Interception: Dissecting BokBot’s “Man in the Browser”

code from BokBot’s proxy module

BokBot: Proxy Module

This article is a continuation of CrowdStrike’s recent blog, “Digging Into BokBot’s Core Module,” and provides a detailed analysis of the inner workings of the BokBot proxy module — a complex piece of code written to trick victims into sending sensitive information to a command and control (C2) server. 


The BokBot banking Trojan —  also known as IcedID — was first observed in April 2017 and CrowdStrike has been tracking this threat ever since. BokBot has been used against financial institutions worldwide and is able to augment its capabilities by retrieving several modules, including one that runs a local proxy server. This proxy module is able to intercept and potentially manipulate web traffic with the goal of facilitating financial fraud.

The BokBot core module downloads the proxy module, injects it into a spawned svchost child process, and the proxy module initializes itself in the target process. The following is a step-by-step analysis of how this process unfolds.

Module Initialization

Prior to standing up the proxy server thread, this module goes through an initialization process. Most of it is similar to the other BokBot modules — covered in the previous blog — and includes making errors less noisy, building a list of C2s, and setting up named events for communicating with the parent process. The following steps are taken prior to intercepting any traffic from a victim’s browser.

Webinject Updates

User-mode asynchronous procedure call (APC) objects are used to trigger update events for the webinject data that exists in process memory. During initialization, separate user APC objects are sent to the APC queue, one for each of the webinject DAT files (see previous blog). Each DAT file is decoded and stored in process memory, and will be passed as a parameter to the APC callback function. Once the APC queue is processed, the web configs will be parsed and loaded into process memory.

C2 Communication Thread

The communication thread is signaled whenever there is collected data to be sent back to the C2. A signal event occurs when the injected malicious javascript sends a specific type of request to the proxy server (see: Browser Perspective section) . The data that is sent back can consist of harvested personal information, snapshots or proxy-related errors.

Proxy Server Initialization

The proxy server is bound to on TCP port 57391. After the listener is set, a Windows Socket API (WSA) event handler is registered using this socket to handle all connect/send/receive network requests.

SSL Certificates

In order to perform a man-in-the-middle (MITM) attack on SSL connections, the proxy server needs to generate an SSL certificate, and insert it into the cert store. The certificate is created by calling CertCreateSelfSignCertificate, using the following hard-coded distinguished name (DN) values:

BokBot Proxy: DN values code

A temporary cert store is created in memory and will eventually be written out to the following location, with a filename that is generated using the Bot ID:

  • C:\Users\jules\AppData\Local\Temp\D38D667F.tmp

The certificate store contains an SSL certificate for the webinject and C2 servers.

BokBot Proxy: SSL Cert for webinject and C2 servers

Once these steps have been completed, the proxy server is ready to intercept requests from the browser, but the browser has not been reconfigured to point to the proxy server.

Proxy Connections

Now, whenever the browser attempts to connect to a website, that request is hijacked and first processed by the proxy server. This section covers how the connection states are managed, how SSL MITM works, and what actions are taken by the proxy server.

Managing Connection State

All connections are managed by a series of data structures tied to WSA callback events that keep track of internal communication with the client, external communication with the target website, and to ensure the integrity of all the requests handled by the proxy server.

Proxy Connection State Architecture

Figure 1: Proxy Connection State Architecture

The architecture diagram in Figure 1 summarizes the layout of relations between the proxy server components used to maintain the state of a proxy request.  

SSL Man in the Middle

Any SSL request, where the URL matches a URL from the list of webinject targets, is hijacked by the proxy inserting itself into the communication using its own SSL certificate. The proxy server receives the request from the victim and establishes an SSL connection using the proxy server’s SSL certificate. After that, the proxy server sends the request to the target website, and establishes an SSL connection between the proxy and the target website. The response from the target is decrypted, and then encrypted using the proxy’s certificate. This response is sent to the victim, where the traffic will be decrypted using the proxy’s certificate.  An SSL context data-structure, similar to Figure 1, is used to maintain the state of the SSL traffic.

