hunt-auth-bypass— agent skill

Crown Jewel Targets

Auth bypass is consistently one of the highest-paying vulnerability classes in bug bounty because it directly violates the most fundamental security control. High-value targets include:

• SSO/SAML implementations at enterprise SaaS companies (Slack, Okta, OneLogin integrations) — payouts regularly in the $5K–$25K+ range
• Admin panels and partner/internal portals — subdomain-separated admin surfaces like partners.shopify.com, admin.company.com
• Third-party auth plugin integrations — WordPress plugins (OneLogin, WP-SAML-Auth), Drupal SSO modules, any CMS with pluggable auth
• XMLRPC endpoints on WordPress — often forgotten, bypasses standard WP auth flows entirely
• OAuth callback flows — state parameter mishandling, redirect_uri mismatches
• API authentication layers — especially where auth was bolted on after the fact

Asset priority: Targets with federated identity (SAML, OAuth, OIDC) connected to large user populations. Partner/reseller portals are particularly juicy because they often have elevated permissions and less security scrutiny than the main product.

Attack Surface Signals

URL patterns to hunt:

/xmlrpc.php
/wp-login.php
/saml/
/sso/
/auth/saml/callback
/oauth/callback
/partners.*
/admin.*
/?wc-api=
/api/v*/auth
/login?redirect=
/accounts/login

Response headers signaling SSO:

X-Frame-Options: SAMEORIGIN (common on SSO portals)
Set-Cookie: SAMLResponse=
Location: https://idp.company.com/saml
WWW-Authenticate: Bearer realm="partners"

JS patterns indicating federated auth:

// Look for in page source
samlRequest
RelayState
SAMLResponse
onelogin
shibboleth
okta
passport.js authenticate

Tech stack signals:

• WordPress + any SSO plugin → check XMLRPC separately
• Shopify Partner API exposure → cross-tenant privilege escalation risk
• Any app advertising "SSO enabled" or "Login with [Enterprise IdP]"
• Separate subdomains for admin/partner that share session cookies with main domain
• Applications using SimpleSAMLphp, ruby-saml, python-saml

Burp passive scan triggers:

• SAMLResponse in any POST body
• openid_connect or id_token in responses
• Cookie domains set to .company.com (wildcard)

Step-by-Step Hunting Methodology

1. Map all authentication entry points

• spider the target for every login surface: main login, admin login, API login, partner portal, mobile API endpoints
• check robots.txt, JS files, and the wayback machine for forgotten endpoints like /xmlrpc.php

1. Identify the auth mechanism per entry point

• Is it forms-based, SAML, OAuth, API key, session token?
• For WordPress: always probe /xmlrpc.php even if the main login is SSO-protected

1. Test XMLRPC independently of SSO

• If site uses SSO (e.g., OneLogin), manually POST to /xmlrpc.php
• XMLRPC uses WordPress-native credentials, not SSO — test with system.listMethods first, then wp.getUsersBlogs

1. Enumerate SAML implementation

• Capture a valid SAMLResponse via Burp
• Decode the Base64 payload, inspect the XML
• Test signature stripping, comment injection, and XML wrapping attacks
• Test if SP validates the signature at all (send unsigned assertion)

1. Test cross-portal session/token reuse

• Log into partners.shopify.com type portals
• Attempt to use the issued token/cookie against the main admin portal
• Look for shared cookie domains, shared JWT secrets, or API tokens that work across contexts

1. Fuzz auth parameters

• Null/empty passwords, password[]=array, SQL in username field
• Try admin/admin, test/test on staging subdomains
• Modify role, is_admin, user_type in JWTs (none algorithm, weak secret)

1. Check redirect and state parameters

• Does removing state from OAuth break anything?
• Can you change redirect_uri to an open redirect target?
• Does the RelayState in SAML get validated?

1. Verify impact by escalating privileges

• Don't stop at login — prove you can access admin functions, other users' data, or sensitive configuration
• Screenshot the highest-privilege action you can perform

Legacy-Protocol Matrix (Probe These First on Any Custom-Branded Login)

When a target has a custom, branded login UI (e.g. customlogin.aspx, /auth/signin, /account/login), always probe the platform's legacy protocol endpoints with native credentials in parallel. These endpoints frequently outlive the custom UI's protections and accept native credentials with NO rate limit, NO MFA challenge, NO CAPTCHA, NO anti-automation. This is the WordPress XMLRPC pattern generalised across CMS / portal / framework stacks.

