Tag: hardening

Back on track as for one of our favorite rant subjects: desktop security. This stuff, commonly called the “LNK vulnerability”, has gained quite some momentum in the last days, including the release of a Metasploit module and a temporary raise of SANS Internet Storm Center‘s Infocon level to yellow (it’s back on green in the interim).

CVE-2010-2568 has been assigned and some technical details can be found here and here.

To give you a rough idea how this piece works, here’s a quote from the US-CERT advisory:

“Microsoft Windows fails to safely obtain icons for LNK files. When Windows displays Control Panel items, it will initialize each object for the purpose of providing dynamic icon functionality. This means that a Control Panel applet will execute code when the icon is displayed in Windows. Through use of an LNK file, an attacker can specify a malicious DLL that is to be processed within the context of the Windows Control Panel, which will result in arbitrary code execution. The specified code may reside on a USB drive, local or remote filesystem, a CD-ROM, or other locations. Viewing the location of a LNK file with Windows Explorer is sufficient to trigger the vulnerability.”

In short, as the Microsoft Malware Protection Center puts it: “[S]imply browsing to the removable media drive using an application that displays shortcut icons (like Windows Explorer) runs the malware without any additional user interaction.”

So actually, there is no exploitation in the sense of a buffer overflow of the LNK handling routines. The flaw just triggers downloading (and executing) some binary code. It’s basically about downloading some piece of code (to be executed on the local “compromised” box) from some location.
Figuring that brings us to an immediate discussion of potential mitigation strategies, which is – next to “some usual rant” 😉 – the main intent of this post.

As so often the US-CERT’s advisory provides good guidance on mitigating controls. It lists the following (most with a comment from my side):

a) Disable the displaying of icons for shortcuts.

Do not do this! It will most probably break your (users’) desktop experience in a horrible way.
Chester Wisniewski has a nice image of a treated desktop in yesterday’s post on the subject.

b) Disable AutoRun

This is pretty much always a good idea (security-wise), but it – partially – limits only one attack vector (removable media).
Interestingly enough the MS KB article on disabling Autorun functionality in Windows was last reviewed on 2010/07/01 and the CVE number for the LNK vulnerability was assigned 2010/06/30. There’s strange coincidences out there… 😉

And, of course, not allowing untrusted USB devices to be connected to the organization’s machines usually is a good thing as well.
[for the record: yes, I know, there are organizations out where business tells you this is not possible for some reason or another. which might be true; I will not go into this debate right here and now. I just want to remind you, dear reader, of some good ole basics ;-)].

In the meantime people expect (and see) the vulnerability being exploited over network shares so focusing on removable media/USB devices alone might be too narrow-focused anyway.

c) Use least privilege

Aka: do not work as admin. I don’t really have to comment on this here, do I? It’s a no-brainer. Well, let’s say: it should be a no-brainer… when gathering with some infosec people from a >100K user environment recently I learned they still have about 30% of users with local admin privs…
[and don’t get me wrong here: there might be good reasons for this. and those guys are not the only ones with such a landscape.]
To be discussed in more detail at another occasion.

d) Disable the WebClient service.

While I like this very much – given it’s a preventative control [“minimal machine approach”] and, btw, addresses other (past and potential future) vulnerabilities as well, e.g. those of MS10-045 published one week ago – it should be noted that this potentially breaks MS SharePoint (and other stuff as well).
So, unfortunately, again this will not be a feasible in quite some environments. Still, it might remind people that WebDAV is a technology that can be (ab-) used to access “network drives” in completely untrusted/untrustworthy locations.

e) Block outgoing SMB traffic

Well, yes…

The (today) updated MS advisory provides another one:

f) Blocking the download of LNK and PIF files

Again, this is soo obvious that I refrain from any comment.

Those of you following this blog or our public statements on desktop security (like this one) regularly might have noticed that – as so often – the two main ones quite some organizations rely on are not mentioned here:

Patching: for the simple reason there’s no patch as of today (and the imp is out of the bottle, to public knowledge, five days now).

Antimalware “protections”: not sure how this could prevent downloading and executing arbitrary binary code. The announcements of major AV vendors “we now have a signature for this” mostly address the Stuxnet stuff found in the initial exploit, nothing else. So this is mostly window-dressing.

So far so good (or bad), the main point of this post is – and yes, I’m aware I needed a long warm-up today 😉 – the following: the security problem discussed here can be broken down to: “there’s a vulnerability that triggers an exploit that goes somewhere, downloads some code and executes it”.

Which, in turn, raises the fundamental question: why should some average corporate desktop computer be allowed to go to some arbitrary location, download code, and – above all – run this code.
Restricting where to get executable code from or, even better, just allowing a specified set of applications to run could (more or less) easily solve the kind of problems vulnerabilities like this one impose. And the technologies needed like – in MS space – Software Restriction Policies , Applocker or just (as part of SRPs) “path rules” restricting where to download executables from have been available for a long time.

