By Cluster25 Threat Intel Team
May 13, 2022


Cozy Bear (aka Nobelium, APT29, The Dukes) is a well-resourced, highly dedicated and organized cyberespionage group that is believed to work in support of the decision-making process of Russian government since at least 2008. Nobelium primarily targets western governments and related organizations, with a particular focus on government, diplomat, political and think tank sectors.  Recently we analyzed several spear-phishing campaigns linked with this adversary that involve the usage of a side-loaded DLL through signed software (like Adobe suite) and legitimate webservices (like Dropbox) as communication vector for Command and Control (C&C).  The misuse of legitimate webservices is in attempt to evade the detection from automatic analysis software. Recently, third-party researchers have also reported it used Trello and its REST API to simulate a first-level Command & Control server. In addition to this evasion attempt, as we are going to discuss later, the side-loaded DLL tries to unhook the windows libraries loaded in the process memory to evade possible EDRs.

To maximize the chances of success, Nobelium, in at least two cases, sent spear-phishing emails from spoofed or compromised government addresses. As initial access we identified the following attack vectors:

  1. The first approach involves the distribution of an IMG file which, when mounted, contains an LNK shortcut and the signed software with the other DLLs and a decoy PDF as hidden files. This attack vector lures the user through a masquerading technique by changing the LNK file icon to a folder icon in order to convince the user to click on it. In fact, once triggered, the cmd.exe utility is invoked to run the signed executable and to start the side-loading of the malicious DLL (i.e. AcroSup.dll).
  2. The second approach involves the usage of the EnvyScout dropper that is basically an HTML file with an embedded JavaScript designed to decode and drop the next-stage payload (HTML Smuggling). In fact, once the HTML file is executed, the JavaScript code decodes a bytes array and saves the result under an archive in the Download directory. In this case, the user is responsible to unzip the archive (that contains the signed software, the relative DLL’s and the lure PDF) and to run manually the executable to start the chain (even if the JavaScript code contains unused snippet for automating the process).

The EnvyScout dropper was used by this threat actor in different campaigns. From mid-January 2022 Cluster25 internally reported differet Nobelium-linked campaigns against European entities that leveraged fairly complex kill-chain started with EnvyScout as well.


In the reported case, the signed executable is represented by WCChromeNativeMessagingHost.exe from the Adobe Create PDF module of the Adobe suite. It’s a plugin for Google Chrome. Since the malware bundle contains a local copy of vcruntime140.dll, once the abused software is executed, the local copy of this Windows library is loaded into the program memory from the PE import table. Analyzing the local copy of vcruntime140.dll, we noticed that the PE imports of this library have been modified: it contains an entry to the AcroSup.dll delivered through the malware bundle.

So, this import chain, leads to the side-loading of the malicious AcroSup.dll and the execution of its DllMain export before the execution of the signed Adobe executable. To evade possible debuggers the execution of the malicious AcroSup.dll starts with a thread hijacking by overwriting the thread context of the main thread (updating the RIP register) in the signed executable space. To avoid the DLL execution in suspected processes, before the thread context overwriting the malware checks for the process image name is currently matching with the name of the signed expected executable, through the K32GetProcessImageFileNameA API.

After that the malware iterates on the loaded Windows DLLs through the K32EnumProcessModules APIs to unhook each DLL and evade active EDRs on the system. Basically, for each loaded DLL, the .text section of each of them is freshly mapped to the virtual address of the possible hooked DLL.

From this point the malware enters a pseudo-infinite loop where, each second, goes to contact the Dropbox service to communicate the victim identifier and receive next-stage payloads.

First of all, the endpoint is contacted at the /oauth2/token/ URI through an HTTP POST request to receive a refresh token, necessary to contact the Dropbox APIs.

For this request the following combination of  API key and API secret are used to represent the Dropbox account used by the threat actor:

  • API key: fm09ogco339u0a9
  • API secret: scqekoaqqj98sze

In addition, for all the network-related requests the malware uses a fixed user-agent:

  • User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.75 Safari/537.36 AtContent/91.5.2444.45

If the refresh token is received from the Dropbox APIs and successfully parsed, the implant proceeds with the integration of some masquerading and persistence techniques. More in details, a new subprocess is created to open the lure PDF document (an empty PDF) contained in the bundle and used to make the user think that he has opened the legit Adobe Acrobat application.

In the meanwhile, all the files involved in the bundle (signed software and relative DLL’s) are copied under the %APPDATA%\AcroSup\ directory and a new registry key under HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run\ is created to achieve persistence.

The lure PDF document is not copied under the above mentioned directory so this document will not be opened again after a restart of the victim system. To register the victim entry on Dropbox, a victim identifier is created through the hex-encoded combination of the current username and current computer name (e.g., john::windows10). The communication of the new victim is then completed through a push of a new .mp3 file (named Rock_[VICTIM_ID].mp3) via the Dropbox APIs on /2/files/upload/ URI.