See the “Ensuring Valid Certificates” section to understand how the browser is hooked to ensure that it sees these certificates as valid.

Proxy Action

If the requested URL hostname does not match one of the webinjects, then the proxy server passes the HTTP requests/responses between the infected host and the web server. However, if a URL matches one of the many websites targeted by the webinjects, then additional action is taken (see “Traffic Manipulation Proxy Perspective”).

Redirecting Browsers to the Proxy

After the proxy server has been initialized, any currently executing browser process will have to be configured to use the proxy. To do this, BokBot injects code into the browser process. The injected code adds hooks into key functions, allowing it to hijack browser traffic.

Browser Process Selection

A looping thread is spawned to iterate over all of the currently executing process names. To get a list of current processes, a list of BASIC_PROCESS_INFORMATION structures is generated using ZwQuerySystemInformation.

Process Identification

Once the list is generated, the module will attempt to identify browser processes. A hash of the process name is used to identify whether the process is one of the specific browser processes listed in Table 1.

BokBot Proxy: Browser Identification by Hash

Table 1: Browser Identification by Hash

The hash is generated using a custom method, and then XORed with a constant value that varies between samples, so that the hash values differ between campaigns.

Proxy Configured Check

Once the browser has been identified an additional check is made to determine if this browser process has already had the proxy configured by BokBot. To keep track of every browser process that has been injected into, a linked list data structure is created (Figure 2). The entire list is walked, checking to see if both the process ID and the process creation time exist in the list. If the browser is not in the list, then a new list item is created that contains the target process ID and target process creation time.

Linked List of Browser Processes using the BokBot Proxy

Figure 2: Linked List of Browsers Processes Using the Proxy

An additional check is made by attempting to open up a named event. After the code is injected, a named event is created by the browser process. The naming scheme is generated in a similar manner as was discussed in the previous blog about BokBot’s main module, however, the process ID is appended to the end of the name. The module calls  OpenEvent and if the returned error code is ERROR_FILE_NOT_FOUND, the injection code continues.

Opening the Process and Additional Checks

OpenProcess is called to open a handle to the browser process. BokBot checks to see if the process is WOW64, and if so, different procedures are used that will yield the same result.

To be thorough, another check is made to determine if the process has already been configured:

  • ZwQueryInformationProcess and ReadProcessMemory are used to get the process environment strings.
  • Each string is checked to see if this exists: v313235373937=true
    • The 3132353739 is an ascii representation of the BotID.
  • No injection occurs if this environment string exists.

Browser Process Injection and Code Execution

The proxy configuration code is injected in the same manner as BokBot’s child process injection method (see the blog on BokBot’s main module), except for two things:OpenProcess is called to connect to the process, and a hook is added for ZwWaitForSingleObject. This means that as soon as the browser executes ZwWaitForSingleObject, the injected code executes:  The hook is removed, the proxy is configured and the ZwWaitForSingleObject call is completed to maintain process execution.

Context Structure

The context data structure is injected into the browser process and provides the proxy configuration code with the necessary information to properly configure the proxy.

Browser Proxy Configuration Code

The “configuration code” is actually a series of procedure hooks that are used to insert the proxy module into the communication channel:

  • CertVerifyCertificateChainPolicy
  • CertGetCertificateChain
  • connect (ws2_32.dll)
  • Browser specific functions:
    • Internet Explorer: MSAFD_ConnectEx
    • Firefox: SSL_AuthCertificateHook
    • Chrome:  ws2_32.WSAEventSelect

Inserting the Hooks

Most of the hooks are inserted by walking the export table of the target module, hashing the procedure name, and then comparing that hash against a static value. If these hashes match, then the hook can be placed.

BokBot Proxy: Hash for Hooked Function Names

Table 2: Hash for Hooked Function Names

To get the address for MSAFD_ConnectEx, a different method is used and is discussed in the “Internet Explorer: MSAEFD_ConnectEx” section.

