.org/wiki/Document_type_definition DTD]. Note, that the message has to be a SAML token. However, this token does not have to be signed with a valid key nor the signature needs to be valid.
=Graphical representation of attack =
The attacker prepares a SAML token ''t'' and creates an XML Signature for it. Note, that it is not important to have a correctly computed signature value – the ''XSLTA'' only requires a well- formed XML document. The attacker adds a '''Transform''' element to the XML Signature and places the '''XSLT Payload''' in it as shown in Figure.
The attacker reads an arbitrary file using ''XSLT'' (in this example by using the ''unparsed-text()'' function). Afterwards, he forwards the contents of the file to his own server via a ''GET'' parameter.
=Attack mitigation / countermeasures=
The attack targets the [https://en.wikipedia.org/wiki/Single_sign-on SSO] Verificator. The SSO Verificator should mitigate the usage of ''XSLT'' within the token.
=Practical Attack Examples=
In 2014, Mainka et al. analyzed 22 Software as a Service cloud providers and found out that one framework was vulnerable to this attack: Instructure
C. Mainka, V. Mladenov, F. Feldmann, J. Krautwald, J. Schwenk (2014): Your Software at my Service: Security Analysis of SaaS Single Sign-On Solutions in the Cloud. In The ACM Cloud Computing Security Workshop (CCSW).