Why is the OS obfuscation defense against "It's a Unix system!" not widely implemented?

Question

The Jurassic Park scene referenced in the title is infamous for how ludicrous it sounds to those who are tech literate. But it also illustrates what seems to me to be a glaringly huge hole in web security, particularly IoT devices--as soon as attackers find out a server or camera or baby monitor is running linux, they instantly know volumes about how it works. They know that commands like sudo are big juicy targets and they know that shell access will bring with it gobs of useful tools like ls and cat.

So why isn't OS obfuscation more of a thing? I'm not talking about just hiding the version in web headers. Similar to JavaScript minification or obfuscation, I'm talking about changing the names of binaries and filepaths in the OS itself. Wouldn't entire classes of attacks be practically useless if the OS had ha7TrUO and RRI6e29 commands instead of sudo and ls? Imagine a hacker that somehow gained remote root access--what are they even going to do if they don't know any commands?

Implementation would be fairly easy for compilers. Take the simplest case of "rename this function and all calls to it." You could give an OS compiler and an application compiler the same randomized names and they'd be able to talk to each other. But even if the application has poor security and is vulnerable to bash injection, such attacks would be fruitless.

Obviously this technique can't be used in all scenarios. Setting aside scenarios like servers maintained by human sysadmins, it seems to me that any device or server managed by automation is a prime candidate for this defense.

I guess the question(s) needs to be a bit more concrete:

1. Is OS obfuscation as described used widely and I just haven't encountered it?
2. If not used widely, what are the practical or technical barriers to usage?

Answer 1

Before I tear your idea apart, let me say that it's a really interesting idea and it was super fun to think about.

Please continue to think outside the box and ask interesting questions!

Alright, let's do this!

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Let's take a step back and ask why that baby monitor is running Linux in the first place? What if there was no operating system and the application was written in bare microcontroller code (think arduino code)? Then there would be no sudo or ls or even a shell for the attacker to use, right?

I'm not an expert here, but I expect that we, as an industry, have gravitated towards putting Linux on anything big enough to run it largely for developer convenience:

1. Reducing dev time: When building your WiFi-and-bluetooth-capable web-administered cloud-syncing self-patching whizpopper with companion Android and iOS apps, Linux comes with all the libraries, utilities, and drivers you need to do that.
2. Increasing testability: If the device is running bash or busybox with an SSH port, then it's super easy to connect in and figure out what went wrong during your product testing phase.

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For your obfuscation idea to work, you'd need to obfuscate not only the names of command-line utilities like sudo and ls, but also every Linux kernel API to prevent the attacker from dropping in their own compiled binary that calls the kernel directly. So let's take another look at your idea:

Implementation would be fairly easy for compilers. Take the simplest case of "rename this function and all calls to it." You could give an OS compiler and an application compiler the same randomized names and they'd be able to talk to each other.

You'll need to do this randomized compilation yourself; otherwise someone could look up the mappings on google.

So, you'll need to build the kernel from source with your "obfuscating compiler" so that only you know the mappings of the obfuscated kernel APIs. (ever built the linux kernel from source? It's certainly more of a chore than docker pull alpine, which is the direction that dev culture seems to be going).

But an operating system is more than just the kernel. You want drivers for the Broadcom BCM2837 wifi chip that comes on that mini-pc device? You'll need to build that driver against your obfuscated kernel with your compiler, if Broadcom will even give you the source code. Then you'll need to build the entire GNU wifi and networking software stacks. How many other things will you need to find source for and add to your build pipeline before you have a functioning OS?

Oh, and if the upstream repos of any of those things issues a patch, you're now responsible for re-building it (assuming you saved the compiler obfuscation mapping files that match your kernel binary) and pushing it out to your devices because - by design - your devices can not use patch binaries produced by the vendor.

Oh, and in order to foil hackers, there'll be none of this "Here's the binary files for Whizpopper 1.4.7", oh no, you'll need to build a uniquely obfuscated version of everything from the kernel up per device that you ship.

