Tag: analiza

Backswap malware analysis

Date of publication: 19/06/2018, Hubert Barc

    Backswap is a banker, which we first observed around March 2018. It’s a variant of old, well-known malware TinBa (which stands for “tiny banker”). As the name suggests, it’s main characteristic is small size (very often in the 10-50kB range). In the summary, we present reasoning for assuming it’s the same malware.
    Read more

    Sage 2.0 analysis

    Date of publication: 14/02/2017, Jarosław Jedynak



    Sage is a new ransomware family, a variant of CryLocker. Currently it’s distributed by the same actors that are usually distributing Cerber, Locky and Spora.

    In this case malspam is the infection vector. Emails from the campaign contain only malicious zip file without any text. Inside zip attachment there is malicious Word document with macro that downloads and installs ransomware.

    After starting the ransomware, Windows UAC window is shown repeatedly until the user clicks yes.

    At the end the encryption process is started and all files are encrypted:

    Ransom message directs us to panel in the Tor network, but before we can log in we have to solve a captcha:

    And finally we are greeted with “user-friendly” panel:

    We can even chat with malware creators:

    Interestingly, this ransomware doesn’t remove itself after encryption, but copies itself to %APPDATA%\Roaming directory and re-encrypts all files after every reboot (until the ransom is paid).

    Technical analysis

    After this short introduction, We’ll focus on the technical side (because Sage 2.0 is not completely a generic ransomware, few things are rather novel).

    Main function of binary looks like this:

    As we see, there is a lot of fingerprinting and checks, though most of them are quite standard. More interesting features include:

    Debug switch

    Probably something didn’t work on the first try, so there is a debug command line parameter to test that configuration data is set correctly:

    And surely enough, this debug parameter does what it should:

    Someone probably forgot to remove this from the final version, because this is clearly a debugging feature.

    Locale Check

    Sage 2.0 creators like some nations more than others:

    This checks user keyboard layouts:

      • next == 0x23 -> Belarussian
      • next == 0x3F -> Kazakh
      • next == 0x19 -> Russian
      • next == 0x22 -> Ukrainian
      • next == 0x43 -> Uzbek
      • next == 0x85 -> Sakha

    We’re a bit disappointed that Polish didn’t make it on the exception list (If Sage creators are reading this: our locale is 0x15).

    Location fingerprinting

    Sage is trying to get it’s host location by querying maps.googleapis.com with current SSID and MAC:

    Canary file

    Before encryption Sage checks for existence of a special debug file:

    Thanks to this, malware creators don’t have to worry about accidentally running the executable and encrypting their own files.

    Finally, if the file is not found, encryption is initiated.

    Extension whitelist

    Of course, not every file is encrypted – only files with whitelisted extension are touched:


    As usual, this is the most interesting thing in ransomware code. Sage 2.0 is especially unusual because it encrypts files with elliptic curve cryptography.

    The curve used for encryption is y^2 = x^3 + 486662x^x + x over the prime field defined by 2^255 – 19, with base point x=9. These values are not arbitrary – this curve is also called Curve25519 and is the state of the art in modern cryptography. Not only it’s one of the fastest ECC curves, it’s also less vulnerable to weak RNG, designed with side-channel attacks in mind, avoids many potential implementation pitfalls, and (probably) not backdoored by any three-letter agency.

    Curve25519 is used with hardcoded public key for shared secret generation. The exact code looks like this (with structures and function names by us):

    This looks like properly implemented Elliptic Curve Diffie-Hellman (ECDH) protocol, but without private keys saved anywhere (they are useful only for decryption and malicious actors can create them anyway using their private key).

    This may look complicated, but almost all those functions are just wrappers for ECC primitive – named CurveEncrypt by us. For example, computing matching public key is curve25519(secretKey, basePoint) – where basePoint is equal to 9 (one 9 and 31 zeroes).

    Shared key computation is very similar, but instead of using constant base point we use public key:

    Due to the design of Curve25519, converting between any sequence of random bytes and a secret key is very easy – it’s enough to mask few bits:

    And, also because of this, secret key generation is completely trivial (it’s enough to generate 32 random bytes and convert them to the secret key):

    That’s all for the key generation. What about file encryption? Files are encrypted with ChaCha (unconventional algorithm, again) and key is appended to output file – but after being encrypted with Curve25519:

    AppendFileKeyInfo fucntion appends sharedKey and pubKey to the file:

    ChaCha is not very popular algorithm among ransomware creators. It’s very closely related to Salsa20 which was used in Petya ransomware. We don’t know why AES is not good enough for Sage – probably it’s only trying to be different.