Interestingly, the pushed .MP3 files identifying the victims, contains always the following string that likely represents the malware family:


Finally, another request is performed to the /2/files/download/ path of Dropbox APIs that tries to download a file named Rock_VICTIM_ID.mp3.backup. The response is parsed to determine if a next-stage payload exists for the current registered victim. If one does, the malware will allocate a new heap space to store the downloaded payload and will execute it in the executable space by overwriting again the thread context of the main executable.


In recent months Cluster25 had evidence of Cozy Bear‘s campaigns that potentially impacted at least Greece, Italy, Turkey and Portugal especially in government and foreign affairs sectors.


NOBELIUM confirms its interest in government and foreign affairs by targeting organizations in Europe and possibly other parts of the world. The campaigns and the payloads analyzed over time show a strong focus on operating under the radar and lowering the detection rates. In this regard, even the use of legitimate services such as Trello and DropBox suggest the adversary’s will to operate for a long time within the victim environments remaining undetected. It is possible to foresee that this actor will also try to change TTPs (Technical and Tactical Procedures) in the near future in order to make any mitigation action aimed at its contrast more difficult. In this regard, we provide a wide set of detection rules useful for verifying potential malicious activities attributable to this threat actor (see DETECTION AND THREAT HUNTING SECTION).



Initial Access
T1566.001 Phishing: Spearphishing Attachment
Execution T1204.001 User Execution: Malicious Link
Execution T1204.002 User Execution: Malicious File
Execution T1059.007 Command and Scripting Interpreter: JavaScript
Defense Evasion T1036 Masquerading
Defense Evasion T1622 Debugger Evasion
Defense Evasion T1140 Deobfuscate/Decode Files or Information
Defense Evasion T1027 Obfuscated Files or Information
Defense Evasion T1055.003 Process Injection: Thread Execution Hijacking
Defense Evasion T1553.002 Subvert Trust Controls: Code Signing
Defense Evasion T1562.001 Impair Defenses: Disable or Modify Tools
Defense Evasion T1112 Modify Registry
Defense Evasion T1202 Indirect Command Execution
Defense Evasion T1497 Virtualization/Sandbox Evasion
Defense Evasion T1620 Reflective Code Loading
Discovery T1082 System Information Discovery
Discovery T1057 Process Discovery
Persistence T1098 Account Manipulation
Persistence T1547.001 Boot or Logon Autostart Execution: Registry Run Keys / Startup Folder
Command and Control T1105 Ingress Tool Transfer
Command and Control T1071.001 Application Layer Protocol: Web Protocols
Command and Control T1102 Web Service


PAYLOAD SHA256 5292c0f5a7ea80124cf7584eacea1881cf2f0814fa13dcc0de56624e215aaba2
PAYLOAD SHA1 32792827c14075cc3091244425e302b1ebe3259c
PAYLOAD MD5 2fbccfc5a1b91b2609e3ae92a93ff7cb
PAYLOAD SHA256 9d063a05280fbce6ff0fd62a877f3fd1e80f227522e16918e6bede2e6ee398de
PAYLOAD SHA1 05241afa180d70e17647b2d8cbc1660adbe3af88
PAYLOAD MD5 d86283af2d5888b0ce3ea63eb26f60f7
PAYLOAD SHA256 4c68c840ae1a034d47900ebdc291116726fd37b3ab0b7e026fad90eaab84d820
PAYLOAD SHA1 c9a5314eb247c7441a5262a7cd22abbe1fcba7b6
PAYLOAD MD5 110c4ae194e7b49ed3e3b254d599f7f4
PAYLOAD SHA256 7f96d59cb02229529b14761f979f710bca500c68cc2b37d80e60e751f809475e
PAYLOAD SHA1 489c36c9ea3fb90f61209d43efffd8d997a362c6
PAYLOAD MD5 9ec1fcb11b597941bec03078cccab724
PAYLOAD SHA256 23a09b74498aea166470ea2b569d42fd661c440f3f3014636879bd012600ed68
PAYLOAD SHA1 ad33bab4bc6232a6666c2190b3bf9fc2ab2a720a
PAYLOAD MD5 454f59dc7d3d7f228bbd4ddd4c250ed8
PAYLOAD SHA256 729fb24b6c18232fc05ccf351edaeaa8a76476ba08cba37b8a93d34f98fa05ed
PAYLOAD SHA1 900cba1d73ddca31a7bb7b7af5b3b7f1a0bc6fbf
PAYLOAD MD5 6bc8be27898e1e280e402a7981be55ae


alert http $HOME_NET any -> $EXTERNAL_NET any (msg:”CLUSTER25 NOBELIUM Registration via Dropbox API”; flow:established,to_server; http.uri; content:”/2/files/upload”; http.header; content:”|22|path|22|”; content:”|22|/Rock_”; fast_pattern; distance:0; content:”.mp3|22|”; distance:0;; content:””; bsize:22; reference:url,; reference:md5,3f400f30415941348af21d515a2fc6a3; classtype:trojan-activity; sid:7704250; rev:1;)