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So to your questions:

1. Is OS obfuscation as described used widely and I just haven't encountered it?
2. If not used widely, what are the practical or technical barriers to usage?

I think the answer is that what you're describing pretty much completely defeats the purpose of using pre-existing software components if you need to find and build literally everything from source. It might actually be less effort to ditch the operating system entirely, pretend it's 1960, and write your application directly in CPU microcode.

I like security more than most developers, but like f* that.

Answer 2

Mike's answer says basically everything I have to offer about why this is a bad idea from a development perspective (and, as Ghedipunk's comment says, an unusable security feature provides no security). So instead, I'm going to talk about why from a security perspective, you would never do this.

The answer is actually surprisingly simple: it's a waste of time, and there are strictly better options. Every stupid IoT doodad (remember, the "s" in "IoT" stands for Secure) that doesn't bother to implement such features sure as hell wouldn't go with an approach like you suggest.

1. The whole idea just won't work for restricting system calls. An attacker can just set a few registers and invoke an opcode and boom, they're in the kernel executing the syscall of their choice; who cares what the symbolic name of it is? Sure, you can tamper with the syscall table to complicate this (if you don't mind needing to recompile everything and making debugging your custom kernel a form of utter hell) but it's like obfuscating the OS in use; why bother when there are so few candidates, anyhow? Even if the attacker doesn't want to reverse engineer existing code on the system, brute-forcing should be possible unless the search space of usable call indices is wider than I've ever seen on an embedded system.
2. Why obfuscate command names when you can just make them entirely inaccessible? A chroot won't work for shell built-ins if you're running a shell, but it works fine for everything else and really, why would your custom-built single-purpose app-in-a-box be running a shell? I mean, dev units would have one installed, for test purposes, and maybe that doesn't get removed in your retail image because you're lazy or think you'll need it again. But an attacker wouldn't be able to run it from the context that its app runs as. A simple chroot (or a more complicated sandbox/jail/container) can make the program unable to run - or even access - any files beyond those required for its job.
3. Why obfuscate the kernel APIs when you can just remove access to them? There are a number of sandboxing systems that can restrict what calls a process (or its descendants... if it's even allowed to create any) can make. See https://stackoverflow.com/questions/2146059/limiting-syscall-access-for-a-linux-application

Answer 3

If your objective is to deprive an attacker of ls and cat, there's an even better alternative to obfuscation: just don't install those utilities.

While I wouldn't say this is a widely implement approach, it is at least implement. For example consider distroless, a collection of docker images with pretty much nothing in them. Some of them (like the one for go) literally have nothing in them. An attack on a system running in such a container can't get shell access because there isn't any shell to run.

The only way then to get shell access by attacking an application in such a container is then to circumvent the container runtime, which is designed to prevent precisely that.

While I gave an example of a docker image, the same concept can be applied to operating systems generally. For example, ls and cat are part of the coreutils package in Debian. You could run apt-get remove coreutils and be confident an attacker won't be able to use ls or cat as part of an attack. Of course this means you can't use them either, and there's probably a lot of other stuff that depends on coreutils that will also have to be removed, but for an embedded device or a server that does just one thing that may be OK.

The general principle is reducing "attack surface": the more "stuff" a target has, the easier it is to compromise. The stuff could be open network ports, lines of code, or binaries installed. If increasing security is the objective, then removing all unnecessary "stuff" is a good start.

Answer 4

Because obfuscation isn't security and because OS obfuscation is basically nonsense.

There's only so many common OSes, and so many ways to make educated guesses. If you detect that I'm running IIS or MSSQL Server, you have one guess at what OS is running underneath.

Even if I somehow manage to run a stack that tells you nothing about my underlying OS, and also mask every other hint at it (fingerprinting is a thing), I still haven't won much.