    In other words, there are two sets of keys + one key pair for every encrypted file:

    After ransomware finishes we know only my_public, sh_public, fl_shared, but we need chachakey to actually decrypt the file.

    This encryption scheme is quite solid because it makes offline encryption possible – there is no need to bother connecting with C&C and negotiating encryption keys – the public key is hardcoded in binary and because of asymmetric cryptography decryption is impossible. Assuming that malware creators didn’t make any drastic implementation mistakes (and we have no reason to suspect that they did), recovery of encrypted files is impossible. Of course, it’s always possible that master encryption key will eventually be leaked or released.

    Additional information

    Yara rules:

    Hashes (sha256):

      • sample 1, 362baeb80b854c201c4e7a1cfd3332fd58201e845f6aebe7def05ff0e00bf339
      • sample 2, 3b4e0460d4a5d876e7e64bb706f7fdbbc6934e2dea7fa06e34ce01de8b78934c
      • sample 3, ccd6a495dfb2c5e26cd65e34c9569615428801e01fd89ead8d5ce1e70c680850
      • sample 4, 8a0a191d055b4b4dd15c66bfb9df223b384abb75d4bb438594231788fb556bc2
      • sample 5, 0ecf3617c1d3313fdb41729c95215c4d2575b4b11666c1e9341f149d02405c05

    Additional information:

    Nymaim revisited

    Date of publication: 30/01/2017, Jarosław Jedynak



    Nymaim was discovered in 2013. At that time it was only a dropper used to distribute TorrentLocker. In February 2016 it became popular again after incorporating leaked ISFB code, dubbed Goznym. This incarnation of Nymaim was interesting for us because it gained banking capabilities and became a serious threat in Poland. Because of this, we researched it in depth and we were able to track Nymaim activities since then.

    However a lot of things have changed during the last two months. Most notably, Avalanche fast-flux network (which was central to Nymaim operations) was taken down and that struck a serious blow to Nymaim activity. For two weeks everything went silent and even today Nymaim is a shadow of its former self. Although it’s still active in Germany (with new injects), we haven’t observed any serious recent activity in Poland.


    This topic is really well researched by other teams, but it’s still interesting enough to be worth mentioning. Nymaim is heavily obfuscated with a custom obfuscator – to the point that analysis is almost impossible. For example typical code after obfuscation looks like this:

    But with some effort we can make sense of it. There are a lot of obfuscation techniques used, so we’ll cover them one by one:

    First of all, registers are usually not pushed directly onto the stack, but helper function “push_cpu_register” is used. For example push_cpu_register(0x53) is equivalent to pushing ebx and push_cpu_register(0x50) is equivalent to pushing eax. Constants are not always the same, but registers are always in the same order (standard x86 ordering).

    . register constant
    0 eax 0x50
    1 ecx 0x51
    2 ebx 0x52
    3 edx 0x53
    4 esp 0x54
    5 ebp 0x55
    6 esi 0x56
    7 edi 0x57

    Additionally, most constants in code gets obfuscated too – for example mov eax, 25 can be changed to:

    The constant used in the example is 8CBFB5DA, but there’s nothing special about it – it’s a random dword value, generated just for the purpose of obfuscating this constant. The only thing that matters is the result of the operation (0x25 in this case).

    Additionally there other similar obfuscating functions are used sometimes – for example sub_*_from_eax and add_*_to_eax.

    Last but not least, the control flow is heavily obfuscated. There are a lot of control flow obfuscation methods used, but all boil down to simple transformation – call X and jmp X are transformed to at least two pushes. This obfuscation is in fact very similar to previous one – instead of jumping to 0x42424242, malware calls function detour with two parameters: 0x40404040 and 0x02020202. The detour adds it’s parameters and jumps to the result. In pseudoasm instead of:

    we have:

    There exists also a slight variation of this method – instead of pushing two constants, sometimes only one constant is pushed and machine code after a call opcode is used instead of a second constant (detour uses return address as a pointer to the second constant).