Winsock2 Connect Hook

The Winsock2 hook intercepts all AF_INET network traffic that uses the connect API to send network traffic (Firefox and IE). The socket is set to non-blocking mode, using ioctlsocket and a new connect call is sent to the proxy server. Once a connection is established, a 12-byte packet containing the following data is sent to the proxy server:

BokBot Proxy: Winsock2 Connect Hook code

These 12 bytes are parsed and stored in the PROXY_SESSION_CONTEXT data structure (Figure 1). The result of the hooked call is a network file descriptor. Any network call that uses this file descriptor will be sent to the proxy server.  Whichever browser made the call will be unaware of the proxy intercepting the traffic.

Hooking Browser-Specific Functions

The next set of hooks are dependent on the target browser. Essentially, whether or not it is a browser-specific library or a MS shared module, each browser handles requests in a different manner.

Internet Explorer: MSAFD_ConnectEx

Similar to the previous connect hook, this function swaps out the original socket with a socket that contains the connection data for the proxy server. The procedure address is not located in the export table of mswsock.dll, so the address is acquired by calling WSAIoctl socket with the IO Control Code of SIO_GET_EXTENSION_FUNCTION_POINTER (0xC8000006).

FireFox: SSL_AuthCertificateHook

Firefox uses the SSL_AuthCertificateHook callback function to authenticate a certificate. SECSuccess (null) is returned if the certificate is authenticated. BokBot attempts to hook this function in the “nss3.dll” module and if that fails, it will patch the same function in “ssl3.dll.”  The hook always returns SECSuccess.

Google Chrome: WSAEventSelect

In addition to the ws2_32.connect procedure being hooked, BokBot adds an additional hook in the ws_32.dll module in WSAEventSelect. The hook grabs the socket and the event object for every connection event (FD_CONNECT). This data will be processed by the call to the hooked connect procedure.

Google Chrome: connect

Essentially, this hook does the same thing as what is covered in the previous section on the connect hook for IE and Firefox. The main difference is that all connection events collected by the WSAEventSelect hook are processed by this hook.

Ensuring Valid Certificates

Once browser traffic is redirected to the proxy, the malware must prevent browser certificate errors from informing the user that requests are being intercepted. To ensure that the certificates are both verified and trusted, two procedures are hooked: CertVerifyCertificateChainPolicy and GetCertificateChain.

Certificate Chain Verification

Certificate chains are verified by calling CertVerifyCertificateChainPolicy. This procedure returns a Boolean function to signify whether or not a specific chain is valid. BokBot hooks this function to ensure that all attempts to verify SSL certificate chains (CERT_CHAIN_POLICY_SSL) will always return TRUE.

Certificate Chain Context Trust

In order to ensure that the browser sees that the certificates are trusted, BokBot hooks the GetCertificateChain procedure. GetCertificateChain will construct a CERT_CHAIN_CONTEXT structure that contains an array of CERT_SIMPLE_CHAIN structures.  Each one of these CERT_SIMPLE_CHAIN structures contains an array of CERT_CHAIN_ELEMNT data structures.

These data structures all contain a field, TrustStatus, used to convey potential issues with the certificate chain. To ensure success, the TrustStatus field needs to be modified to ensure all certificates in the chain are trusted.

TrustStatus is a structure comprised of a field that identifies errors with the certificate (ErrorStatus) and a field that contains an information status for the ticket (InfoStatus).  Patching these two fields within each structure will trick the browser into believing that the certificates are all trusted.

First, ErrorStatus is set to indicate that there is no error with the certificate or chain:

This value is set the same for all of the data structures. The InfoStatus fields, however, are different between the CERT_CHAIN_ELEMENT structure, CERT_CHAIN_CONTEXT and CERT_SIMPLE_CHAIN structures:

BokBot Proxy: Cert_Chain Structures

Once these two values are set, the certificate chain will be seen as trusted by the browser.