Security-wise, you knowing that I'm running Linux, or even that I'm running Debian 8, doesn't give you much to work on nor me much to worry about. If I'm properly hardened and patched, you could know whatever you want. If I'm on a patch level that has applied to the software museum yesterday, your suite of attacks will break me wide open simply through trying them all, and obfuscating my OS only forces you to try a couple useless exploits more. In an automated attack, it'll slow you down all of a few seconds.

Obfuscation doesn't work. People can scan you, fingerprint you or just throw their entire library of exploits to see what works.

If you waste time on that which you could've spent on actual hardening, you're actually harming your security.

Proper hardening works. I said above that "you could know whatever you want". I've posted my IP address and root password at security conferences where I gave a speech. SSH with remote root login and a bunch of services enabled. Seriously hardened SELinux machine. Nobody has ever managed to interrupt my speech, though one person once managed to drop a text file into the root directory when my policy wasn't yet perfect.

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Addendum: Your starting point was a movie. Obfuscation is a great movie device because a revelation like that tells the viewers that the hero (or villain) found some piece of information and is thus making progress. It's the computer equivalent to finding out where the safe is, even though you still need the code. It doesn't matter if it's factually correct, if it conveys the proper emotional message to the audience.

Answer 5

For an extremely widespread example, Intel Management Engine, which has its own OS, does something like that, where there exists a firmware update mechanism, but the firmware has to be in a very specific format the details of which are confidential. It appears to involve Huffman encoding with unknown parameters. Similarly to your proposal (which is basically symmetric encryption of firmware), ME requires specific modifications at firmware preparation time and has a matching deviation from standard mechanisms at execution time.

Answer 6

What you are describing is called "Security through Obscurity" and is a widely known security antipattern. Meaning it's not a new idea, rather it's an old, bad idea, that people who are uninitiated in security philosophy must be educated so as not to fall into.

Imagine you were designing a house to be secure, and thought obscurity was a promising strategy. All houses are constructed out of hallways and rooms, doors with doorknobs, light switches, etc. Since these are commonly known, you might reason this is insecure since an intruder could easily navigate the house by these commonly known construction elements. You might try to redesign the principles of construction from scratch, replace door knobs with rubiks cubes, make half height ceilings to confuse the intruder etc. You end up with a house that is terrible to live in, cannot be maintained because no contractor wants anything to do with it, and worst of all it's for nothing because any intruder, once inside, can look around with his eyes and use his brain to figure it out. Hackers are puzzle addicts at heart.

The solution to securing your house is not to have a non-standard construction, it is to have better locks, secured windows, a proper security system, etc. You don't want to hide your gold in a secret passageway, because eventually Abbot and Costello will lean on that candlestick and open it accidentally. Put your gold in a safe with a good secret combination and a tamper alarm. The computer equivalent is having restricted credentialed access by public key cryptography, limited user access roles, monitoring systems, reducing exposed surface area, entry threat vector mitigation, etc.

Efforts to make a computer system more obscure only make your system farther from support, harder to get security patches, harder to use in a secure way.

Edit: Sometimes, some security through obscurity is acceptable, when used in addition to actual security. A common example is running SSH on a high port like 30000 something. SSH is encrypted, and access is behind a credentialed authentication, that's your real security. But having it on a high port just makes it less noticeable to begin with if someone is doing quick scans. Anything more convoluted than this, such as trying to obfuscate your OS, would just make it an operational nightmare, that would make actual security measures (like being up to date on patches) more difficult.

Answer 7

Wouldn't entire classes of attacks be practically useless if the OS had ha7TrUO and RRI6e29 commands instead of sudo and ls? Imagine a hacker that somehow gained remote root access--what are they even going to do if they don't know any commands?

A better alternative would be to simply not install these commands in the first place. I don't believe you actually need a shell to run a Linux kernel, although this implies that your start-up process is something other than sysV-init or systemd.

