Attackers typically try to pass off malware as legitimate applications or as potentially unwanted programs that users deliberately for and download, such as cheats or cracks. They often rely on ad and affiliate networks to deliver their creations to victims’ devices. This post examines a less conventional case: a well-known backdoor distributed under the guise of adware. The attackers may have chosen this distribution method because the adware was signed by the developer. On top of that, users often manually add these apps to exclusions, so their useful features don’t get blocked.
Some time ago, a client asked us to analyze a file with the MD5 hash c24e99f9437feacaa63766a3cde3fe3d and add it to our detection database. We initially classified it as adware, but a cursory analysis turned up suspicious network activity, which prompted us to dig deeper. It turned out the sample did far more than serve ads. In fact, its advertising functionality doesn’t even work; instead, it triggers an infection chain that delivers the ValleyRAT backdoor.
The file the client shared with us turned out to be an installer that performed different actions depending on the two-letter suffix used in the file name, positioned just before the numeric string.
These actions are most likely designed to divert the user’s attention away from the sample’s malicious functionality. Regardless of the file name, the installer deploys a modified Chinese desktop wallpaper management tool called QN Wallpaper (hxxps://qnwallpaper[.]keansoft[.]cn/) and adds it to the registry’s autorun entries.
The original version of QN Wallpaper is genuine adware: on installation, it delivers bundled partner apps to the device and then displays ad banners to the user. In this case, however, the attackers use it to carry out DLL sideloading , a technique that allows malicious code to run under the guise of a signed process by way of a malicious DLL.
The QN Wallpaper modules, along with the malicious components, are unpacked to C:\Program Files\QNWallpaper\5.4.0.1662\ . The following files are saved in that directory:
After unpacking, the installer uses the DisableAntiSpyware registry key to disable Windows Defender and then launches QnWallpaper.exe.
Disabling Windows Defender
QnWallpaper.exe has dependencies in libcef.dll, so this library gets loaded when the process starts. QnWallpaper.exe also launches QnwPlayer.exe, which likewise calls libcef.dll.
QnWallpaper and QnwPlayer won’t actually function correctly, because the functions exported from libcef.dll are put into an infinite sleep. However, in case that sleep is ever interrupted, the attackers have implemented a function that loads all the necessary functions from the original library into memory, provided it can locate that library on the system.
Example of an exported function
Loading functions from the original libcef.dll
The malicious functionality in libcef.dll is invoked by a call to DllMain, which runs automatically when the library is loaded. That said, alongside the original exports, the library also contains a function named RunDLL, which likewise initiates execution of the malicious code. QnWallpaper never calls this function. We suspect the attackers intended to invoke it manually via rundll32 or planned to use a separate executable for this purpose, one that wasn’t included in the package downloaded by the sample.
When the library is loaded, code runs that ensures QnWallpaper.exe persists at startup: it adds a file extension association and drops a file with the corresponding extension in C:\Documents and Settings\ \Start \Programs\Startup\.
This is followed by a chain of wrapper functions whose main job is to call the one. Execution eventually reaches the function that contains the actual malicious code. For convenience, we’ll refer to it as mw_entry.
Inside mw_entry, the malware checks two things:
Checking for administrator privileges
If the user isn’t a member of the Administrators group, the program attempts to obtain administrator privileges by using the runas utility.
Relaunching the process to obtain administrator privileges
Once it has administrator privileges, the malicious code determines which process the DLL has been loaded into, and selects the payload accordingly:
Retrieving the payload from a resource
Retrieving the payload from a resource
Both payloads are AES-encrypted DLLs that contain the ValleyRAT backdoor. The only difference between them is their configuration, specifically, the C2 server addresses. After decryption, libcef.dll checks the magic signatures in the resulting PE file’s headers to confirm the sample is valid. If this check fails, the library releases its resources and takes no further action.
Validating the PE file headers after decryption
If the headers check out, libcef.dll loads the payload into the process’s memory space and hands control over to the backdoor by calling DllMain.
ValleyRAT begins its operation by parsing its configuration, which consists of key:value pairs concatenated into a single string. To obfuscate this configuration, the attackers wrote the string in reverse.
Obfuscated configuration
During parsing, the backdoor restores the correct character order and reads the key values one by one. The set of keys is the same regardless of which process the backdoor is running in.
Parsing the configuration
Some of the configuration fields are listed below:
The backdoor uses several techniques to protect its process. Some are configuration-dependent, while others are always applied:
Injecting code into svchost
The function injected into the process has a single job: restart the backdoor if its execution is interrupted for any reason.
Setting its own process as critical
Setting its own process as critical
Restarting on exceptions
Restarting on exceptions
The backdoor also has spyware functionality. While running, it tracks keystrokes and the currently focused window by using functions from the DirectInput8 library. It also captures clipboard contents. All collected data is saved to a file on disk.
Capturing clipboard data
If the ll key in the configuration is set to 1, ValleyRAT periodically checks for active windows belonging to applications that could be used to analyze processes or traffic. Window enumeration is done via the EnumWindows function, using the following callback:
After completing these checks, the backdoor collects system information, including:
On command, the backdoor can perform the actions typical of this malware category:
Snippet of the command handler
Let’s take a closer look at the module-loading functionality. Upon receiving the corresponding command with a link from its operator, the backdoor downloads the file at that link and executes it. The download can come from either the C2 server or a third-party address.
The DownloadPeFile function is responsible for downloading a PE file
The DownloadAndExecute function calls DownloadPeFile, then launches the downloaded module
Additional modules can take the form of purpose-built dynamic libraries or shellcode. If the payload is shellcode, the backdoor uses process hollowing with svchost to launch the module.
Implementation of the process hollowing technique
If the module is a dynamic library, the backdoor loads the PE file into its own process, calls DllMain, and searches for a Main function among the exported functions. Once Main has been called, the library is unloaded from memory.
Calling DllMain after the backdoor loads the PE file
Over the course of 2026, we detected the ValleyRAT backdoor and its associated malware more than 100,000 times, with more than 1500 unique users affected, primarily in China and India.
This attack geography, combined with the use of the ValleyRAT backdoor, points to Silver Fox , a known operator of this malware family, as the likely group behind the campaign.
This case is a clear example of how adware and affiliate networks can turn out to be far more dangerous than they appear. ValleyRAT is a sophisticated backdoor capable of collecting sensitive data such as keystrokes and clipboard contents, taking screenshots, and delivering additional malicious modules. The attackers exploited a well-known adware application to run the backdoor under the guise of a signed process, which complicates detection.
Motivated by both cyberespionage and financial gain, Silver Fox targets organizations across multiple countries. To stay protected, organizations should keep employee cybersecurity awareness up to date and enforce clear policies on the use of third-party software on work devices.
For individual users, we recommend avoiding the installation of software with a questionable reputation, and, even more importantly, never adding such software to your security solutions’ exclusion lists.
07ddbbe2c71c45577a7a4fbcdba0df91 c24e99f9437feacaa63766a3cde3fe3d 8a626d844943da3456b044f38deae3a2
103.45.66.18:441 103.45.66.18:442 103.45.66.18:443 192.253.225.173:6666 192.253.225.173:8888
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