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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:
```ruby
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:
In **XML Signature Wrapping attacks (XSW)**, adversaries exploit a vulnerability arising when XML documents are processed through two distinct phases: **signature validation** and **function invocation**. These attacks involve altering the XML document structure. Specifically, the attacker **injects forged elements** that do not compromise the XML Signature's validity. This manipulation aims to create a discrepancy between the elements analyzed by the **application logic** and those checked by the **signature verification module**. As a result, while the XML Signature remains technically valid and passes verification, the application logic processes the **fraudulent elements**. Consequently, the attacker effectively bypasses the XML Signature's **integrity protection** and **origin authentication**, enabling the **injection of arbitrary content** without detection.
The following attacks ara based on [**this blog post**](https://epi052.gitlab.io/notes-to-self/blog/2019-03-13-how-to-test-saml-a-methodology-part-two/) **and** [**this paper**](https://www.usenix.org/system/files/conference/usenixsecurity12/sec12-final91.pdf). So check those for further details.
* **Strategy**: A new root element containing the signature is added.
* **Implication**: The validator may get confused between the legitimate "Response -> Assertion -> Subject" and the attacker's "evil new Response -> Assertion -> Subject", leading to data integrity issues.
* **Strategy**: An Extensions element is inserted with the copied Assertion as a child.
* **Implication**: This exploits the less restrictive schema of the Extensions element to bypass schema validation countermeasures, especially in libraries like OpenSAML.
You can use the Burp extension [**SAML Raider**](https://portswigger.net/bappstore/c61cfa893bb14db4b01775554f7b802e) to parse the request, apply any XSW attack you choose, and launch it.
SAML Responses are **deflated and base64 encoded XML documents** and can be susceptible to XML External Entity (XXE) attacks. By manipulating the XML structure of the SAML Response, attackers can attempt to exploit XXE vulnerabilities. Here’s how such an attack can be visualized:
You can also use the Burp extension [**SAML Raider**](https://portswigger.net/bappstore/c61cfa893bb14db4b01775554f7b802e) to generate the POC from a SAML request to test for possible XXE vulnerabilities and SAML vulnerabilities.
Extensible Stylesheet Language Transformations (XSLT) can be used for transforming XML documents into various formats like HTML, JSON, or PDF. It's crucial to note that **XSLT transformations are performed before the verification of the digital signature**. This means that an attack can be successful even without a valid signature; a self-signed or invalid signature is sufficient to proceed.
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.
You can also use the Burp extension [**SAML Raider**](https://portswigger.net/bappstore/c61cfa893bb14db4b01775554f7b802e) to generate the POC from a SAML request to test for possible XSLT vulnerabilities.
The **XML Signature Exclusion** observes the behavior of SAML implementations when the Signature element is not present. If this element is missing, **signature validation may not occur**, making it vulnerable. It's possibel to test this by altering the contents that are usually verified by the signature.
You can also use the Burp extension [**SAML Raider**](https://portswigger.net/bappstore/c61cfa893bb14db4b01775554f7b802e). Intercept the SAML Response and click `Remove Signatures`. In doing so **all** Signature elements are removed.
Certificate Faking is a technique to test if a **Service Provider (SP) properly verifies that a SAML Message is signed** by a trusted Identity Provider (IdP). It involves using a \***self-signed certificate** to sign the SAML Response or Assertion, which helps in evaluating the trust validation process between SP and IdP.
2. If the response contains a signature, send the certificate to SAML Raider Certs using the `Send Certificate to SAML Raider Certs` button.
3. In the SAML Raider Certificates tab, select the imported certificate and click `Save and Self-Sign` to create a self-signed clone of the original certificate.
4. Go back to the intercepted request in Burp’s Proxy. Select the new self-signed certificate from the XML Signature dropdown.
5. Remove any existing signatures with the `Remove Signatures` button.
6. Sign the message or assertion with the new certificate using the **`(Re-)Sign Message`** or **`(Re-)Sign Assertion`** button, as appropriate.
7. Forward the signed message. Successful authentication indicates that the SP accepts messages signed by your self-signed certificate, revealing potential vulnerabilities in the validation process of the SAML messages.
Token Recipient Confusion and Service Provider Target Confusion involve checking whether the **Service Provider correctly validates the intended recipient of a response**. In essence, a Service Provider should reject an authentication response if it was meant for a different provider. The critical element here is the **Recipient** field, found within the **SubjectConfirmationData** element of a SAML Response. This field specifies a URL indicating where the Assertion must be sent. If the actual recipient does not match the intended Service Provider, the Assertion should be deemed invalid.
For a SAML Token Recipient Confusion (SAML-TRC) attack to be feasible, certain conditions must be met. Firstly, there must be a valid account on a Service Provider (referred to as SP-Legit). Secondly, the targeted Service Provider (SP-Target) must accept tokens from the same Identity Provider that serves SP-Legit.
The attack process is straightforward under these conditions. An authentic session is initiated with SP-Legit via the shared Identity Provider. The SAML Response from the Identity Provider to SP-Legit is intercepted. This intercepted SAML Response, originally intended for SP-Legit, is then redirected to SP-Target. Success in this attack is measured by SP-Target accepting the Assertion, granting access to resources under the same account name used for SP-Legit.
This revealed that the `base` parameter accepts a URL. Considering this, the idea emerged to substitute the URL with `javascript:alert(123);` in an attempt to initiate an XSS (Cross-Site Scripting) attack.
The [**SAMLExtractor**](https://github.com/fadyosman/SAMLExtractor) tool was used to analyze subdomains of `uberinternal.com` for domains utilizing the same library. Subsequently, a script was developed to target the `oidauth/prompt` page. This script tests for XSS (Cross-Site Scripting) by inputting data and checking if it's reflected in the output. In cases where the input is indeed reflected, the script flags the page as vulnerable.
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