As others have noted, however, it's a trade-off between security and ease of development. Unfortunately, many device makers care much more about the latter than the former.

Implementation would be fairly easy for compilers. Take the simplest case of "rename this function and all calls to it." You could give an OS compiler and an application compiler the same randomized names and they'd be able to talk to each other. But even if the application has poor security and is vulnerable to bash injection, such attacks would be fruitless.

I'm not sure you need the compiler's help at all. I think you can mostly achieve this by modifying the linker¹ to apply a randomization to all symbol names. Probably just applying a hash function with a salt that is known only to the manufacturer would suffice. I'm not sure you even need source code to do this, i.e. I think you could apply the mangling to already compiled code.

(¹ I think this is something of a lie. You may need to modify the object code in order to replace the symbol names, but this is still more like the assembler level, which is to say, a) you aren't doing all the hard bits of compiling C/C++/etc. code and b) you don't need the C/C++/whatever source code. OTOH I'm not sure you can do this sort of thing at all if your device code is instead something like Python.)

There is still some possibility to reverse-engineer the process, however, and moreover this may violate the GPL² (especially GPLv3) unless you give out the salt, which would defeat the purpose.

In fact, "because of the GPL" is probably the main reason why you don't see this; it would be hard to implement in a way that is actually useful aside from making each device different. OTOH, that would at least mean that attackers can only target specific devices rather than being able to exploit a vulnerability on "any device running Linux x.y.z".

(² For simplicity, I will just use "GPL" throughout, but note that this generally applies even for LGPL'd stuff.)

That said, note that the GPL doesn't require you to publish the salt because you "modified" the sources. If the symbol name randomization happens at compile time, you haven't modified the sources. The reason you would need to publish the salt is because the GPL requires that users may substitute their own versions of a GPL'd library, which they can't do unless they know the salt. (As noted, you may be able to weasel out of this with GPLv2, but the technical effects are similar to "only run signed software", which GPLv3 was specifically written to address.)

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Ultimately, there could be some advantage here, but you aren't going to make any particular system notably more secure (it's well known that "security through obscurity" generally is no security at all). What you can accomplish is making it harder to target many systems via a single vector.

Answer 8

Fair Disclosure: I'm the CTO of a company that builds just this. De-bias for that all you want.

It IS indeed possible to completely build a system kernel-up, device drivers, packages, the whole stack PER host, multiple times per day, and it can be quite effective. That's exactly what we do, and we help our customers do. We're not the only ones - we can infer that at least Google does this for all their machines as well (reference coming soon.)

Now if you had the ability to rebuild per-machine from scratch what are some of the things you can change?

The Kernel Self Protection Project already allows for this through random reordering of kernel structures: https://lwn.net/Articles/722293/. This article also points to the famous exchange where Linus Torvalds calls it security theatre, but the author of the project (who works at Google) comments, "Well, Facebook and Google don't publish their kernel builds. :)"

This leads credence to the inference that at least Google does this and considers it useful. Can we do MORE types of scrambling over a closed set? At the most fundamental level why doesn't a Windows virus run on Linux or a Mac? Because the formats are different. It's all x86 underneath, and yet it's not the same. Well what if two Linuxes were different in a similar fashion? Windows vs Linux isn't "obfuscation" merely because we don't have a lot of ways to make Linuxes as different as Windows is to Linux. But it's not impossible, and it's not really even that hard. Take the KSPP's approach and apply it to syscalls, then recompile everything on top against those syscalls. That's going to be hella difficult to break - at least not in a flyby fashion.

Your question though was about symbol renaming (names of executables, libraries, etc.) This has two aspects: (a) is it useful? (b) Can it be done reliably?

We were looking for a way to solve PHP code injections once and for all. Despite what HackerNews would have you believe, PHP code injections are not a solved problem outside of internet message boards. Real life PHP developers, admins and users are exposed to a continuous number of code injections.