    To sum up, previously pasted obfuscated code should be read like this:

    With this in mind, we created our own deobfuscator. This was quite a long time ago and since then other solutions have shown up. Our deobfuscator probably isn’t the best, but is easily modifiable for our needs and it has some unique (as far as we know) features that we need, for example it imports recovery and decrypting encrypted strings stored in binary. Other deobfuscators include mynaim and ida-patchwork Nevertheless, with our deobfuscator we are able to untangle that messy code to something manageable:

    When it comes to Nymaim obfuscation capabilities it’s not nearly over. For example external functions are not called directly, instead of it an elaborate wrapper is used:

    This wrapper pushes hash of function name on the stack and jumps to the next dispatcher (even though call opcode is used, this code never returns here):

    A second dispatcher pushes hash of a dll name on the stack and jumps to the helper function:

    And finally real dispatcher is executed:

    Additionally, real return address from API is obfuscated – return address is set to call ebx somewhere in the ntdll (real return address is somewhere in ebx by then, of course). Most tools are very confused by it. Let’s just say, it’s very frustrating when debugging and/or single stepping.

    But wait, there’s more! As we have seen, short constants are obfuscated with simple mathematical operations, but what about longer constants, for example strings? Fear not, malware authors have a solution for that too. Almost every constant used in the program is stored in a special data section. When Nymaim needs to use one of that constants, it is using special encrypted_memcpy function. At heart it is not very complicated:

    Inner workings of memcpy_and_decrypt are not that complicated either. Our reimplementation of the encryption algorithm in Python is only few lines long:

    We only need to extract constants used for the encryption (they differ between executables) – they are hidden in these portions of code:

    (These functions are not obfuscated, so extraction can be done with simple pattern matching).

    But encryption of every constant was not good enough. Malware authors decided that they can do better than that – why don’t encrypt the code too? That’s not very often used, but few critical functions are stored encrypted and decrypted just before calling. Quite an unusual approach, that’s for sure. Ok, let’s leave obfuscation at that.

    Static Configuration

    After deobfuscation, the code is easier to analyze and we can get to interesting things. First of all, we’d like to extract static configuration from binaries, especially things like:

      • C&C addresses
      • DGA hashes
      • Encryption keys
      • Malware version
      • Other stuff needed for communication

    How hard can that be? Turns out that harder than it looks – because this information is not just stored in the encrypted data section.

    Fortunately, this time the encryption algorithm is rather simple.

    We just need to point nymaim_config_crypt to the start of encrypted static config and everything will just work.

    How do we know where static config starts? Well… We tried few clever approaches (matching code, etc), but they weren’t reliable enough for us. Finally, we solved this problem with a simplest possible solution – we just try every possible index in binary and try to decrypt from there. This may sound dumb (and it is), but with few trivial heuristics (static config won’t take 3 bytes of space, neither will it take 3 megabytes) this is quite fast – less than 1s on typical binary – and works every time.

    Despite this, after decrypting static config we get a structure, which is is quite nice and easy to parse. It consists of multiple consecutive “chunks”, each with assigned type, length and data (for those familiar with file formats, this is something very similar to PNG, or wav, or any other RIFF).

    Graphically this looks like this:

    And chunks are laid consecutive in static config block:

    So we can quickly traverse through all chunks of a static config with a simple five-liner:

    Snippet from process_chunk (hash == chunk_type):

    After initial parsing the static config looks like this:

    static config example

    (By the way, in this article chunk types are usually represented byte-order, i.e. big endian)

    And in a more human readable form with most interesting chunks interpreted:

    static config example

    Infection timeline

    There is more than one “kind” of Nymaims. As of now we distinguish between three kinds:

      • dropper – first Nymaim that gets executed on the system. This is the only type distributed directly to victims.
      • payload – module responsible for most of the “real work” – web injects for example
      • bot_peer – module responsible for P2P communication. It tries to become supernode in the botnet.

    These are all one kind of malware and all of them share the same codebase, except few specialized functions. For example our static config extractor works on all of them, just like our deobfuscator and they all use the same network protocol.