Proxy C2 Communication

The majority of the communication passed between the proxy server and the C2 will be comprised of either exfiltrated data or error messages for debugging. Table 3 contains the URI  (Uniform Resource Identifier) parameters that are passed with every request. The data sent will be covered in the following section.

Table of C2 URI Parameters

Table 3: C2 URI Parameters

The following is an example of the request headers:

BokBot Proxy: C2 request headers code

In this case, the request body contains the following Zlib compressed data:

BokBot Proxy: C2 Zlib compressed code

The request body decompresses to the following:

BokBot Proxy: C2 request body decompressed

Traffic Manipulation: Proxy Perspective

BokBot’s proxy module relies on traffic manipulation to steal a victim’s sensitive information. Web traffic generated by a victim’s browser is matched against a list of target URLs (webinjects), and if matched, the proxy takes one of the following actions: redirect to a decoy website (webfake); scrape the page; screenshot the page; or ignore. In addition, the response to the request is matched to determine if either HTML or Javascript is injected into the page and served back to the victim.

BokBot WebFake Process Overview

Figure 3: BokBot WebFake Process Overview

Web Injection DAT Files

The DAT files downloaded by the BokBot main module are structured binary files that contain a list of target URLs, target HTML/Javascript code blocks, and the Javascript/HTML code blocks to be injected. During the initialization process, this structured data is broken into multiple lists for each of the webinject categories. The webinject category lists are built out of a series of webinject types. Once parsed, each element has the following structure:

BokBot Proxy: Webinject DAT files code

Page Scraping

The page-scraping processing functions perform a match on either the URL or on the HTML body to determine if the webpage’s information should be scraped and sent to the C2. Page scraping targets banking account display pages to grab the target’s account information.

Types 33, 34

Either an exact URL string or a regex is used to match the victim’s requested URL. Once a match is made, both the HTML body and the matched URL are sent to the C2. Each of the targeted URLs are related to pages that contain the victim’s account information, for example:


Type 64

This scans the webpage body looking for text (“Account Balance,” “Current Balance,” etc.) related to account balances, security challenge questions and other personal data. If located, the URL, the HTML body and the matched text are compressed and sent to the C2.


Screenshots are generated when a URL matches on a Type 49 inject. Screenshots are taken using the Windows GDI+ API. A bitmap is generated, written to a tmp file, read into a buffer, compressed, and then sent to the C2. Bitmaps are written to the local temp directory under AppData and the filename is a unique string of alpha characters.

  •  Example: AppData\Local\Temp\alksfjlkdsfk.tmp

Although no concrete examples came with the webinjects of the BokBot versions used to generate this report, it is believed that these screenshots will contain sensitive details related to commercial and banking accounts.

Code Injection

Code injection works by either matching  a URL, or doing a match and replace on an HTML element.

Types 17 and 19

Types 17 and 19 are used to hide elements within a page, grab form data, inject code, replace code, and to make the webfake experience more believable for the victim. The difference between these two types is that Type 17 doesn’t rely on regex to match and replace HTML and Javascript.

BokBot Proxy: Type 17 code

Type 81

Type 81 is a list of URLs that are ignored entirely by the proxy server. The purpose is to avoid complications that could come up when injecting into advertisements and chat or email clients. Rather than deal with handling this code, the requests and responses are forwarded to the victim or the target by the proxy, without modification.   

Webfake Phishing Site

BokBot uses the webinjects to create a replica of the original target website. These replica websites are called webfakes. URIs are rewritten to forward the traffic to the webfake site. This section covers each of the injection types used to redirect traffic to a webfake. The web browser is not aware that traffic is being redirected to the webfake.

BokBot WebFake Example

Figure 4: BokBot WebFake Example

Type 50

Type 50 requests perform exact matches (no regex) on URIs received from the client. Whenever a match occurs, the URL is rewritten to point toward the webfake site, and the request is sent to the replica page. The response to that request is parsed by the proxy server and sent back to the victim.