How to use:

1. Identify the tech stack from headers + paths (use hunt-misc Attack Surface Signals).
2. Find the row above that matches.
3. Probe the legacy endpoint anonymously to confirm it's reachable and not 403/404.
4. Test with synthetic credentials to confirm it accepts native credential format and returns differential responses (success vs failure).
5. Verify there is no rate limit, no lockout, no CAPTCHA — burst 10 requests at the same user, confirm uniform timing.
6. Report as Critical / High depending on chain to ATO: an anonymous + unlimited credential brute-force endpoint is consistently Critical on bug-bounty programs.

Lesson from a authorized engagement: A an enterprise dealer portal on SharePoint 2013 had a custom branded customlogin.aspx. The hunt-auth-bypass skill was loaded but the matrix above did not exist in this document — and the WordPress XMLRPC pattern was not connected to the SharePoint equivalent. /_vti_bin/Authentication.asmx was reachable anonymously, accepted unlimited credential attempts with no rate limit and no lockout, and was the highest-impact finding in the engagement. Walking this matrix on the first pass would have surfaced it immediately.

Payload & Detection Patterns

XMLRPC auth probe (bypasses SSO):

curl -s -X POST https://target.com/xmlrpc.php \
  -H "Content-Type: text/xml" \
  -d '<?xml version="1.0"?>
<methodCall>
  <methodName>system.listMethods</methodName>
  <params></params>
</methodCall>'

# If 200 with method list → XMLRPC is enabled, test auth:
curl -s -X POST https://target.com/xmlrpc.php \
  -H "Content-Type: text/xml" \
  -d '<?xml version="1.0"?>
<methodCall>
  <methodName>wp.getUsersBlogs</methodName>
  <params>
    <param><value><string>admin</string></value></param>
    <param><value><string>password</string></value></param>
  </params>
</methodCall>'

SAML signature stripping (send unsigned assertion):

import base64, re

# Decode captured SAMLResponse
saml_b64 = "BASE64_FROM_BURP"
saml_xml = base64.b64decode(saml_b64).decode()

# Strip the Signature element entirely
stripped = re.sub(r'<ds:Signature.*?</ds:Signature>', '', saml_xml, flags=re.DOTALL)

# Re-encode and submit
print(base64.b64encode(stripped.encode()).decode())

SAML XML comment injection (username confusion):

<!-- Original NameID -->
<NameID>[email protected]</NameID>

<!-- Injected to confuse parser -->
<NameID>[email protected]<!---->[email protected]</NameID>

<!-- Or namespace confusion -->
<NameID xmlns:evil="http://evil.com">[email protected]</NameID>

Partner/cross-portal token reuse test:

# Get token from partner portal
TOKEN=$(curl -s -X POST https://partners.target.com/login \
  -d '[email protected]&password=pass' \
  -c cookies.txt | grep -o 'token=[^;]*')

# Replay against admin portal
curl -s https://admin.target.com/dashboard \
  -H "Authorization: Bearer $TOKEN" \
  -H "Cookie: $TOKEN"

JWT none algorithm attack:

import base64, json

header = base64.b64encode(json.dumps({"alg":"none","typ":"JWT"}).encode()).decode().rstrip('=')
payload = base64.b64encode(json.dumps({"user_id":1,"role":"admin","email":"[email protected]"}).encode()).decode().rstrip('=')
token = f"{header}.{payload}."
print(token)

Grep patterns for auth bypass surface:

# Find XMLRPC in scope
grep -r "xmlrpc" scope_urls.txt

# Find SSO indicators in JS
grep -rE "(SAMLResponse|samlRequest|RelayState|onelogin|shibboleth)" *.js

# Find partner/admin subdomains
subfinder -d target.com | grep -E "(admin|partner|internal|sso|auth|login)"

Common Root Causes

1. SSO bypasses local auth entirely at the UI layer, but not at the API layer — developers disable the login form but forget that API endpoints (/xmlrpc.php, REST API, mobile API) have their own auth handlers that still accept native credentials.

1. SAML signature validation is skipped or optional — library defaults often don't enforce signature checking; developers use wantAssertionsSigned: false or fail to configure the IdP certificate correctly.

1. Shared session infrastructure across different trust levels — partner portals and admin portals reuse the same session cookie or JWT secret because they're built on the same internal framework, assuming access control at the application layer is sufficient.

1. Trust inheritance in multi-tenant architectures — a token issued in a lower-privilege context (partner, reseller) is accepted in a higher-privilege context because the verification only checks signature validity, not the issuance context.

1. Plugin/module auth is independent of application auth — every WordPress plugin that handles auth (contact forms, REST API extensions, WooCommerce) may implement its own auth handler inconsistently with the main site's SSO.