alert http $HOME_NET any -> $EXTERNAL_NET any (msg:”CLUSTER25 NOBELIUM Backdoor Download via Dropbox API”; flow:established,to_server; http.uri; content:”/2/files/download”; http.header; content:”|22|path|22|”; content:”|22|/Rock_”; fast_pattern; distance:0; content:”.mp3.backup|22|”; distance:0;; content:””; bsize:22; reference:url,; reference:md5,3f400f30415941348af21d515a2fc6a3; classtype:trojan-activity; sid:7704251; rev:1;)


import “pe”
rule APT29_Loader_87221_00001 {
        author = “Cluster25”
tlp = “white”
        description = “Detects DLL loader variants used in Nobelium kill-chain”
        hash1 = “6fc54151607a82d5f4fae661ef0b7b0767d325f5935ed6139f8932bc27309202”
        hash2 = “23a09b74498aea166470ea2b569d42fd661c440f3f3014636879bd012600ed68”
        $s1 = “%s\\blank.pdf” fullword ascii
        $s2 = “%s\\AcroSup” fullword ascii
        $s3 = “vcruntime140.dll” fullword ascii
        $s4 = “ME3.99.5UUUUUUUUUUU” fullword ascii
        $c1 = “Rock” fullword ascii
        $c2 = “.mp3” fullword ascii
        $c3 = “%s.backup” fullword ascii
        $sequence1 = { C7 45 ?? 0B 00 10 00 48 8B CF FF 15 ?? ?? ?? 00 85 C0 74 ?? 48 8D 55 ?? 48 89 75 ?? 48 8B CF FF 15 ?? ?? ?? 00 85 C0 74 ?? 48 8B CF FF 15 ?? ?? ?? 00 } // Thread contect change
        $sequence2 = { 0F B6 0B 4C 8D 05 ?? ?? ?? 00 89 4C 24 ?? 4D 8B CD 49 8B CD BA 04 01 00 00 E8 ?? ?? ?? ?? 48 8D 5B 01 48 83 EF 01 75 ?? } // encoding cycle
        $sequence3 = { 4C 8D 8C 24 ?? 00 00 00 8B 53 ?? 44 8D 40 ?? 48 03 CD 44 89 A4 24 ?? 00 00 00 FF 15 ?? ?? ?? 00 8B 43 ?? 44 8B 43 ?? 4A 8D 14 38 48 8D 0C 28 E8 ?? ?? 00 00 8B 4B ?? 4C 8D 8C 24 ?? 00 00 00 8B 53 ?? 48 03 CD 44 8B 84 24 ?? 00 00 00 FF 15 ?? ?? ?? 00 } //DLL Unhook
        $sequence4 = { 42 0F B6 8C 32 ?? ?? ?? 00 48 83 C2 03 88 0F 48 8D 7F 01 48 83 FA 2D 7C E7 } // get domain name string
        uint16(0) == 0x5a4d and filesize < 200KB
            and pe.imports(“kernel32.dll”, “SetThreadContext”) and pe.imports(“kernel32.dll”, “ResumeThread”) and pe.imports(“kernel32.dll”, “K32GetModuleFileNameExA”)
            and 3 of ($s*)
            and all of ($c*)
            and 3 of ($sequence*)


rule APT29_HTMLSmuggling_ZIP_82733_00001 {
author = “Cluster25”
description = “Rule to detect the EnvyScout HTML smuggling with ZIP payload used in the APT29/Nobelium APT29 chain”
date = “2022-05-12”
hash = “d5c84cbd7dc70e71f3eb24434a58b2f149d0c39faa7e4157552b60c7dbb53d11”
$s1 = “new Blob(“
$s2 = “new Uint8Array(“
$s3 = “application/octet-stream”
$t1 = “saveAs(“
$t2 = “download(“
$r1 = { 66 6F 72 28 76 61 72 20 69 20 3D 20 30 78 30 3B 20 69 20 3C 20 64 5B 27 6C 65 6E 67 74 68 27 5D 3B 20 69 2B 2B 29 20 7B 0A 20 20 20 20 64 5B 69 5D 20 3D 20 64 5B 69 5D }
condition: (filesize > 500KB and all of ($s*) and ($t1 or $t2) and $r1)


title: Potential NOBELIUM APT persistence by detection of registry key events (via registry_event)
status: stable
description: This rule detects potential NOBELIUM APT persistence via registry event
author: Cluster25
date: 2022/04/27
– internal research
– attack.persistence
  product: windows
  category: registry_event
    – ‘\Software\Microsoft\Windows\CurrentVersion\Run\’
    – ‘\AppData\Roaming\AcroSup\Acro.exe’
  condition: selection
– unknown
level: high


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