BokBot Proxy: Type 50 request code

Type 51

To match individual icon/gif/bitmap/other names, regex match and replace is leveraged:

  • Regex to match: ^https:\/\/www\.amazon\.[a-z]{2,5}\/.*\/style\/images\/(.*)\.(png’|gif)$
  • Replacement URI: hxxps://hospirit[.]com/amazon/style/images/$1.$2
    • IP Address for hospirit[.]com:

This regex will replace the values “$1” and “$2” with the name of the PNG or GIF file. This rewritten request is what will be sent out of the proxy server.

BokBot Proxy: Type 51 request code

Type 52

Type 52 requests appear to check a URL for a substring and then extract that data, but the BokBot samples used to build this report did not contain a Type 52 rewrite.

URL Rewrite Bypass

It would become an issue if the same webfake process is used every time a victim attempts to log into a targeted website. Once the victim’s information has been collected, there is no longer a need to rewrite URLs to point towards the webfake site. Instead, the request should be sent, unmodified, to the real website. To ensure this happens, BokBot, maintains a list of subkeys located under the HKCU\Software\Classes\CLSID registry key.

For this to work, a unique name needs to be generated for each of the target sites. Each of the inject types included in this category contains a Webfake Template field, for example:

BokBot Proxy: Webfake Tempate code

The highlighted value is used as a seed to generate a unique registry key name (covered in more detail in the “Verification Requests” section). Each of the target sites contains either a value similar to the one highlighted above, or a simple identifier that uses the URL hostname (e.g., “amazon”).  

If the registry subkey exists, the proxy server sends a request to the C2 to determine if the request should be rewritten. If the response from the C2 is anything other than one of the two bytes — “0x2D” or “0x2B” — the legitimate URL will not be rewritten to point to the webfake site.

Verification Requests

Verification requests are generated by the Bot API javascript (covered further down) and are used to either send data to the C2, or to insert/delete/query a value in the registry. Multiple verification request types are used, each one is processed by the proxy server.

BokBot verification commands

Table 4: Verification Commands

Type 96

In order to either display a “Site Down for Maintenance” message or present the victim with a legitimate website (bypass the URL rewrite request), the Bot API injected javascript code can generate these Type 96 banknameS requests.

BokBot Proxy: Type 96 banknameS request code

The values passed to the body of this request will be written to the registry and checked both by the Bot API and during execution of the code that handles the URL redirection request types.  

Each site is provided a unique ID that is sent to the proxy server via the banknameS parameter. This value is used to generate a UUID, which will be the name of the registry key. All of these entries are created under the following registry path: HKCU\Software\Classes\CLSID.

After the key is created, the value located in the body of the Type 96 request is hashed and written to the (Default) value field in the registry subkey.

Types 97 and 98

Both Type 97 and 98 verifications will be generated by the Bot API’s injected javascript, and will perform an action on the registry key that was created by the Type 96 request. Type 97 queries the registry to see if the value exists, and Type 98 will delete the key.

Type 100

Verification Type 100 is either passed directly to the C2 (Type 2) or the request tells the proxy server to interact with the C2 (Type 1) in some manner.  These requests are either HTTP POST or HTTP GET requests that contain the information collected from the malicious javascript code.

Putting it All Together

Let’s take an example using the cashanalyzer website inject: First, the proxy server receives a request from a web browser to access The proxy cycles through the various webinject lists to match the URL. A match is found.

BokBot Proxy: Victim request code

The proxy server takes the 2299737dfa5c070dc29784f1219cd511 value from the first part of the Webfake Template field and uses it to generate a UUID to determine if a “Site Down For Maintenance” page is set (see the “Verification Requests” section). If it exists, a request is sent to the C2 to determine if the block page should be updated with a new time or removed.

