Didier Stevens

Monday 31 July 2006


Filed under: Reverse Engineering — Didier Stevens @ 17:24

A friend faced the following problem: his company has to provide confidential data to a financial company. To maintain the confidentiality of the data, this financial company provided my friend with a custom-made program to “protect” the data to be provided.

But my friend doesn’t trust unknown programs, he wanted to know exactly what protection this program offered. The financial company didn’t want to provide further details about their program, so my friend called me for help.

To remain confidential, data transferred on public channels must be protected with strong encryption, the implementation of the cryptographic process must be free of errors and the cryptographic keys must be managed securely.

First we get acquainted with the program. In fact, it’s very simple: you start the program, open the file to be protected and save the resulting file with another extension.
So there’s no password to be provided. This is an indication that the cryptographic key is stored in the program. This is no problem, as long as public key cryptography is used. However, if secret-key cryptography is used, the secret key can be retrieved from the program by reverse engineering and can then be used to decrypt the data.

The protected file is much smaller (around 4 times), so compression is involved. A first glance at the protected file with a hex editor (like XVI32) doesn’t reveal much, there’s nothing readable.

One can follow 2 paths to identify if cryptographic methods are used in a program: you can analyze the program and you can analyze the data.

When analyzing the program , the goal is to identify cryptographic algorithms. The cryptographic library can be linked statically or dynamically. For Windows programs, you can use a dependency viewer (like Dependency Walker) to view the imported DLLs. For statically linked programs, you can use FindCrypt2 by Ilfak. It’s an IDA Pro plugin that looks for cryptographic constants in the disassembled code.

We decide to proceed with the analysis of the protected data. Reverse engineering will come later, but I can’t resist a quick peek at the strings in the program (with BinText). These strings stand out to me:

deflate 1.1.4 Copyright 1995-2002 Jean-loup Gailly 
inflate 1.1.4 Copyright 1995-2002 Mark Adler

They come from the zlib library, an open source library for GZip compression.

We encrypt the same data file a second time, and save it with another name. The size of the 2 protected files is the same. Comparing these 2 files with JojoDiff (a binary comparison program) shows that the files are almost identical:

jdiff-w32 -lr test1.prt test2.prt 
       1        1 EQL 10 
      11       11 MOD 256 
     267      267 EQL 2380

The –lr options displays ASCII output with regions (sequential parts of the binary files).
This result shows us that the first 10 bytes and the last 2380 bytes are the same, there’s only a region of 256 bytes that differ. Because most of the file is the same, we can deduce that the protection always uses the same encryption key (the 256 different bytes are probably a structure of status fields like filename, timestamps and other stuff). So this program doesn’t use “fancy” stuff like session keys, salting, initialization vectors, …

Now we protect several other files and compare them: the size is different and only the first 10 bytes are the same.
We formulate our hypothesis for the file format:
Bytes 1-10: header (magic bytes)
Bytes 11-266: status data
Bytes from 267 on: encrypted data

Now we will concentrate on the encrypted data. Strong ciphertext should be hard to distinguish from a series of random bytes. CrypTool is a freeware program which enables you to apply and analyse cryptographic mechanisms. It’s an excellent educational program. We will use it to see how “random” the encrypted data is.

The Analysis / General menu option in CrypTool has several tools to analyze ciphertext (like calculating the entropy), but because we have no clear idea of the results we can expect with strong encryption, we do the following:

We take our data file and use several methods to generate a “transformed” file:

  1. We protect it with the provided program
  2. We ZIP it
  3. We GZip it
  4. We password protect it with ZIP but don’t use compression, just store.
  5. We encrypt it with RSA
  6. We encrypt it with Ncrypt

We analyze each file with the CrypTool cryptanalysis tools and compare the results. I won’t detail each result here, but we have 2 important results.

First, the entropy of our protected file is in the same range as the entropy of the compressed files, rather than the encrypted files:

File Entropy
File 1 7.92
File 2 7.89
File 3 7.93
File 4 7.98
File 5 7.97
File 6 7.97

The maximum entropy is 8.

Second, we find the same periodicity cycle in the protected file and the GZipped file, but at a different offset:

Periodicity analysis of test1.prt: 
No.    Offset    Length    Number of cycles    Cycle content 
1    2637    1    2        .     00
Periodicity analysis of test.gz: 
No.    Offset    Length    Number of cycles    Cycle content 
1    2388    1    2        .     00

The difference in offset is 249 bytes, almost the size of the header and status data (265)!
This is a strong indication that the protected data is just compressed, not encrypted, and that it’s GZip compressed.