As Marcus Ranum is much more eloquent than me – especially when it comes to ranting where he’s nearly unbeatable 😉 – I allow myself to quote him literally, from the “Schneier-Ranum Face-Off” on “Is antivirus dead?”:

“Of course, most organizations don’t know (or haven’t got the courage to discover) what programs they allow–and, ultimately, isn’t that the root of their security problems? When I read the security news and hear that thus-and-such government agency is trying to decide if Facebook is a necessary application, it makes my head spin. In Marcus-land, where I come from, you decide what is a necessary application first, not after you have 40,000 employees who have gotten so used to it that they now think Twitter is a constitutionally protected right. Isn’t a virus or malware just unauthorized execution that someone managed to sneak onto your machine? If we adopt a model whereby there are programs that are authorized (i.e., on a whitelist) and the operating system should terminate everything else, then malware and viruses are history, unless their authors can somehow fool the administrator into authorizing them to run.
[…]
Whenever I talk about execution control/whitelisting with corporate types, someone says, ‘But we don’t really have a way of determining all the applications that we use!’ Really? Wow. That sounds like a policy that’s basically, ‘We have no idea what our computers are for.’ In other words: ‘We’ve given up, and as far as we’re concerned, our computers are an unmanaged mess.’ Or to put it another way, malware heaven. Can anyone even calculate the cost of malware and viruses (as well as the occasional office time spent playing online games) to businesses? That cost, ultimately, is paid solely in order to avoid the difficulty of determining what programs are authorized — what’s the purpose of the computer an employee is provided to use?
…
Here’s why I keep talking about execution control: it’s actually ridiculously easy compared to dealing with antivirus and antimalware. So why isn’t everyone doing it? Because it’d dramatically cut down on our ability to goof off. If executives knew how easy it was to cut back on productivity-wasting goof-off-ware, don’t you think it would be happening all over the place by now? If, instead, we tell them it’s hard to know what executables we use in the office…well, what nobody knows won’t hurt anyone.”

Well said, Marcus. Nothing to add here.

So, please please please, just take a small amount (e.g. 1%) of the yearly budget you spend on antimalware software/support/operational cost, get a student intern in and have her start testing application whitelisting on some typical corporate desktops. This might contribute to a bit more sustainable security in your environment, one day in the future.

thanks for your time,

Enno

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This is the first post of a – potential – series of rants on ubiquitous pieces of crap (security-wise), bothering pretty much every ISO I know.
I’m talking about “common desktop applications” and today’s topic is going to be the beloved Adobe Flash Player. Some of you who had the opportunity (or imposition 😉 to listen to one my talks covering “modern enterprise security space” (e.g. this one) might remember me saying sth like “If a fairy godmother turned up and asked me for three things to get rid of in order to enhance overall corporate information security in a sustainable way, my answers would be…” and then giving Adobe Flash as the first mention. (before you ask: amongst the other candidates are Apple Quicktime, Windows GDI and “Javascript in Acrobat Reader”).

And, yes, I can already hear all the yelling “But we absolutely need Flash on our corporate desktops.”. Maybe that’s really really the case. Maybe not. I’ve fought that fight in many environments, and usually lost it. Kind-of been there, done that. I’d just like to point out that – from a security point of view – this is a risky thing.
On a personal level I still do not get why Flash is needed. I can certainly be regarded as a “typical executive user”, being online most parts of the day and performing all sorts of (what I think) “typical actions” like travel booking, online financial services etc. All this can be done with my 64-bit browser that just has no associated Flash player. Seems my mileage as for “corporate browser use” still varies from the one in many of those – “we absolutely need Flash on our corporate desktops” – organizations…
And even if your company’s marketing dept is powerful enough to ask for large scale deployment of that fancy technology (some of you certainly know the “We have our own Youtube channel” argument) I still have to understand why it’s needed on the desktops in the engineering or R+D departments. But oh well…

Still, all this ranting is a bit outside the intended scope of this post. Actually the trigger for the post was this advisory titled “Security Advisory for Flash Player, Adobe Reader and Acrobat” and released by Adobe some days ago.
Here’s a little quote from the summary:

“A critical vulnerability exists in the current versions of Flash Player […] for Windows, Macintosh, Linux and Solaris operating systems, and the authplay.dll component that ships with Adobe Reader and Acrobat v9.x for Windows, Macintosh and UNIX operating systems. This vulnerability […] could cause a crash and potentially allow an attacker to take control of the affected system. There are reports that this vulnerability is being actively exploited in the wild via limited, targeted attacks against Adobe Reader v9 on Windows.”