Once the block page check has been made, the proxy server takes the rest of the Webfake Template and replaces the front:// string with the web protocol (HTTP/HTTPS) and the replacement URL The URL path and query, content/main?a=2299737dfa5c070dc29784f1219cd511&b=#gid#&c=#id#, is appended to the end of the URL. The Project ID and the Bot ID values replace the #gid# and #id# parameter value tokens, respectively.

BokBot Proxy: Project ID/Bot ID values change code

This request is sent to the webfake. A response is sent and additional checks are made to determine if anything matches one of the inject types. In this case, no match was found and the response will be forwarded back to the victim’s browser without modification. The browser is not aware of the redirection and the original requested URL is still visible in the browser address bar.

As the browser renders the response, additional requests will be sent to load dependant javascript files from the webfake. In this case, main.js is loaded from the webfake. The browser requests to download the javascript file from the redirected website and, once again, the proxy server performs the webinject match routines. As a result, a Type 19 match is found.

BokBot Proxy: Type 19 URL match code

The “Code to Inject” value will be injected into the main.js code when the redirected website responds to the request. Now all of the necessary components are in place to collect the target’s account information.

Once form data is collected, Validation Type 100 requests are sent and processed by the proxy server. These requests are POST requests with the form data contained within the request body. Once processed by the proxy, the form data will be sent to the C2 by signaling the C2 communication thread.

Bokbot Proxy: C2 from data code

The request parameters can be parsed by referring back to the “Proxy C2 Communication” section.

There are multiple variations of how webinjects are handled by the proxy server. However, this example is sufficient to showcase the entire workflow.

Traffic Manipulation: Browser Perspective

Due to the multitude of variations for the injected javascript API, and rather than attempting to abstract and present the high-level processes each inject has in common, this section will focus only on continuing the example introduced in the previous section. This analysis can be used to understand how the other injects work.

The first couple of sections will cover how the site interacts with the victim, the proxy and the C2. After that is covered, the “Putting it All Together” section will connect the separate pieces.

Bot API: Core Javascript Module

As previously mentioned, the  main.js file contains the Bot API code. This code provides everything necessary to interact with the proxy, the C2 and the webpage HTML elements, to ensure that the victim has a seamless experience while entering the account information.

Token States

A token state is a numerical indicator used to identify what action the Bot API should take next. There are five token states:

BokBot Token States

Table 5: Token States

As an example, if there is some sort of error, the token state is set to either 3 or 4, and this will trigger the logic in the javascript to call the viewBlock method (next section) and load the “Site Down for Maintenance” page.

Page View Architecture

In order to ensure that the correct information is collected in the proper order, the Bot API relies on a traditional view/controller architecture. In this case, the controller is the attacker-controlled webfake site. The Bot API client notifies the C2 that it is ready to receive a command, the server responds with a command, and this command is used to load the next page view.

BokBot Page View Commands

Table 6: Page View Commands

Once the command is received by the Bot API, the CSS display field of the relevant HTML element is modified to display this code block. A typical session using the webfake will take the following steps:

  1. Login page view is loaded
    1. Victim enters the account information and submits the form
    2. A wait page is displayed as the login form is processed
  2. Security token page view is loaded
    1. Victim enters the security token
    2. A wait page is displayed as the security token form is processed
  3. First name/last name/phone number page view is loaded
    1. Victim enters this information
    2. Another wait page is displayed as this form is processed

An example of this will be covered in the “Putting it All Together” section below.

Wait Pages

After the victim attempts to log in to the website, there is an idle period as the proxy forwards the request to the webfake site, and that site forwards the request to the legitimate website.  To avoid causing any concern, the Bot API displays a wait page.


Example of a BokBot Wait Page

Figure 5: Post-Login Wait Page

There are multiple versions of this site, one for each step of the account information collection process: login information; security token information; security question answers; and the form that collects the victim’s name and address.

Communication: Bot API Requests to the C2

Commands are forwarded to the C2 on behalf of the Bot API by the proxy server using Validation Type 100 requests. Contained within these requests is a base64-encoded string. This string decodes to a series of parameters in JSON format.