The binary comparison of the protected data and the GZipped opens our eyes:

jdiff-w32 -lr test1.prt test.gz 
       1        1 MOD 598 
     599      599 DEL 129 
     727      598 EQL 1791

Both files share the same sequence of 1791 bytes!

We review our hypothesis for the file format:
Bytes 1-10: header (magic bytes)
Bytes 11-266: status data
Bytes from 267 on: encrypted GZipped data

I know that jdiff can be confused when comparing files which start differently but then continue identically, so we decide to compare them starting from the end. We binary reverse both files and compare them again:

jdiff-w32 -lr reverse-test1.prt reverse-test.gz 
       1        1 EQL 2370 
    2371     2370 MOD 19

Wow! The GZipped file is almost completely included in the protected file, except for 19 bytes (this is very likely the GZip header which contains, among other things, the original file name).

To test our hypothesis, we strip the first 266 bytes from the protected file (with the tail command), name it test.gz and decompress it with the gzip command. Success! We have recovered our original file, and we prove that the so-called “protection” provided by the program is not encryption, just standard compression! It can easily be defeated in a few seconds with 2 simple commands: tail and gzip.

This analysis has taken us about 2 hours. My friend has his answer about the protection level provided by the program. Now it’s up to him to report this to his manager and decide how to proceed.

Later on, I started reverse engineering the program.
The first 10 bytes are a fixed string, the so-called magic bytes, used to identify the file type.
The next 256 bytes are just random bytes generated by the program, and have no meaning whatsoever! The program seeds the RNG with the current time, explaining why protecting the same file twice gives a different 256 byte sequence.

By now I knew enough to formulate a final, proven hypothesis about the file format:
Bytes 1-10: header (magic bytes)
Bytes 11-266: status data garbage
Bytes from 267 on: encrypted GZipped data.

Yet Another Case of Security Through Obscurity. Or, quoting Bruce Schneier, “Snake Oil”!

Monday 24 July 2006

ROT13 is used in Windows? You’re joking!

Filed under: Reverse Engineering — Didier Stevens @ 21:50

We where telling “encryption jokes” (like ROT26) at the office, until a colleague mentioned that a part of the Windows Registry is ROT13 encrypted.

It appeared to be true, Windows Explorer will store info about the programs you run in registry key
The value names stored in this key are ROT13 encrypted.

Google for UserAssist and you’ll find several pages where this is explained in detail, like this one. Some of these pages mention programs to decrypt ROT13, but I didn’t find a program to display and manage the UserAssist data. So I decided to write my own, just for the fun of it.

I wanted this program to have a GUI and I didn’t want to spend much time coding low-level details, so I decided to code it with the Microsoft .NET 2.0 Framework.
I posted my program (source code and binaries) here on the gotdotnet site.

Download the ZIP file, you’ll have to extract UserAssist\UserAssist\bin\Release\UserAssist.exe to get my program. There is no setup, it’s just one executable.

I used Microsoft Visual C# 2005 Express Edition because it’s free, so you can examine and modify my program. But it’s not needed to run my program, you’ll only need the .NET Framework 2.0 runtime to run my program (download it only if you have a problem running my program, if you have an up-to-date version of Windows XP, the .NET 2.0 Framework will already be installed).

My program displays the decrypted UserAssist entries as a treeview:


ROT13 is a monoalphabetic substitution cipher, these ciphers are very easy to decrypt, e.g. by frequency analysis. The namespace System.Security.Cryptography in .NET 2.0 does not implement the ROT13 cipher, I had to implement my own method.

While I developed my program, I became intrigued with the binary data. Because I had no access to the Internet at that time, I had to resort to the good old trial and error technique to discover the meaning of this data (I tested my program on Windows XP SP2).
For all entries starting with UEME_RUN, the binary data is 16 bytes long. The first 4 bytes at first always remained zero, and the fifth byte increased with one each time I ran the corresponding program (like notepad.exe). Because the sixth, seventh and eight byte are zero, I surmized that the 4 bytes starting at byte 5 are a 32 bit counter. Data on Intel machines is usually stored in little-endian format, this means that the least significant byte is stored first, e.g. a 32 bit integer with value 9 is stored as 09 00 00 00. When I ran a program the first time, this counter was initialized to the value of 6. This was also the case when I deleted the entry and ran the program again.
The remaining 8 bytes changed apparently at random each time I ran the corresponding program, but in fact only the least significant bytes changed. I hypothesized that the remaining 8 bytes are a timestamp. I ran notepad, noted the value of the 8 bytes, ran notepad exactly one minute later and noted the new value of the 8 bytes. The difference was 0x23B9D020, or 599380000, which is almost equal to 60 seconds times 10.000.000. Hence it definitely was a timestamp, I tried to convert the 8 last bytes to a timestamp with the C# method DateTime.FromFileTime, and Bingo!, I got the date and time when I last ran notepad.