So we set out to try Polyscripting (permissively MIT-licensed Open Source): https://github.com/polyverse/polyscripted-php

We share this publicly to solicit feedback, and we run two websites, polyscripted.com and nonpolyscripted.com, which do exactly what you'd expect based on their names. We also hired pentesters to try to break it.

And I'm just beginning to experiment with executable, shared libraries and exported-symbol renaming over a closed set (in a docker container, for instance). I don't personally think this adds as much value, but will it add a little? I think so. So it comes down to cost. If you can get little value for even littler cost, why not just do it? That's exactly why we all have ASLR - it's not exactly the greatest defense since sliced bread, but if you already have relocatable code, and you're going to reorder it anyway, why NOT push it to the limit and randomize it?

In short, the approach you're describing is being attempted by many people (including Google, Facebook, the Linux kernel, etc.), along with many academicians under the field of Moving Target Defense, and a handful of companies like ours who're trying to make it trivially consumable like ASLR.

Answer 9

Think usability

• Try to explain to your new sysadmin that he have to hit ha7TrUO instead of sudo, RRI6e29 instead of ls, and so on... Yes, there is a list of translation you just have to learn!

• Browsing local network must not be possible too: you have to maintain paper list in order to keep an overall view!?

• If you rename all critical commands, you will have to drive this for making system upgrade!

• And as Nick2253 comments out, if you try to build embedded system, then do obfuscation before publishing end product, you will have to test final product.

  • You have to create home made script to bind everything for obfuscation.
  • If something goes wrong, debugging will become tricky.
  • You have to create home made deobfuscation scripts, in order to be able to do some debugging.
  • Custom feedback (with log files) will have to be deobfuscated too.

Doing so, you will add a layer of important work with new potential bugs.

For very few security improvement, see further.

Obscurity could harm yourself.

Think lower level

• Even if you try to rename files, function at application and OS level, all this will use standard libraries who will be called directly if an attacker sends binary executable files. You could even consider obfuscating all standard libraries! (Including filesystem, network protocols...)

• While you use unmodified kernel, they will use standard variables and namespaces. So, you will have to create obfuscated version of kernel...

... Then bind everything together!

Well, even when everything is obfuscated, the application will have to deal with the internet. Obfuscation won't prevent from conceptual bugs about this!

Obscurity, if not at all levels, won't improve security.

Full obscurity is not really possible, due to the need of using standard protocols in order to be able to deal with the internet.

Think license

If you plan to distribute GNU/Linux, you have to share source code as described in GNU GENERAL PUBLIC LICENSE GPLv3 and/or GNU LESSER GENERAL PUBLIC LICENSE LGPLv3 (depending on application and libraries used). This will imply publication of obfuscation method along with every distributed product.

Strictly answering to your two questions:

I'm something an expert, but with a very small focus. Working on many different small business infrastructures, I've done some choices some years ago. Evolution and history seem to confirm my choices, but it's just my personal point of view.

Is OS obfuscation as described used widely and I just haven't encountered it?

I really don't know in which proportion obfuscation is widely used, but I don't use security by obscurity and recommend not to.

If not used widely, what are the practical or technical barriers to usage?

No barriers! Just it's counter-productive. Time cost quickly become gigantic and security improvement is a lure.

Of course, some minimal things are to do:

• Don't start ssh server if not needed.
• Don't install sudo. Use correct permissions, groups and coherent structure.
• Keep your global infrastructure up-to-date !!!

Little sample when light come over obscurity

• Securing ssh: two way.

  • Move ssh port from 22 to 34567

    • Light and quickly done, but

    If an attacker found them, they could engage smooth brute force against this port a lot of time until end user discover them.

  • Install firewall

    • Stronger, require more knowledge, but

    More secure while up-to-date.

Answer 10

I would tell how I see this from hacker point of view.

Definitely, if you will rename all utilities - it will make things much harder and less comfortable for me, but what I could do?

