11:32 a.m.
Hello everyone,
there has been a report on incident-response at idpy.org about a security
issue in PySaml2.
Alexey Sintsov and Yuri Goltsev from HERE Technologies reached out and
reported a XML Signature Wrapping (XSW) vulnerability. The issue
affects responses with signed assertions. PySaml2 can be tricked to
think that an assertion had been signed and use the assertion
information, when in reality the Signature points to another part of
the xml document that is controlled by another party.
The issue was assigned CVE-2020-5390 and is now fixed in the latest
pysaml2 release.
The relevant code commit that fixes is the issue:
https://github.com/IdentityPython/pysaml2/commit/5e9d5acbcd8ae45c4e736ac521…
Release v5.0.0 contains more changes, including:
- Add freshness period feature for MetaDataMDX
- Fix ipv6 validation to accommodate for addresses with brackets
- Fix xmlsec temporary files deletions
- Add method to get supported algorithms from metadata
- Add mdstore method to extract assurance certifications
- Add mdstore method to extract contact_person data
- Start dropping python2 support
Pointers to the release with changelog and more information, below:
- the relevant release commit:
https://github.com/IdentityPython/pysaml2/commit/f27c7e7a7010f83380566a219f…
- the github release:
https://github.com/IdentityPython/pysaml2/releases/tag/v5.0.0
- the pypi package:
https://pypi.org/project/pysaml2/5.0.0/
+ + + + + + + +
In more detail, regarding the XSW vulnerability:
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.
--
Ivan c00kiemon5ter Kanakarakis >:3