Now back to the first 4 bytes. I noticed a trend: they are set to the value of the last 4 bytes of the UEME_CTLSESSION, and these 4 bytes appear to be a 32 bit counter that increases each time the user logs on (only after a reboot?). I need to analyze this further.

To summarize, the 16 data bytes are organized as:
• 4 bytes, meaning unknown, probably a 32 bit integer, appears to be a session counter
• 4 bytes, a 32 bit integer, counts the number of times the corresponding program was executed, is equal to 6 for the first run
• 8 bytes, a 64 bit timestamp, last time the corresponding program was executed

When you select an entry in the treeview, the binary data will be decoded and displayed at the bottom of the form.

My program should be self-explanatory.

Right-click an entry to clear it:


Clear All will delete the root keys, thus deleting all entries and also preventing Windows Explorer to record program execution until you perform a new logon (in fact, the entries are (re)created when Windows Explorer is started).

Caution: the UserAssist entries are used by Windows to display programs you use frequently in the Start menu:


Clearing the UserAssist entries will impact the Most Frequently Used Programs portion of your Start menu.

Reverse-Engineering the UserAssist entries was relatively easy, but I can’t explain why they are ROT13 encrypted!

Thursday 13 July 2006

Do you have Mailslots on your Windows PC?

Filed under: Vulnerabilities — Didier Stevens @ 20:42

Tuesday July 11th, second Tuesday of the month. IT professionals working for a Microsoft shop know the drill: patch Tuesday.

MS06-035 Vulnerability in Server Service Could Allow Remote Code Execution. One vulnerability fixed by this patch is the “Mailslot Heap Overflow Vulnerability – CVE-2006-1314″. According to the Microsoft Security Bulletin, a mitigating factor for this vulnerability is “Microsoft Windows XP Service Pack 2 and Microsoft Windows Server 2003 Service Pack 1 do not have services listening on Mailslots in default configurations“. Good, but what about non-default configurations? When do you have mailslots on your machine?

Maislots are an Inter-Process Communication (IPC) protocol. It can be used by processes (running programs) to communicate with each other.

It’s easy to create programs using mailslots.
Your server program listens to a mailslot by creating a file starting with \\.\mailslot followed by the name of the mailslot (e.g. \\.\mailslot\listener) and starts reading from that file.
Your client program talks to a mailslot by creating a file starting with \\server\mailslot followed by the name of the mailslot (e.g. \\MyServer\mailslot\listener) and writing a message to it. The Server Service will transport your message from your client program to your server program.

More details can be found on MSDN and sample code is available on The Code Project.

Hence any program designed to use mailslots can open a mailslot on your Windows PC, making your Windows XP SP2 machine vulnerable. You can list the mailslots opened on a machine by enumerating the files in the \\.\mailslot directory.

I wrote a simple C# 2.0 console application to do this:

using System;
using System.Collections.Generic;
using System.Text;
using System.IO;

namespace ListMailSlots
    class ListMailSlots
        static void Main(string[] args)
            foreach (string file in Directory.GetFiles
(@".mailslot", "*.*", SearchOption.AllDirectories))

Mail me or post a comment if you want the compiled program.

Running this program on a fresh Windows XP SP2 install shows nothing: as stated by Microsoft, a default install has no mailslots.

But on a Windows Server 2000 SP4, the result is different:


The mailslot \\.\mailslot\messngr is used by the Messenger service (the service that displays a popup when you issue a NET SEND command).
Alerter is used by the Alerter service to display administrative alerts.

These services are disabled on Windows XP SP2 and Windows 2003 SP1. In fact, when you enable and start these services on a default install, the mailslots will be created and my program will list them.

53cb31a0\UnimodemNotifyTSP is used by the Telephony service.

There is another way to list mailslots using Process Explorer by Sysinternals: start PE and search (File Handle or DLL…) for \Device\Mailslot:


This will also show you the process that opened the mailslot. svchost.exe is a generic process to host Windows services, you’ll have to open the properties of the process and select the Services tab to view which Services are hosted by the process.

I’ve also discovered (with my program) that McAfee uses a mailslot.

This gives you a method to check if a Windows machine has mailslots and hence if it’s vulnerable.

Few details have been published about this vulnerability, the best I found is by TippingPoint. I wonder when H D Moore will publish an exploit module for his Metasploit framework.
Cybertrust has issued an alert for this vulnerability, warning for a possible new worm like Slammer. Wait and see…

Update: Restoring Safeboot

Filed under: Malware,Update — Didier Stevens @ 18:06

Chris explains how to recover the Safeboot key with a Live CD directly from the System Volume Information directory: Repairing Safe Mode (Safeboot)

Clever idea Chris!

