World of Warcraft – Scan.dll is not a Virus

Howdy,

As I logged on today on WoW, my antivirus raised a virus alert for the file Scan.dll in WoW’s installation folder. I am using Avast 4.8 Home Edition (Virus Database 081028-0 dating from today). It is probably a false positive, but it is also possible that my WoW install has been infected with something (since some malware specifically targets WoW accounts). Since I don’t want to lose all my precious gold, I preferred checking a few things before logging in.

Here is a VirusTotal scan of the suspicious file Scan.dll: http://www.virustotal.com/fr/analisis/2b23aa39412bdfffdc8e8f34215de118. Here is what we can learn from this:

  • only 3/36 antiviruses flag scan.dll as suspicious (which is low but still more than one)
  • all raise generic alerts such as “suspicious file” and “Win32:Trojan-gen {Other}”. In plain English, this means “I have no idea what this file does, however it is protected against analysis and I am better off raising a scary alert”.
  • according to multiple tools the file is packed with UPX.

Hopefully, UPX is a really weak protection and the official release can unpack the file for us. So, all we have to do is download the latest UPX release, install it and unpack Scan.dll:

D:\test\upx303w>upx -d Scan.dll -o UnpackedScan.dll
                       Ultimate Packer for eXecutables
                     Copyright (C) 1996 - 2008UPX 3.03w
        Markus Oberhumer, Laszlo Molnar & John Reiser   Apr 27th 2008

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
        90372 <- 39684    43.91%    win32/pe     UnpackedScan.dll

Unpacked 1 file.

Now that we have an unpacked Scan.dll, let’s check again what Virus Total has to tell us: http://www.virustotal.com/fr/analisis/fdfe09829e1dd865144d6f80313209c4

This time, we have 0/36 detection rate, and the file does not seem to include additional layers of protection (which means it can now be fully analysed by antivirus engines).

Conclusion: it was the UPX protection layer that caused the new Avast and eSafe alert. Therefore this alert is a false positive, Scan.dll is not infected and you can go back to murloc farming. What is strange however is that UPX is a really common packer, I’m wondering exactly what part of it is confusing Avast’s heuristics.

Ok I didn’t understand anything above, what should I do/know ?

  • at the time of writing, Scan.dll is not a virus
  • the alert only comes from Avast and related antiviruses
  • when Avast asks you what to do, choose Nothing. On the next virus database update, the alert will probably be removed.
  • if you chose Quarantine, try to restore the file since WoW will probably complain that you messed with one of its DLLs.

Here is the post dealing with this issue on the official forums.

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A Quick Survey on Automatic Unpacking Techniques

This is a non-comprehensive list of papers and tools dealing with automated unpacking. Please let me know if I’ve missed another technique or if I misunderstood any of the techniques below.

Ring0/Ring3 components, using manual unpacking and heuristics

OllyBonE:

OllyBonE (Break on Execution) uses a Windows driver to prevent memory pages from being executed, and an OllyDbg plugin communicating with the driver. As such it is not an automatic unpacker and requires manual tagging of the pages in which the unpacked code is expected to be found.

Technology used: Windows driver to prevent memory page execution, debugger plugin

Handles unknown packers: no.

Drawbacks: requires a priori knowledge of the memory location of the unpacked code, vulnerable to anti-debugging techniques, modification of the integrity of the host operating system due to the driver.

Code Available: yes, http://www.joestewart.org/ollybone/.

Original Site

(Updated) Dream of Every Reverser / Generic Unpacker:

It is a Windows driver used to hook ring 3 memory accesses. It is used in a project called Generic Unpacker by the same author to find the original entrypoint. The tool then tries to find all import references, dumps the file and fixes the imports. It is reported to work against UPX, FSG and AsPack, but not against more complex packers.

Technology used: Windows driver to hook userland memory access

Handles unknown packers: no.

Drawbacks: requires a priori knowledge of the memory location of the unpacked code, modification of the integrity of the host operating system due to the driver.

Code Available: yes, http://deroko.phearless.org/GenericUnpacker.rar.

Original Site

(updated) RL!Depacker

No description for this one, however it looks similar to Dream of Every Reverser / Generic Unpacker.

Code Available: yes,  http://ap0x.jezgra.net/RL!dePacker.rar.

Original Site

(updated) QuickUnpack

Again, no real description, but it looks similar to RL!Depacker and DOER / Generic Unpacker. It is a scriptable engine using a debugging API. It is reported to work against 60+ simple packers.

Code Available: yes, http://www.team-x.ru/guru-exe/?path=Tools/Unpackers/QuickUnpack/

Original Site (in Russian)

Universal PE Unpacker:

This is an IDA Pro plugin, using the IDA Pro Debugger interface. It waits for the packer to call GetProcAddress and then activates single-stepping mode until EIP is in a predefined range (an estimate for the OEP). It only works well against UPX, Morphine, Aspack, FSG and MEW (according to the authors of Renovo).

Technology used: Debugging and heuristics.

Handles unknown packers: no, needs an approximation of the OEP and assumes that the unpacker will call GetProcAddress before calling the original code.

