SAML Vulnerabilities

SAML (Security Assertion Markup Language) is used for SSO in enterprise environments. Its XML-based structure creates several unique attack vectors.

XML Signature Wrapping (XSW):

SAML responses are XML-signed. A wrapping attack duplicates the signed assertion and adds a malicious assertion. Vulnerable parsers process the signed (legitimate) assertion for signature verification but then use the malicious (unsigned) assertion for authorization.

<!-- Original signed assertion for user: user@corp.com -->
<SAMLResponse>
  <SignedAssertion>...uid=user@corp.com...<Signature>...</Signature></SignedAssertion>
  <!-- Injected malicious assertion (unsigned) -->
  <Assertion>...uid=admin@corp.com...</Assertion>
</SAMLResponse>

Comment injection attack:

<!-- Injected comment in username attribute -->
<NameID>admin<!--comment-->@attacker.com</NameID>
<!-- Some parsers strip comments before processing,
     resulting in: admin@attacker.com → "admin" -->

Use vetted SAML libraries that handle these edge cases correctly, and enforce strict schema validation.

Key defensive requirements:

• Use a library with a strong security track record (OneLogin, python-saml, Spring Security SAML)
• Keep the library updated — most XSW vulnerabilities are patched in library updates
• Validate the signature against the correct element ID — do not allow any signed element to satisfy the assertion verification
• Strip XML comments before processing NameID values
• Canonicalize XML before signature verification to prevent normalization attacks

Configuration validation (python-saml):

settings = {
    "strict": True,  # Enforce strict security validation
    "security": {
        "wantAssertionsSigned": True,
        "wantMessagesSigned": True,
        "signatureAlgorithm": "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256",
        "digestAlgorithm": "http://www.w3.org/2001/04/xmlenc#sha256",
    }
}

Defense checklist:

• Use an actively maintained SAML library, never roll your own
• Enable strict mode in SAML library settings
• Require assertions to be signed (wantAssertionsSigned)
• Validate signatures against specific assertion IDs
• Strip comments from NameID before processing