XML Signature Wrapping

XML Signature Wrapping (XSW) is an attack against XML digital signatures where the attacker manipulates the document structure to make the application process unsigned content while signature validation passes on different content.

How It Works

XML signatures reference specific elements by ID. If an application validates the signature on one element but processes a different element with the same ID (or in an expected location), attackers can inject malicious content.

Attack Variants

XSW #1: Cloning signed element

<Response>
  <Signature>
    <Reference URI="#original"/>  <!-- Validates this -->
  </Signature>

  <Assertion ID="original">
    <Subject>admin@example.com</Subject>  <!-- MALICIOUS -->
  </Assertion>

  <Assertion ID="original">  <!-- Original moved here -->
    <Subject>user@example.com</Subject>
  </Assertion>
</Response>

XSW #2: Wrapping in new element

<Response>
  <MaliciousAssertion>
    <Subject>admin@example.com</Subject>
  </MaliciousAssertion>

  <OriginalWrapper>
    <Signature>...</Signature>
    <Assertion ID="legit">
      <Subject>user@example.com</Subject>
    </Assertion>
  </OriginalWrapper>
</Response>

Why It Works

Signature validation uses XPath/ID reference
Application logic uses different selection method
XML allows multiple elements with same ID
Libraries may not enforce strict document structure

Prevention

Ensure signed element is exactly what's processed
Reject documents with duplicate IDs
Use strict schema validation
Verify assertion is direct child of signed response

PentesterLab Exercises

SAML Exercises