Oops, sorry, in fact the quote above was from this advisory, initially released on july 22, 2009.
The current one (from 06/04/2010) goes like this (as for the summary):

“A critical vulnerability exists in Adobe Flash Player 10.0.45.2 and earlier versions for Windows, Macintosh, Linux and Solaris operating systems, and the authplay.dll component that ships with Adobe Reader and Acrobat 9.x for Windows, Macintosh and UNIX operating systems. This vulnerability (CVE-2010-1297) could cause a crash and potentially allow an attacker to take control of the affected system. There are reports that this vulnerability is being actively exploited in the wild against both Adobe Flash Player, and Adobe Reader and Acrobat.”

Note the difference?
There’s practically none: same products affected, same component to blame, same workaround [deactivating authplay.dll], same “Adobe’s quality assurance element” [discovery of the stuff being exploited in the wild] responsible for public statement.
In short: SSDD.

Mitigation Approaches

Given I try to be a responsible citizen [and, for that matter, responsible security practitioner too ;-)] I’d like to discuss potential approaches as for the efficient mitigation of the risk of being attacked “actively in the wild” due to (not only) this vulnerability.
At ERNW, for many years we’ve been using sth we called “The small catechism of IT security” which was essentially a set of simple fundamental rules as for securing complex systems. This piece included, amongst others, these ones:

Minimal Machine.
Least Privilege.
Patching.

Following these lines some approaches come to mind and I’ll discuss some of those.

a) Do not run Flash at all. Yes, we had this discussion already. And, no: I do not live in a ivory tower. And I mainly consult to very large organizations.
Sure, this might be one of the fights you (as an ISO) just can’t win. But, heck, I still dare to post this on our very personal and ranty blog: Running Flash on corporate desktops is simply asking for trouble. Asking for trouble loudly. Very loudly.

It should be noted that, according to this, removing Adobe Flash (e.g. in the way described here) will not remove the instances of Flash Player that is installed with Adobe Reader 9 or other Adobe products.

There is always a lot of trade-offs in managing complex IT environments. There are business requirements – and, as we security folks know: business pretty much always wins (and this is fully ok, as security is not the most important thing in corporate life) -, there’s “cost considerations”, all sorts of politics and in the end of the day there’s our mission of getting the best possible security stance given all these considerations and trade-offs. Running vulnerable software to provide some business functions (while at the same time inducing the risk of getting owned) obviously is such a trade-off, and it’s a common one.

As for Flash one should just be aware that – in most environments – there’s only little business value of running it, but – in all environments – there’s quite some associated risk.

b) Do not run Flash embedded in PDFs (by deactivating authplay.dll as described in the advisories).

I think this is – security-wise – a very feasible approach (following that good ole security principle called “minimal machine”). Only problem might be that the stuff gets re-deployed/re-enabled next time you patch Adobe Reader. So operational processes might have to be adjusted to ensure it does not re-appear.
And, of course, this is an ugly one (deleting a dll), which might not be “aligned with your sw management and deployment processes” 😉
This document mentions that deleting another dll as well avoids the crashes when invoking a file with SWF code in it. Haven’t tested this though.

Btw: this is a preventative control. Whereas patching is a reactive one. Most probably I don’t have to tell any reader of this blog that preventative controls tend to have a better cost-impact ratio than reactive ones, do I? 😉

c) Patching. Hmm, unfortunately there is no patch as of today. And the stuff is “exploited in the wild” (Adobe, thank you! for letting us know, once more. What about just adding a checkbox somewhere in “Preferences” that allows to disable playing embedded SWF stuff at all?).
Furthermore patch cycles for Adobe products are quite long in most environments (due to the number of integration aspects and side effects).

So, dear reader who’s still sympathetic to patching (as for Adobe stuff): do not pass go, do not collect $200, but maybe re-read the last sentences of the two former points.

d) Use of an alternate PDF reader, like Foxit Reader. Looking at this I’m not sure if this is really better (security-wise) and most probably it’s not an option for most corporate environments anyway (for reasons outside the security realm).

e) Security measures/approaches from the “Least privilege” space like “running Adobe stuff on a low integrity level” (on Windows systems disposing of integrity levels, that are Win Vista or Windows 7). While this can certainly help and can be regarded as a nice preventative control, it has the big disadvantage that taking the route of “least privilege” usually has, that is added complexity and high operational cost… (which is, btw, why it practically never works out to a satisfactory degree).

f) Gateway-based controls. In a number of environments there will be quite some praying that “our malicious content protection saves us”. This may happen. or not. Taking the “detective/reactive way” (which is what most anti-malware controls do) has well-known weaknesses…
Sanitizing Flash (like Blitzableiter does) could be a much better approach. Hopefully technologies like this will gain some deployment in the near future.

And hopefully in the upcoming world of HTML5 we won’t see that high risk software piece called Flash player anymore (alas, experience tells there will be other similarly awful stuff. but that’s another story…)

have a great day,

Enno

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