    Dropper role is simple. It performs few sanity checks – for example:

      • Makes sure that it’s not virtualized or incubated
      • Compares current date to “expiration time” from static config
      • Checks that DNS works as it should (by trying to resolve microsoft.com and google.com)

    If something isn’t right, the dropper shuts down and the infection doesn’t happen.

    The second check is especially annoying, because if you want to infect yourself Nymaim has to be really “fresh” – older executables won’t work. Even if you override check in the binary, this is also validated server-side and the payload won’t be downloaded.

    If we want to connect to a Nymaim instance, we need to know the IP address of peer/C&C. Static config contains (among others) two interesting pieces of information:

      • DNS server (virtually always it’s and
      • C&C domain name (for example ejdqzkd.com or sjzmvclevg.com).

    Nymaim is resolving that domain, but returned A records are not real C&C addresses – they are used in another algorithm to get a real IP address. We won’t reproduce that code here, but there is a great article from Talos on that topic. If someone is interested only in the DGA code, it can be found here:


    When dropper obtains C&C address, it starts real communication. It downloads two important binaries and a lot more:

      • payload – banker module (responsible for web injects – passive member of botnet)
      • optional bot module (it is trying to open ports on a router and become an active part of a botnet. When it fails to do so, it removes itself from a system).
      • few additional malicious binaries (VNC, password stealers, etc – not very interesting for us).


    Payload is very different from dropper when it comes to network communication:

      • No hardcoded domain
      • But has DGA
      • And P2P

    The payload’s DGA algorithm is really simple – characters are generated one by one with simple pseudo-random function (variation of xorshift). Initial state of DGA depends only on seed (stored in static config) and the current date, so we can easily predict it for any given binary. Additionally, researchers from Talos have bruteforced valid seeds, simplifying the task of domain prediction even more.


    First of all, few examples why we suspected from the start that there is something else besides DGA:

    We have taken one of our binaries that hadn’t behaved like the payload, unpacked it, deobfuscated and reverse engineered it. But even without in-depth analysis, we’ve found a lot of hints that P2P may be happening. For example we can find strings typical for adding exception to Windows Firewall (and of course – that’s what malware did when executed on a real machine).

    Another suspicious behavior is opening ports on a router with help of UPNP. Because of this, infected devices from around the world can connect to it directly.

    And finally something even more outstanding. As we have seen, the malware presents itself as the Nginx in the “Server” header. Where does this header come from? Directly from the binary:

    We implemented tracker for the botnet (more about that later) and with the data we obtained, we concluded that this probably is a single botnet, but with geolocated injects (for example Polish and US injects are very similar). Distribution of IPs we found is similar to what other researchers have determined (we have found more PL nodes and less US than others, but that’s probably because the botnet is geolocated and we were more focused on Poland).

    49.9% (~7.5k) of found supernodes were in Poland, 30% (~4.5k) in Germany and 15.7% (~2.2k) in the US.

    Network protocol

    And now for something more technical. This is an example of a typical Nymaim request (P2P and C2 communication use the same protocol internally):

      • Host header is taken from the static config
      • Randomized POST variable name and path
      • POST variable value = encrypted request (base64 encoded)
      • User-Agent and rest of the headers are generated by WinHTTP (so headers are not very unique and it’s impossible to detect Nymaim network requests by using only them).

    Typical response:

      • This isn’t really Nginx, just pretending.
      • Everything except the data section is hardcoded
      • Data = encrypted request

    Encrypted messages have very specific format:

    A lower nibble of the first byte is equal to a length of the salt and a lower nibble of the second byte is equal to the length of the padding. Everything between the salt and the padding is the encrypted message. To decrypt it, we need to concatenate the key with the salt – and use that password with the rc4 algorithm.

    It can be easily decrypted using Python (but we had to reverse engineer that algorithm first):

    After decrypting a message, we get something with a format very similar to the static config (i.e. a sequence of consecutive chunks):

    Each chunk has its type, length and raw data:

    We can process decrypted message with almost exactly the same code as code for static config:

    And this is the basic code used for parsing the message. Each chunk type needs to be processed a bit differently. Interestingly, parsing message is recursive, because some chunk types can contain other lists of chunks, which in turn can contain other lists of chunks, etc. Unfortunately, important chunks have another layer of encryption and compression. At the end of an encrypted chunk we can find special RSA encrypted (or rather – signed) header. After decryption (unsigning) of the header, we can recover a md5 hash and length of the decrypted data and most important of all – a Serpent cipher key used to encrypt the data.