Update: Viewing strings in executables

Filed under: Reverse Engineering,Update — Didier Stevens @ 17:59

Ryan was inspired by my post Viewing strings in executables and explains how to unpack Packed Executables.

Good work Ryan!

Friday 7 July 2006

Viewing strings in executables

Filed under: Reverse Engineering — Didier Stevens @ 8:30

I was having an interesting chat with a colleague. He is developing a VB6 application for his personal use, and has some of his friends beta-testing it. At one point in the conversation he asked me if it’s possible to view the strings in the compiled application, because he tried and didn’t see them. It turns out he implemented a popup message to remind his beta testers to check for an update, and wondered if this would be easy to circumvent.

It’s usually easy to circumvent, I’ll explain it here for my colleague and you.

Strings used in the source code of VB6 are stored as UNICODE strings in the compiled executable. UNICODE strings are usually encoded in UTF-16 on Windows, as UTF-16 is the standard format for the Windows API. ASCII characters are encoded with 2 bytes in UTF-16.

So for example, the string “Hello” is represented as 48 00 65 00 6C 00 6C 00 6F 00 in UTF-16. It’s because of the 2 bytes per character that my college didn’t remark the string.

There are many free utilities on the net to view strings in executables, I use these:

Consider this simple VB6 program (it will display a message box “Hello” when executed after 1/1/2007):

    If Now > DateValue("1/1/2007") Then 
        MsgBox "Hello" 
    End If

You can view the strings in the compiled program, hello.exe, with the Sysinternals tool strings like this (it’s a command-line tool):

strings hello.exe

The result will be this:

    Strings v2.2 
    Copyright (C) 1999-2005 Mark Russinovich 
    Sysinternals - www.sysinternals.com                         


You can see strings “1/1/2007″ and “Hello” listed at the beginning.

The BinText utility has a GUI, has more options and provides you with more info. Start BinText, use the Browse button to select the executable to analyze, and press Go to display the strings:


The red U indicates that it’s a UNICODE string. The first hexadecimal number is the position of the string in the file (hello.exe), and the second hexadecimal number is the position of the string in memory when the executable is executed.

So you will find string 1/1/2007 at position 000016D0 in the file and position 004016D0 in memory.

BTW, this technique to dump files will almost always fail when analyzing malware, because these files are often packed or encrypted. A simple trick to view the strings of such malware code is done with Process Explorer by Sysinternals. When the malware is running (you’ll want to run it on an isolated machine, like a virtual machine), start Process Explorer and display the properties of the running malware. The Strings tab will show you the strings in the file (Image) and in memory (Memory). Since the malware as unpacked/decoded itself when it started, you’ll be able to view the strings in memory.

The strings in an executable will give you an idea of what this program will do: you’ll recognize registry keys, URLs, filenames, …

Back to our example, hello.exe. We recognize a date, 1/1/2007. When you run the program (on a day before 1/1/2007), nothing happens. When you run the program after 1/1/2007 (by changing your computer clock), the Hello message box appears. So now we have clearly established that the string 1/1/2007 is indeed the date after which the message box is displayed.

How would someone modify this program to avoid the message box? A simple trick, which requires no programming skills, is to change the date 1/1/2007 to a later date, say 1/1/3007.

You’ll need a hex editor to do this, like the free XVI32.

  1. Start XVI32 and open the file hello.exe
  2. We know that string 1/1/2007 is at file position 000016D0: type CTRL+G, select hexadecimal, type 000016D0 and click OK
  3. The cursor is know positioned at the beginning of string 1/1/2007.
  4. Select digit 2 in the right pane:
  5. Type 3, this will replace digit 2 (32 hexadecimal) with digit 3 (33 hexadecimal)
  6. Save the file hello.exe

One can argue that this technique can only be applied to very simple programs, which have few strings and which store dates as strings. This is partially true, because it’s still easy to find dates in programs that don’t use dates for their normal operation. Like the program of my colleague. It’s almost 1 Mb large, but it only uses dates to decide when to display the message box. So they are easy to find, and easy to modify.

In fact, I didn’t use this string patching technique to show him how his message box could be disabled, but I changed the program logic by analyzing the assembler code and patching a few bytes. I will explain this in a next post.
But this shows my colleague how easy it is to disable his message box.

He can make it more difficult by hiding the dates:

  1. don’t use strings to represent dates
  2. use string manipulation to hide the date
  3. encrypt the strings and decrypt them at runtime
  4. pack the executable
  5. protect the executable with specialized software (e.g. ASprotect)

But this will not stop a determined attacker, there are even generic unprotection tools for ASprotect.

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