BokBot Proxy: Type 100 requests with base64-encoded string

Table 7 describes the meaning of each of these parameters. In this case, the Bot API is notifying the C2 that the page has not been initialized (invalid).

BokBot URL Parameters for Bot API C2 Communication

Table 7: URL Parameters for Bot API C2 Communication

Heatbeat (Type 1) requests are sent out at regular intervals by the Bot API. These requests are used as a means to maintain constant communication with the C2 and provide a medium for the Bot API to receive commands.

Communication: C2 Responses to the Bot API

Responses from the C2 contain commands specifying what actions the Bot API should take, based on the initial request. After each form submission, a command is received from the C2 telling the Bot API which page to load next. These responses are also base64-encoded JSON.

BokBot Proxy: C2 Responses to the Bot API code

In this case, the command ID is “99” and it is telling the Bot API to reload the original site. This will reset all of the forms, clear any site blocks and load the original page. Table 8 covers the various fields of the C2 response:

BokBot Proxy:C2 Response Fields

Table 8: C2 Response Fields

As can be seen, the id field lines up with the view commands table (Table 6) covered in the “Page View Architecture” section.

Site Maintenance Page

Whenever the token state of the Bot API is set to “3” or “4,” an error page stating that there are technical issues is displayed (Figure 6). The token is set to one of those two states whenever there is an issue communicating with the C2, the webfake, the proxy server, or if an error occurs when the webfake communicates with the real website.

Example of a BokBot Site Maintenance Page

Figure 6: BokBot Proxy “Site Down for Maintenance” Page

The fix date is generated by adding one hour to the current time. After displaying the website, this timestamp is set in the registry by sending a Type 96 Verification request. Once the timer expires, a Type 98 Verification request is sent to delete the block time from the registry.


The Bot API is constantly sending logging data back to the C2 as base64-encoded JSON. The following is the decoded JSON object:

BokBot Proxy: Bot API sends logging data back to C2 code

This chunk of data has a series of key:value pairs, where the h key contains the log, as a value. The logs are descriptive and useful for the malicious actor.

Putting it All Together

Initial Browser Request

The victim’s browser sends a request for, and that request is intercepted by the proxy, rewritten, sent to the webfake site, and the response is sent from the proxy to the victim. The page content also contains the following javascript code to load a javascript file from the attacker-controlled site:

BokBot Proxy: code to load javascript from C2 site

This main.js request is forwarded by the proxy and has the following code injected into the response:

BokBot: code injected into javascript response

This code will call the String.prototype.init method that is defined in main.js. In this case, the init method contains the malicious Bot API javascript module, and will end up calling the main function to initialize and load the webfake site.

Example of a BokBot Webfake Login Page

Figure 7: Cashanalyzer Fake Login Page

After checks are performed to determine if a block page needs to be loaded, the Bot API calls the viewStep method and passes it the string “login.” The string passed to this method is the element id of the HTML <div> object that contains the login form.

BotBok Proxy: Bot API passes string login code

This display field of this element will be switched from none to block and the login page is displayed (Figure 7). Now the victim can enter the login information and click “continue.” All of the data entered is placed into a C2 request structure and sent to the webfake site.

BokBot data in C2 request structure code

The first highlighted section above is the user account credentials, and the second is the logging information. This information notifies whoever is on the other end that the login view was loaded, and that the last view is the wait page.

The wait page that the victim is seeing churns as the Bot API continuously sends heartbeat requests, waiting for the response from the webfake site.

BokBot API heartbeat request code

After processing is finished, the webserver responds with a command to disable the login page, and make the next page viewable. In this case, the command is to load the user contact information form.

BokBot Proxy command code to load user contact info form

The rest of the steps are self explanatory: The subsequent actions consist of forms leading to a wait time, followed by additional forms until the last step is reached.