    After the decryption we will stumble upon another packing method – decrypted data is compressed with APLIB32. This structure is very similar to the one used by ISFB – firstly we have magic ‘ARCH’, then length of compressed data, length of uncompressed data and crc32 – all of them are dwords (4 bytes).

    Again, it’s nothing Python can’t deal with. We quickly hacked this function to recover real data hidden underneath:

    With this function we finally managed to hit the jackpot. We decrypted all of the interesting artifacts passed over the wire, most importantly additional downloaded binaries, web filters and injects.


    An example request, after dissection, may look like this:

    As we can see, quite a lot of things is passed around here. There are a lot of fingerprinting everywhere and some information about current state.

    Responses are often more elaborate, but for the sake of presentation, let’s dissect a simple one:

    An infected machine gets to know its public IP address, IP addresses (and listening ports) of its peers and the active domain. Additionally it is usually ordered to sleep for some time (usually 90 seconds when some files are pending to be transmitted and 280 seconds when nothing special happens).

    Here is the list of types of chunks that we can parse and understand:

    chunk hash short description
    ffd5e56e fingerprint 1
    014e2be0 fingerprint 2 + timestamps
    f77006f9 fingerprint 3
    22451ed7 crcs of last received chunks of type be8ec514 and 0282aa05
    b873dfe0 probably “enabled” flag (can be only 1 or 0)
    0c526e8b nested chunk (decrypt with nymaim_config_crypt, unpack with aplib, recursively repeat parsing)
    875c2fbf plain (non-encrypted) executable
    08750ec5 nested chunk (decrypt with nymaim_config_crypt, unpack with aplib, recursively repeat parsing)
    1f5e1840 injects (decrypt with serpent, unpack with aplib, parse ISFB binary format)
    76daea91 dropper handshake (marker, without data)
    be8ec514 list of peer IPs
    138bee04 list of peer IPs
    1a701ad9 encrypted binary (decrypt with serpent, unpack with aplib, save)
    30f01ee5 encrypted binary (decrypt with serpent, unpack with aplib, save)
    3bbc6128 encrypted binary (decrypt with serpent, unpack with aplib, save)
    39bc61ae encrypted binary (decrypt with serpent, unpack with aplib, save)
    261dc56c encrypted binary (decrypt with serpent, unpack with aplib, save)
    a01fc56c encrypted binary (decrypt with serpent, unpack with aplib, save)
    76fbf55a padding
    cae9ea25 nested chunk (decrypt with nymaim_config_crypt, unpack with aplib, recursively repeat parsing)
    0282aa05 nested chunk (decrypt with nymaim_config_crypt, unpack with aplib, recursively repeat parsing)
    d2bf6f4a state informations
    41f2e735 web filters
    1ec0a948 web filters
    18c0a95e web filters
    3d717c2e web filters
    8de8f7e6 datetime (purpose is unknown, it’s always few days ahead of current date)
    3e5a221c list of additional binaries that was downloaded
    5babb165 payload handshake (marker, without data)
    b84216c7 public IP of infected machine
    cb0e30c4 number of seconds to sleep
    f31cc18f additional CRC32s of downloaded binaries
    920f2f0c injects (decrypt with serpent, unpack with aplib, parse ISFB binary format)
    930f2f0c injects (decrypt with serpent, unpack with aplib, parse ISFB binary format)

    This may seem like a lot, but there are a lot of things we didn’t try to understand (we ignored most of dword-sized or always-zero chunks).

    After extracting everything from communication we can finally look at injects. For example Polish ones:

    (304 different injects, as of today)

    Or US injects:

    (393 different injects, as of today)


    Yara rules:

    Hashes (md5):

      • Payload 2016-10-20, 9d6cb537d65240bbe417815243e56461, version 90032
      • Dropper 2016-10-20, a395c8475ad51459aeaf01166e333179, version 80018
      • Payload 2016-10-05, 744d184bf8ea92270f77c6b2eea28896, version 90019
      • Payload 2016-10-04, 6b31500ddd7a55a8882ebac03d731a3e, version 90012
      • Dropper 2016-04-12, cb3d058a78196e5c80a8ec83a73c2a79, version 80017
      • Dropper 2016-04-09, 8a9ae9f4c96c2409137cc361fc5740e9, version 80016

    Repository with our tools: nymaim-tools

    Other research

    Evil: A poor man’s ransomware in JavaScript

    Date of publication: 18/01/2017, Jarosław Jedynak



    Initially Evil was brought to our attention by an incident reported on 2017-01-08. By that time the Internet was completely silent on that threat and we had nothing to analyze.

