Attackers typically try to pass off malware as legitimate applications or as potentially unwanted programs that users deliberately search 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.Malicious installerThe 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.Installer nameWhat it doesFS_SETUP_DD_173.exeInstalls DingTalk, a workplace collaboration platformFS_SETUP_GG_173.exeInstalls Google ChromeFS_SETUP_HY_173.exeOpens hxxps://meeting[.]tencent[.]com/download/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:File nameMD5Purpose1.zip7ad1e3ef4e6d9d636c9e7e967733850eArchive containing the adware files QnWallpeper.exe and QnwPlayer.exe, along with the modules needed to run them7z.dll96b4c1d0683dce22bd3223e1e40689c17z archiver library7z.exe9b86d3ab6cef15c633933fbbeab39c0aArchiverchrome_elf.dlledfdc30cbd85879776b8f735ea7de1f1Library used to launch Electron-based applicationslibcef.dll07ddbbe2c71c45577a7a4fbcdba0df91Malicious libraryPeLoader48826d5ca845979d2e6ebd66dc1aae90File containing the encrypted backdoorQnWallpaper.exe6c158c0f8e029342192d4f0d72e102b7Adware moduleQnwPlayer.exe9a71d6a41cd258b9e89cdc5fc224de73Adware moduleNedca.exec24e99f9437feacaa63766a3cde3fe3dMalicious installer copyAfter unpacking, the installer uses the DisableAntiSpyware registry key to disable Windows Defender and then launches QnWallpaper.exe.Disabling Windows DefenderDLL Sideloading via libcef.dllQnWallpaper.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 functionLoading functions from the original libcef.dllThe 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.The RunDLL functionRunning the malicious codeWhen 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 Menu\Programs\Startup\.This is followed by a chain of wrapper functions whose main job is to call the next 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:Whether the current user belongs to the Administrators groupWhich process the DLL is running insideChecking for administrator privilegesIf 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 privilegesOnce it has administrator privileges, the malicious code determines which process the DLL has been loaded into, and selects the payload accordingly:If the library is running inside QnWallpaper.exe, the payload is loaded from the PeLoader file.Encrypted payloadIf the library is running inside QnwPlayer.exe, the payload is loaded from libcef.dll resources.Retrieving the payload from a resourceBoth 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 decryptionIf 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.Calling DllMainValleyRATValleyRAT 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 configurationDuring 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 configurationSome of the configuration fields are listed below:KeyDescriptionp?C2 server IP addresso?C2 server portt?Protocol (1: TCP, 0: UDP)ddSleep duration before executing the main codeclSleep duration after receiving the corresponding command from the serverbzConfiguration creation datebhWhether to mark the current process as critical (so that terminating it triggers a blue screen of death) Possible values: 1: yes, 0: nollWhether to check for running security/traffic-analysis tools/processes (1: check, 0: do not check)shWhether to inject code into svchost that will restart the malicious process (1: inject, 0: do not inject)The backdoor uses several techniques to protect its process. Some are configuration-dependent, while others are always applied:Injecting code into svchost to restart the process: a configurable option. The backdoor allocates memory inside the svchost process, injects code into it, and sets PAGE_NOACCESS permissions on the memory page containing the injected data. It then creates a suspended thread, waits 60 seconds, grants read, write, and execute permissions on the page, and resumes the thread.Injecting code into svchostThe function injected into the process has a single job: restart the backdoor if its execution is interrupted for any reason.Injected functionMarking its own process as critical (so that terminating it triggers a blue screen of death): a configurable option.Setting its own process as criticalRestarting on an unhandled exception. This protection mechanism is always active, regardless of the backdoor’s configuration.Restarting on exceptionsThe 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 dataIf 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:Window name checksAfter completing these checks, the backdoor collects system information, including:Host nameHost IP addressesUser idle timeDetailed Windows version information (ProductName, EditionId, DisplayVersion)Number of CPU coresFree disk spaceGraphics adapterCurrently focused window and its titleSystem bitnessLanguage settingsPath to the system directoryOn command, the backdoor can perform the actions typical of this malware category:Rebooting the computerShutting down the computerTaking a screenshotWiping logsUpdating its C2 addressesDownloading additional modulesSending keylogger logs along with clipboard contentsSnippet of the command handlerLet’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 fileThe DownloadAndExecute function calls DownloadPeFile, then launches the downloaded moduleAdditional 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 techniqueIf 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 fileTargets and attributionOver 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.ConclusionThis 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.IoCMD507ddbbe2c71c45577a7a4fbcdba0df91c24e99f9437feacaa63766a3cde3fe3d8a626d844943da3456b044f38deae3a2Network103.45.66.18:441103.45.66.18:442103.45.66.18:443192.253.225.173:6666192.253.225.173:8888