1. If I already got access to shell on system, I would just upload either busybox (all basic utilities in one binary) or full set of binaries I need for basic operations.

2. To escalate privileges further, I may need to look for suid-root binaries, and there is just few of them (sudo, mount, etc.). Hacker cannot upload such files (unless he is already root). My simple small linux system has just 24 suid binaries. Very easy to try each of it manually.

But anyway, I agree, this would make hacking this box harder. It would be pain to use (hack) this system, but possible. And hacker works on system for hour or day or month... but admin/user may work on system for years and cannot remember ha7TrUO and RRI6e29 names. Maybe nobody will even try to hack system, but admin will suffer every day.

But... if you make security too high but uncomfortable for user/admin - often you're making security lower in fact. Like if you enforce complex passwords with 20+ characters - most likely passwords will be written on post-it notes on monitor. If you will make system such obfuscated - it will be very hard to work on it for good users. And they will have to do some things to make their life easier. For example, they can upload their busybox binary. Temprorary. And they will forget about it or will leave it there intentionally (because they plan to use it little later).

Answer 11

It actually does happen, because ssh connections are encrypted. Changing the names of some core binaries doesn't accomplish anything more than this. Also it would cause a lot of chaos: e.g. any scripts relying on these utilities are either rendered unusable, or you'll need to have some kind of proxy "deobfuscator", which would surely end up as an attack vector. However I've seen some servers in the wild that don't allow root login, forcing to use sudo instead. The usernames of sudoers remain somewhat secret. I'm not sure how secure it is, but I'm no expert.

Answer 12

Why don't all doors have ten keyholes, only one of which actually worked for keys or lock picks? Answer: mostly the bother isn't worth the ten extra seconds of a thief's time.

If there were millions of unique created in isolation source code operating systems, obscurement might be common. As a practical matter though, we have roughly four unique code bases in common use - all leak info about themselves by event timing, and for several low level OS functions where chip manufactures give prescribed machine bitcode sequences to activate or access certain cpu features, they all likely have short sequences containing exactly the same code.

Answer 13

Compilers (actually, linkers) do this already. It's one of the main reasons reverse engineering is hard.

In your source code you might have these functions:

void print_hello() {
    printf("Hello!\n");
}

void print_hello_twice() {
    print_hello();
    print_hello();
}

Here's how they look after they're compiled into a program:

0x400582 push rbp
0x400583 mov rbp,rsp
0x400586 mov edi,0x400634
0x40058b call 0x400490
0x400590 nop
0x400591 pop rbp
0x400592 ret
0x400593 push rbp
0x400594 mov rbp,rsp
0x400597 call 0x400582
0x40059c call 0x400582
0x4005a1 nop
0x4005a2 pop rbp
0x4005a3 ret

As you can "clearly" see, print_hello is now called 0x400582, printf is now called 0x400490 and print_hello_twice is now called 0x400593. All the calls were also given the same randomized names, so that functions are able to call each other.

However, the linker is only able to do this within a program, since it links one program at a time.

Answer 14

If you obfuscate GNU/Linux on an IoT device distributed to the public you will be forced to publish your code because it is under the GPL, or take the device off the market.

Obfuscating GPLed software — a strategy dependent on secrecy — is not a viable idea.

Not sure why this is downvoted; but if the objections are in the vein of Charles' comments: Simply renaming the core utilities will not work (for a simple start, you'll have to edit the GPLed init scripts). I strongly suspect that you'll also have to re-compile many binaries which interact with other binaries like the gcc compiler driver, ld.so, sh/bash etc. All shell scripts and many configuration files need to know the names and locations of binaries, so they have to be changed.

All this is GPLed and you'll be forced to publish the changed sources, scripts and config files, thus frustrating the obfuscation efforts. If you use the device only in-house there is no obligation to publish the sources, but at the same time the case for obfuscation is much weaker: You'll frustrate mostly yourself.