    We found first working sample day later, on 2017-01-09. In this article we will shortly summarize our analysis and conclusions. Since then, we had relatively high number of infections reported, so we predict that this family of ransomware may become a bigger threat in near future.

    This malware follows recent trend, and doesn’t have any decryption panel (like CryptoMix) – instead of this, an email address is provided. Sure, why complicate things if simple solutions work good enough.

    Read more

    Technical analysis of CryptoMix/CryptFile2 ransomware

    Date of publication: 04/01/2017, Jarosław Jedynak

    skull and crossbones malware


    CryptoMix is another ransomware family that is trying to earn money by encrypting victims files and coercing them into paying the ransom.
    Until recently it was more known as CryptFile2, but for reasons unknown to us it was rebranded and now it’s called CryptoMix.
    It was observed in the wild being served by the Rig-V exploit kit.

    This malware stands out from among others, but not necessarily in a good way.


    First unusual thing about this family is very large amount of money requested – 5 bitcoins is an insane amount of money (especially considering that CryptoMix is really primitive under the hood, but we’ll get to it). We don’t know how many victims have paid, but probably few were desperate enough.

    Additionally we have stumbled upon following comment discouraging anyone from paying the ransom:

    we were infected and they asked for 10 bitcoins, after some negotiations the price was lowered to 6 bitcoins. they provided 1 decrypted file to prove concept. we paid 6 bitcoins and they asked for another .6 as the c&c server will not provide the key due to late payment. after promptly paying another .6 bitcoins (about $4800 in total) there has been no communication from them! its been 2 weeks and nothing.

    We can’t verify if this is true, but it sounds plausible – if someone is desperate enough to pay 6 bitcoins for his files, he probably can be coerced into paying even more. As usual, we discourage anyone from supporting the criminals by paying the ransom.

    Payment portal

    Additionally CryptoMix doesn’t have any payment portal in the Tor network. Or any payment portal, for that matter – victim have to write an email and literally wait some time before malware operators kindly send the decryption keys
    (assuming that they will do it, instead of bargaining for even more money).

    For example, ransom message can look like this (most recent variant):

    Or like this (older variant):

    We don’t think that this strategy was well thought out. First of all, using emails for communication with victims is bothersome and need constant attention.
    Automated portal would be much more reliable and secure for both sides. Additionally, emails are prone to being deleted/locked, effectively cutting malware authors from their “clients”.


    Content of exchanged emails is very unusual too. Actors claim to be a charity organization (!) that is going to sponsor presents and medical help for children. For example:

    That’s really original, but unfortunately also obviously false.

    Technical analysis

    Leaving aside strange quirks of ransomware “interface”, let’s get more technical. In its heart, CryptoMix is just a bare bones encryptor – it doesn’t have any fancy features, it doesn’t have a web portal, it doesn’t change user wallpaper, the only thing it does is encrypting every file on the victim’s disk and on the mounted network drives.

    CryptoMix is protected by a very primitive packer – the real binary is stored in resources, and xored with a hardcoded key. For some reason, Cuckoo has problems with automatic unpacking of cryptomixer, so we had to write our own unpacker. Using pefile and Yara is very easy:

    After decryption ransomware checks whether it’s being debugged – but no antiVM techniques are employed, so everything works as it should under VirtualBox.

    Before file encryption starts, the ransomware checks internet connectivity (using InternetOpenUrl function). If everything is ok, an encryption key is generated on victim’s PC and sent to the C&C server.
    Otherwise, depending on malware version, either a hardcoded encryption key is used or malware is spinning in an infinite loop until the internet connection is restored.