Drawbacks: not fully automatic, very vulnerable to debugger detection, does not necessarily work against all packers or self-modifying code.

Code Available: yes, since IDA Pro 4.9

Original Site

Instruction-level analysis, comparison between written addresses and executed addresses

Renovo:

Built on TEMU (BitBlaze), it uses full system emulation to record memory writes (and mark those memory locations as dirty). Each time a new basic block is executed, if it contains a dirty memory location a hidden layer has been found. Cost: 8 times slower than normal execution. It seems to unpack everything correctly except Armadillon and Obsidium (due to incorrect system emulation ?). It seems to only obtain partial results against Themida with the VM option on.

Technology used: Full system emulation.

Handles unknown packers: yes.

Drawbacks: order of magnitude slowdown, detection of the emulation stage

Code Available: I couldn’t find it.

Original Site, Local Copy

Azure:

Paul Royal’s solution, named after BluePill because it is based on KVM, a Linux-based hypervisor. It uses Intel’s VT extension to trace the target process (at the instruction-level), by setting the trap flag and intercepting the resulting exception. The memory writes are then recorded and compared to the address of the current instruction. According to the paper, it handles every packer correctly (including Armadillo, Obsidium and Themida VM).

Technology used: Hardware assisted virtualization and virtual machine introspection.

Handles unknown packers: yes.

Drawbacks: detection of the hypervisor. Slowdown ?

Code Available: yes, http://blackhat.com/presentations/bh-usa-08/Royal/Royal_Extras.zip.

Original Site, Local Copy

Saffron:

Developed by Danny Quist and Valsmith, a first version uses Intel PIN to dynamically instrument the analyzed code. It actually inserts instructions in the code flow, allowing lightweight fine-grained control (no need for emulation or virtualization), but it modifies the integrity of the packer. A second version modifies the page fault handler of Windows and traps when a written memory page is executed. It has mixed results with Molebox, Themida, Obsidium, and doesn’t handle Armadillo correctly (according to Paul Royal).

Technology used: Dynamic instrumentation, Pagefault handling (with a kernel component in the host operating system).

Handles unknown packers: yes.

Drawbacks: modifies the integrity of the code (with DI) and of the host operating system. It must not work in a virtual machine. The dynamic instrumentation is very slow. The memory monitoring of the pagefault handler is coarse-grained (pages are aligned on a 4k boundary), and therefore some memory access can go unnoticed.

Code Available: dynamic instrumentation available, what about the driver ?

Original Site, Local Copy

(updated) OmniUnpack:

Uses a technique similar to the second version of Saffron: a Windows driver to enforce a W^X policy on memory pages.

Technology used: Pagefault handling  and system call tracing (with a kernel component in the host operating system)

Handles unknown packers: yes.

Drawbacks: modifies the integrity of the host operating system. It must not work in a virtual machine. The memory monitoring of the pagefault handler is coarse-grained, leading to spurious unpacking stages.

Code Available: ?

Original SiteLocal Copy

Pandora’s Bochs:

Developed by Lutz Böhne, it is based on Bochs which is used to monitor memory writes and compare them with branch targets. Interestingly, the assumptions about the program are stated explicitly (which is a GOOD thing) : the unpacking does not involve multiple processes, it does not happen in kernel mode, the unpacked code is reached through a branch instruction (not a fall-through edge), etc… Another interesting point in this approach is that it uses no component in the guest OS (as opposed to Renovo for example), all the information is retrieved from outside the matrix (as with Azure).

Technology used: Full system emulation based on Bochs.

Handles unknown packers: yes.

Drawbacks: As stated in the paper the limitations are speed, compatibility (not all packed samples seemed to run under Bochs), detection of OEP and reconstruction of imports sometimes failed.

Code Available: http://damogran.de/blog/archives/21-To-release,-or-not-to-release-….html

Original Site, Local Copy

Other techniques (comparison with static disassembly or disk image)

Secure and Avanced Unpacking by Sebastien Josse:

The idea developed by Sebastien Josse is to use full system emulation (based on QEMU ?) and to compare the basic blocks that are going to be executed by the virtual CPU with the equivalent address in the file image of the executable. If the memory and the disk version differ, it means that the code has been generated on the fly and therefore a hidden layer has been found. Josse then proposes techniques to rebuild a fully functional executable based on the memory dump. This technique seems to work well (but sometimes requires human intervention) against several packers, including Armadillo, ASProtect, PEtite, UPX, yC…

Technology used:Full system emulation, comparison between memory images and disk images.

Handles unknown packers: yes, manual intervention might be required in some cases.

Drawbacks: slowdown due to the full system emulation, full reconstruction of the unpacked program is not always possible.

Code Available: ?

Original Site

PolyUnpack:

The idea behind PolyUnpack is to address the fundamental nature of unpacking, which is runtime code generation. To identifiy code that has been generated at runtime, PolyUnpack uses a conceptually elegant technique: it first statically analyses the program to build a map of statically accessible code, and then traces the execution of the program. The dynamically intercepted instructions are compared with the static disassembly, if they do not appear in the static disassembly then they have been generated at runtime.