1. XML parsing inconsistencies — different XML parsers (used by SP vs. IdP) handle comments, namespaces, and whitespace differently, enabling confusion attacks where the signed content differs from the evaluated content.

Bypass Techniques

XMLRPC multicall for mass auth bypass:

<methodCall>
  <methodName>system.multicall</methodName>
  <params><param><value><array><data>
    <value><struct>
      <member><name>methodName</name><value><string>wp.getUsersBlogs</string></value></member>
      <member><name>params</name><value><array><data>
        <value><string>admin</string></value>
        <value><string>password1</string></value>
      </data></array></value></member>
    </struct></value>
    <!-- repeat for each credential pair -->
  </data></array></value></param></params>
</methodCall>

Gate 0 Validation

Before writing any report, answer these three questions:

1. What can the attacker DO right now?

Must be: authenticate as another user OR authenticate without valid credentials OR elevate to admin/privileged role. "Partial information disclosure" is not auth bypass.

1. What does the victim LOSE?

Must identify a concrete asset: account takeover of specific user, access to all admin functions, ability to read/modify other tenants' data, or access to privileged APIs. Abstract "security control bypass" without impact is not sufficient.

1. Can it be reproduced in 10 minutes from scratch?

You must be able to: (a) start from a fresh browser/session, (b) follow your exact steps, and (c) arrive at authenticated access to a protected resource. If reproduction requires special preconditions you can't re-create (a specific victim's active session, timing windows), the report needs more work.

Real Impact Examples

Scenario 1 — SSO Enforcement Bypassed via Forgotten Protocol Endpoint A large ride-sharing company enforced SSO (via OneLogin) on all WordPress-based internal/public properties. The XMLRPC endpoint (/xmlrpc.php) remained active and accepted WordPress-native credentials entirely independent of the SSO flow. An attacker with any valid WP-native credentials (obtained via credential stuffing or from a previous breach) could authenticate directly through XMLRPC, bypassing MFA, SSO policies, and IP restrictions enforced on the main login form. Impact: Full authenticated access to all WordPress functions available to that user role, including content management and potentially admin functions.

Scenario 2 — SAML Assertion Forgery via Signature Validation Failure A major enterprise communication platform's SAML SP implementation failed to properly validate assertion signatures in specific edge cases. By manipulating the XML structure of a captured SAMLResponse (specifically through comment injection or namespace prefix attacks), an attacker could modify the NameID value to impersonate any user in an organization — including workspace administrators — without possessing that user's credentials or private key material. Impact: Complete account takeover of any user within a SAML-enabled organization; attacker gains access to all messages, files, and integrations in the workspace.

Scenario 3 — Cross-Portal Privilege Escalation via Shared Auth Backend An e-commerce platform's partner/reseller portal issued authentication tokens that were validated by the same backend service as the merchant admin portal. A partner-level account (lower trust, external-facing) could use its issued credentials or tokens to authenticate directly against admin-tier API endpoints, bypassing the merchant onboarding and permission assignment flow. Impact: A malicious partner could access any merchant's admin panel, modify store configurations, exfiltrate customer PII and payment data, or install malicious scripts — affecting thousands of merchant storefronts.

Related Skills & Chains

• `hunt-idor` — Auth bypass without object-level access is half a finding; pair them. Chain primitive: legacy /v1/users/{id} route missing both auth middleware AND ownership check = unauthenticated cross-tenant data read via direct ID substitution → full PII dump from "I am nobody" starting position.
• `hunt-ato` — Auth-bypass primitives feed the ATO funnel. Chain primitive: XMLRPC native-cred acceptance + no rate limit on wp.getUsersBlogs → credential-stuff with breach corpus from hunt-misc recon → system.multicall batches 1000 cred pairs per request → one valid pair = ATO bypassing the SSO + MFA the UI enforces.
• `hunt-sharepoint` — The SP equivalent of the WordPress XMLRPC pattern lives here. Chain primitive: /_vti_bin/Authentication.asmx anonymous reachable + native Forms-auth credential accepted + zero rate limit = unlimited credential brute-force endpoint bypassing custom-branded customlogin.aspx protections → FedAuth cookie → full SharePoint farm access.
• `security-arsenal` — Pull the JWT-attack payloads section (alg=none, kid path-traversal, JWK injection, RS256→HS256 key confusion) when JWT validation is the auth wall; pull the SAML signature-stripping section when the SP accepts unsigned assertions.
• `triage-validation` — Run the Pre-Severity Gate before claiming Critical on an "auth bypass" that only enumerates usernames or only reveals a 401-vs-403 differential. Username enumeration alone without lockout-amplification is consistently N/A or Informational on H1.