XXE - XML External Entity Attack

1. Web Application Security - Team bi0s © 2017
XXE
XML External Entity
25 February
2017
@Team bi0s 1/25
HEERAJ
Btech, Third Year, Computer Science Engineering
Amrita University

2. whoami
Web Application Security - Team bi0s © 2017 @Team bi0s
➔ Undergraduate Student @ Amrita
➔ Web Security Enthusiast
➔ CTF{flag_seeker}
➔ @HRJ
➔ ww.i4info.in
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3. Agenda
Web Application Security - Team bi0s © 2017 @Team bi0s
➔Intro to XML & DTD
➔XML Entity
➔Parsing XML
➔Attacks Vector
➔Demo
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4. XML
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➔EXtensible Markup Language
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Picture:123RF.COM

5. Where it is used ?
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➔Document Formats
➔Image Formats
➔Configuration Files
➔Network Protocols
➔RSS Feeds … etc . . .
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Picture: c-sharpcorner.com

6. Document Type Definition
Web Application Security - Team bi0s © 2017 @Team bi0s
➔ References an External
DTD
➔ Define structure with the list of legal
elements
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7. XML Entity
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➔ Entities help to reduce the entry of
repetitive information and also allow
for easier editing
Output:
Writer: Donald Duck. Copyright: bi0s.
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8. XML Entity
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XML Entity
Internal Entity External Entity
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9. Parsing
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➔ Character other than < , > , & , ‘ , “ all are parsable.
➔ PCDATA is text that will be parsed by a parser. Tags inside
the text will be treated as markup and entities will be
expanded.
➔ CDATA is text that will not be parsed by a parser.
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10. Attack’s Possible
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➔ LFI
➔ SSRF
➔ Internal scans
➔ Denial of Service
➔ Rce (Not Always!!!)
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11. Attack Vectors
Web Application Security - Team bi0s © 2017 @Team bi0s
Classic XXE
We can view any file which doesn’t contain < , > , & , ‘ , “ as characters.
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12. 12

13. Direct Feedback Channel
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What if
you are Reading
Some configuration files?
13

14. Direct Feedback Channel
Web Application Security - Team bi0s © 2017 @Team bi0s
➔ CDATA very helpful to read web configuration, which contain non
parsable characters.
But this won’t work !!
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15. Direct Feedback Channel
Web Application Security - Team bi0s © 2017 @Team bi0s
➔ We have to use Parameter entities
➢ Parameter.dtd
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16. Out Of Band Channel
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17. Out Of Band Channel
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➔ No Direct Feedback Channel
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Website: http://web-in-security.blogspot.in/2016/03/xxe-cheat-
sheet.html

18. Billion Laughs Attack (Simple Denial of
Service)
Web Application Security - Team bi0s © 2017 @Team bi0s
➔ Works by expansion property (Simple code(<1kb) will expand up to 3
gigabytes of memory.
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19. Different Protocols
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20. OFFICE OPEN XML
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➔ Zip archive file containing XML and media files
➔ *.docx , *.xlsx , *.pptx
➔ Developed by Microsoft
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21. OFFICE OPEN XML
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Open XML File Container
Document
Properties
Custom Defined
XML
Comments
WordML/
SpreadsheetML etc
Embedded
Code/Macros
Images, Video,
Sound Files
Charts

22. OFFICE OPEN XML
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➔ General Parsing XML
◆ /_rels/.rels
◆ [Content_Types].xml
◆ Default Main Document
● /word/document.xml
● /ppt/presentation.xml
● /xl/workbook.xml
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23. Playing With Content Type
Web Application Security - Team bi0s © 2017 @Team bi0s
➔ Server may accept multiple data formats
➔ Results in Json endpoints may be vulnerable to XXE
➔ Content-Type changed to application/xml
➔ JSON has to be converted to XML
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24. Demo
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25. Solution
Web Application Security - Team bi0s © 2017 @Team bi0s
➢ Don’t reflect the XML back to user
➢ Turn off external DTD fetching
➢ Turn off DTD
➢ Disable External Entity Parsing
libxml_disable_entity_loader(true);(PHP)
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