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Fix XML Signature Wrapping (XSW) vulnerabilities
PySAML2 did not check that the signature in a SAML document is enveloped and thus XML signature wrapping (XSW) was effective. The signature information and the node/object that is signed can be in different places and thus the signature verification will succeed, but the wrong data will be used. This specifically affects the verification of assertions that have been signed. This was assigned CVE-2020-5390 Thanks to Alexey Sintsov and Yuri Goltsev from HERE Technologies to report this. + + + + + + + + In more detail: libxml2 follows the xmldsig-core specification. The xmldsig specification is way too general. saml-core reuses the xmldsig specification, but constrains it to use of specific facilities. The implementation of the SAML specification is responsible to enforce those constraints. libxml2/xmlsec1 are not aware of those constraints and thus process the document based on the full/general xmldsig rules. What is happening is the following: - xmldsig-core allows the signature-information and the data that was signed to be in different places. This works by setting the URI attribute of the Reference element. The URI attribute contains an optional identifier of the object being signed. (see "4.4.3 The Reference Element" -- https://www.w3.org/TR/xmldsig-core1/#sec-Reference) This identifier is actually a pointer that can be defined in many different ways; from XPath expressions that need to be executed(!), to a full URL that should be fetched(!) in order to recalculate the signature. - saml-core section "5.4 XML Signature Profile" defines constrains on the xmldsig-core facilities. It explicitly dictates that enveloped signatures are the only signatures allowed. This mean that: * Assertion/RequestType/ResponseType elements must have an ID attribute * signatures must have a single Reference element * the Reference element must have a URI attribute * the URI attribute contains an anchor * the anchor points to the enclosing element's ID attribute xmlsec1 does the right thing - it follows the reference URI pointer and validates the assertion. But, the pointer points to an assertion in another part of the document; not the assertion in which the signature is embedded/enveloped. SAML processing thinks that the signature is fine (that's what xmlsec1 said), and gets the assertion data from the assertion that contains the signature - but that assertion was never validated. The issue is that pysaml2 does not enforce the constrains on the signature validation facilities of xmldsig-core, that the saml-core spec defines. The solution is simple; all we need is to make sure that assertions with signatures (1) contain one reference element that (2) has a URI attribute (3) that is an anchor that (4) points to the assertion in which the signature is embedded. If those conditions are met then we're good, otherwise we should fail the verification. Signed-off-by: Ivan Kanakarakis <ivan.kanak@gmail.com>
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