    The main function can be expressed as follows:

    After encryption key is generated/selected, it is stored in windows registry. Registry key used for malware specific data varies depending on version, but for example SoftWare\Microsoft\Windows\Shell\Nodes Slots, SoftWare\Microsoft\Windows\Shell\FlashPlayerPluginK or Software\Adobe Reader LicensionSoftWare\AdobeLicensionSoftWare can be used (malware probably tries to hide its presence by impersonating another software).

    The list of supported extensions constains more than 1250 entries:

    That’s quite a lot of extensions, but nothing special (for comparsion: CryptXXX supports 933 extensions, CrypMIC 901). Most unusual thing here is inclusion of another ransomware extensions (for example .zepto, .locky, .crypt, .locked, .cryptolocker, .cryptowall, etc).


    Let’s get back to ransom message for a while:

    Malware claims that our files are “encrypted with 2048bit RSA KEY”. Well, it’s not entirely true. Yes, 2048bit RSA key is generated with windows Crypto API – but after RSA key is selected, it is hashed with SHA256 to create a real encryption key and every file on disk is encrypted with that key. Encryption algorithm used is AES 256 in CBC mode without initialization vector.

    Encryption routine can be summarized with this (simplified) code:

    This function is called for every file, so hashing rsaKey and deriving AES key every time doesn’t make much sense. But there is bigger problem with it – there is no need for such things as “public” and “private” keys, because this encryption routine is entirely symmetric – RSA serves here just as (unnecesarily slow) random number generator.

    So yes, in a way RSA is “used for encryption”, but files are not encrypted with RSA and encryption is entirely symmetric.

    UserID given by CryptoMix is not random – it is generated from username and serial number for first disk.

    This doesn’t seem like a good idea, because UserIDs absolutely have to be unique, and neither username nor volume serial number is designed to be unique – so userID collisions are possible and very plausible (after taking low entropy of userID and birthday paradox into account).
    Why is this a problem? Because when UserID collision happens, malware creators have no way of distinguishing two users apart – so they don’t know which encryption key belongs to which user, and can’t send the right one. It’s also possible that in case of collision old key will be overwritten in database and lost.

    Finally, CryptoMix achieves persistence by copying itself to user documents and writing to HKEY_CURRENT_USER\SoftWare\Microsoft\Windows\CurrentVersion\Run registry key.

    As a final measure, all shadow copies are removed (if user doesn’t have admin account, UAC window is shown before):

    Cryptomix Decryptor

    Due to a cryptographic flaw in encryption, we are able to decrypt CryptoMix (and CryptFile2), but only sometimes and only if files were encrypted with a vulnerable version.
    If your files were encrypted by CryptoMix and you don’t want to pay a ransom, you can contact us at [email protected] and we’ll see what we can do.
    Please attach a single encrypted file without changing it’s filename after encryption (for example warnings.h.email[[email protected]]id[7e5973f5e0ce337d].lesli).


    Cryptomix packer (old and new):

    Cryptomix payload (after unpacking):


    c2f30cd537c79b6bcd292e6824ea874e sample0

    befc0e43b38fe467ddc3ddd73150a9fc sample0 decrypted

    8c413e31f39a54abf78c3585444051f7 sample1

    0d1206246bf15c521474cee42f13fc09 sample1 decrypted

    b778bda5b97228c6e362c9c4ae004a19 sample2

    042a38a32cd20e3e190bb15b085b430a sample2 decrypted

    Talking to Dridex (part 0) – inside the dropper

    Date of publication: 10/11/2015, CERT Polska


    Dridex mostly comes to us as spam which contains a .doc with some macros, responsible for downloading a dropper. One can quickly analyze it using oledump.py and looking through vbscript, or naturally, just try to run it in a sandbox and obtain the dropped files.
    Read more

    Ransomware still a threat to Polish users

    Date of publication: 19/09/2013, CERT Polska


    During the summer holidays we observed an increased infection rate of ransomware. We mentioned this type of malware a few times already in the past (here is a description of similar malware and here is information detailing how to remove it from your computer). CERT Polska was able to acquire three samples of this malware from three different sources. In every case we were able to determine the infection vector. Most probably, all of the three samples were created by the same group of cybercriminals. One of the samples came from a hacked .gov.pl website in collaboration with CERT.GOV.PL, second sample was from a hacked website in .eu domain and the last sample was from a malicious advertisement from a .pl website. A case of malware on the governmental website was also a subject of our previous blog post.
    Read more