Technology used: comparison between static disassembly and dynamic tracing. The dynamic trace is extracted with single-step debugging APIs.

Handles unknown packers: yes.

Drawbacks: vulnerable to debugger detection. Note that this is a limitation of the implementation, not of the concept.

Code Available: http://polyunpack.cc.gt.atl.ga.us/polyunpack.zip (updated 26/06/2009)

Original Site, Local Copy

Trusted Computing, change we’re supposed to believe in

I just come back from ETISS08, the 3rd European Trusted Infrastructure Summer School at Oxford. The event was really well organised with a number of high-level speakers, including David Grawrock, Graeme Proudler (HP Labs), Paul Congdon (HP ProCurve CTO), Robert Thibadeau (Seagate Chief Technologist), Paul England (Microsoft) and many others.

For those who don’t know what Trusted Computing is, have a look at the wikipedia page, insight from Bruce Schneier or pure hatred from RMS. The idea is to use a hardware component called the Trusted Platform Module as a secure cryptographic device to secure the boot process of your computer and ensure the integrity of “important components”.

Here are some key concepts and concerns about Trusted Computing and my totally biased opinion about them:

Tamper Resistance: It is quite common to hear that the TPM is a tamper resistant hardware module (that’s what security hardware is for, right ?). Finally the story is quite different, the Trusted Computing Group doesn’t care much about hardware attacks. Mainly because tamper resistance = $$$.

Secure boot: this is one of the core features of TC, it allows you to boot securely by measuring (the rest of the world calls that hashing) each component before executing it and storing the hash securely in the TPM. BitLocker already uses that feature, and if your boot sequence has changed, BitLocker won’t be able to automatically extract the encryption key of your hard disk. This is a nice feature, however notice that we can only “lock” the boot process by checking if “something” has changed.

Attestation: the other Big Thing about TC. Remember that hashes about your boot process, operating system and other are stored in the TPM ? Now the idea of attestation is that some remote party would like to ensure that you are using a trusted OS or piece of software (notice the use of “trusted” and not “secure”). Well, TC can do that for you ! Your TPM contains a secret key called the Endorsement Key which can be used to sign the value of your Platform Configuration Register (i.e. the hash of your system configuration) and send that over the network. The remote party can check that the hash has been signed by a valid EK and is a hash of a valid boot with a trusted OS and software. Now, if you don’t see the obvious implementation problems, think again. Basically, there are so many valid systems, configurations and software that the number of hashes is unmanageable (plus they change each time you apply patches and they depend on the order in which you hash things). And you can just attest remotely that the software was OK when it was launched, not that it hasn’t been exploited in the mean time (and as David Grawrock pointed, hashing a memory image of a running program is HARD).

Privacy: there have been concerns about having a secret key (the EK) in a hardware module, somewhat out of reach of the user, which might be used to aggregate personal information on remote services. Therefore the system has been carefully designed to use the EK in very limited circumstances and to use indirect signing for attestation using Attestation Identity Keys or zero-knowledge proofs such as Direct Anonymous Attestation. There was a consensus that your privacy is more at risk when you purchase something on the internet or have a Facebook account than when you use a TPM, and I think this is true.

Security: you might have noted that the keyword here is trusted, not secure. There have been a lot of philosophical debate at the summer school about what is to be trusted, what is trustworthy, and how all that relates to correctness and security. Overall TC is not a revolution for computer security, it’s more a way to bypass the question.

Open Source Software: a lot of concern about TC comes from the FOSS community, probably because the Trusted Computing Group is manned by large companies and you can somehow feel a big red hovering DRM behind it. However, most practical sessions were based on Linux and Xen and there is an opentc.net research initiative. Plus there was a really hot debate when Graeme Proudler asked “can you trust open source software ?”

Digital Rights Management: this is probably the most common concern about DRM. Why should I trust Trusted Computing if it’s just a tool to implement stronger DRM techniques ? It’s true that DRM is one of the possibilities of TC, but honestly I think than most actual DRM implementations will rely on attestation (to ensure you use a “trusted” mp3 player for example), so as long as attestation doesn’t work in practice, there can be no DRM based on it.

Loss of Control: the other big concern about TC is the idea to have a hardware chip containing some secret code and cryptographic keys that you can’t control (to some extent). Graeme and David insisted that the Platform Owner (TC terminology) remains in control of the TPM, by design. But I think there is still some loss of control due to remote attestation, and this is also by design. Because the whole point of remote attestation and the technologies based on it (such as Trusted Network Connect) is not to protect your computer from malware, but to protect your corporate network from you. Therefore if the remote party doesn’t want to attest your OS/software configuration, you’re out. In Ian Levy’s terminology, it is a way to mitigate the “wetware” risk. This is probably a good way to control your network infrastructure and corporate network, but doesn’t solve the porn surfing workstation problem. Remember, the monkey behind the keyboard really wants to see the dancing bunnies !

Oh and by the way, I also met the cool guy behind Joebox there ! If you have a suspicious executable file to analyse, Joebox is the right tool for you.

(updated: Sven Türpe has posted some nice photos of Oxford and his presentation about BitLocker on his blog)

etiss-group