bigbluebutton-lfi

Overview

This vulnerability was reported by annfalotaibi

Source link for the reported vulnerabilities are as follows:

• https://huntr.com/bounties/aed70a15-b1c6-4eae-9e47-02a1dd86ce7c/

How to Identify Similar vulnerabilities

Path Traversal in UrlMappins.groovy

The problem in this was quite straightforward but to understand it, we will go over two functions, one that is parsing the URL containing a user-controlled parameter which is responsible for referencing files by their id. Even though the application is using id instead of filename for referencing the files to be shown. Following piece of code maps the URL and parameter for the textfiles processing.

    "/bigbluebutton/presentation/$conference/$room/$presentation_name/textfiles/$id"(controller: "presentation") {
      action = [GET: 'showTextfile']

UrlMappings.groovy

Now, from the above code you could make the educated guess that it takes parameters like id, presentation_name etc. and let the "presentation" controller's showTextfile function to handle processing:

def showTextfile = {
    def presentationName = params.presentation_name
    def conf = params.conference
    def rm = params.room
    def textfile = params.id
    log.debug "Controller: Show textfile request for $presentationName $textfile"

    log.error("Nginx should be serving this text file! meetingId=" + conf + ",presId=" + presentationName + ",page=" + textfile);

    InputStream is = null;
    try {
      def pres = presentationService.showTextfile(conf, rm, presentationName, textfile)
      if (pres.exists()) {
        log.debug "Controller: Sending textfiles reply for $presentationName $textfile"

        def bytes = pres.readBytes()
        response.addHeader("Cache-Control", "no-cache")
        response.contentType = 'plain/text'
        response.outputStream << bytes;
      } else {
        log.debug "$pres does not exist."
      }
    } catch (IOException e) {
      log.error("Failed to read text file. meetingId=" + conf + ",presId=" + presentationName + ",page=" + textfile);
      log.error("Error reading text file.\n" + e.getMessage());
    }
  }

PresentationController.groovy

As we can see above that the showTextfile function calls another method presentationService.showTextfile(conf, rm, presentationName, textfile) where all of these parameters are directly taken from the showTextfile function call, in this case URL parameters that we saw earlier.

Digging into the presentationService class, we could see that the showTextFile function just returns a File object by preparing a filepath from the passed parameters, since the textfile parameter is one of the parameters we controlled (in the above function it was referenced as id later assigned with variable named textfile), what can we do here due to lack of any filter on the textfile parameter, any attacker can just point to an existing directory first and then do path traversal attack to read local files, though the problem here is that we are only limited to txt extension file.

	def showTextfile = {conf, room, presentationName, textfile ->
		def txt = roomDirectory(conf, room).absolutePath + File.separatorChar + presentationName + File.separatorChar +
				"textfiles" + File.separatorChar + "slide-${textfile}.txt"
		log.debug "showing $txt"

		new File(txt)
	}

PresentationService.groovy

Patch

The patch is rather way too simple in this case, groovy allows you to have a specific type of parameter by performing a type check on the input, in this case, the developers deployed a fix where they check id parameter to be strictly of integer parameter:

    "/bigbluebutton/presentation/$conference/$room/$presentation_name/textfiles/$id"(controller: "presentation") {
      action = [GET: 'showTextfile']
      constraints {
        id matches: /\d+/
      }
    }

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How to Identify Similar vulnerabilities

As usualy, mapping out the functions which do file based operations can help narrowing down the potential issues that may lead to local file read. Also, checking you might be limited to reading specific extension of a file but that does not mean it is less of a problem for the overall application security.