Wrapping Up the Process

Once all of the information has been collected, the Bot API sends a Validation Type 96 request to the proxy server: However, instead of a timestamp, the value being set in the registry is the string “true,” as highlighted below.

BokBot Proxy: Bot API sends Type 96 validation code

This request instructs the proxy server to create the ‘HKCU\Software\Classes\CLSID\{7570CC99-D32B-6883-1375-9D2881583EFB) registry key with the (Default)value, set to a four-byte binary value.  Once this is set, any further attempt to access bypasses any attempt to rewrite the URL (inject Types 50, 51, 52), and will load the legitimate website.

To load the legitimate website, the page is refreshed and now the victim will be able to login to the legitimate website. Of course, if any of the other webinjects match on the site, other actions will be taken (collecting account balance data, etc.).

How CrowdStrike Falcon® Prevent Stops BokBot

CrowdStrike® CrowdStrike Falcon® Prevent™ next-generation antivirus successfully stops BokBot when process blocking is enabled, as explained in the following.

BokBot spawns a svchost child process, injects the main module, and that svchost process spawns and injects into multiple child processes. The process tree in Figure 8 is an example of what BokBot looks like when viewed using Falcon Prevent with process blocking is disabled. As shown below, several malicious child processes were launched by BokBot’s main module, located inside of the first svchost process.

BokBot Process Tree as seen through Falcon Prevent with blocking disabled

Figure 8: BokBot Process Tree Without Process Blocking Enabled

Without preventions enabled, the customer will still be notified of the malicious activity, but no action will be taken to prevent the behavior automatically.

Suspicious Process Blocking

Falcon has the capability to prevent the execution of BokBot’s main module and all of the child modules. Turning on process blocking in Falcon Prevent kills the BokBot infection at the parent svchost process. Looking at the process tree in the Falcon UI with process blocking enabled (Figure 9), an analyst sees that the svchost process was prevented. The block message (Figure 10) that occurs with this preventative action explains why this process was terminated.

BokBot Process Tree as seen through Falcon Prevent with blocking enabled

Figure 9: BokBot Process Tree With Process Blocking Enabled


Message when BokBot Process is Blocked by Falcon

Figure 10: BokBot Process Block Message

Suspicious process blocking is an example of malware prevention based on behavior. If the malware uses behavior that has not been caught by Falcon’s indicators of attack (IOAs), then Falcon can also prevent malware execution by leveraging either next-generation AV machine learning or intelligence collected by the Crowdstrike Intelligence team.


BokBot Hashes

The following hashes were used in creation of this blog post.

File Hash File
87d37bc073d4d045d29e9c95806c7dcf83677697148e6b901c7a46ea7d5f552e BokBot Container
2c331edaadd4105ce5302621b9ebe6808aecb787dd73da0b63882c709b63ce48 BokBot Container
7e05d6bf0a28233aa0d0abfa220ef8834a147f341820d6159518c9f46f5671b7 BokBot Container
961f7bada0c37c16e5ae7547d9b14b08988942af8d4a155ad28e224ece4fa98e BokBot Container
c992229419759be2ecaddcfd2d0ce26ce3cddca823a4c4875564316b459b05eb BokBot Container
88e41cc6bd4ec57bcabf67f15566475e1ee3ff7667b73f92ac81946f8564e6d9 BokBot Proxy DAT
ec205babdc4422888c3c29daa2f3d477315a2a136a2bd917947cd2184cdce406 BokBot Proxy DAT

File Locations

Path Details
%PROGRAMDATA%\{UUID}\[A-Za-z].exe Bokbot main binary
%PROGRAMDATA%\[A-Za-z]}\[A-Za-z].dat Bokbot DAT File


Path Details
HKCU\Software\Classes\CLSID\{UUID) Block Site and Rewrite bypass data

Network Listeners

Port Service Name Details
TCP  57391 svchost.exe Proxy Server Port

Attacker Controlled Sites


Additional Resources


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