cbcvebase.
CVE-2021-34473
published 2021-07-14

CVE-2021-34473: Microsoft Exchange Server Remote Code Execution Vulnerability

PriorityP1100critical9.8CVSS 3.1
AVNACLPRNUINSUCHIHAH
KEVITWEXPLOITRansomwareInitial access
CISA Known Exploited Vulnerabilitydue 2021-11-17
Exploited in the wild
EPSS
100.00%
100.0th percentile
Microsoft Exchange Server Remote Code Execution Vulnerability

Affected

13 ranges
VendorProductVersion rangeFixed in
microsoftexchange_server
microsoftexchange_server
microsoftexchange_server
microsoftmicrosoft_exchange_server_2013_cumulative_update_23>= 15.00.0 < 15.00.1497.01515.00.1497.015
microsoftmicrosoft_exchange_server_2016_cumulative_update_19>= 15.01.0 < 15.01.2176.01215.01.2176.012
microsoftmicrosoft_exchange_server_2016_cumulative_update_20>= 15.01.0 < 15.01.2242.00815.01.2242.008
microsoftmicrosoft_exchange_server_2019_cumulative_update_8>= 15.02.0 < 15.02.0792.01315.02.0792.013
microsoftmicrosoft_exchange_server_2019_cumulative_update_9>= 15.02.0 < 15.02.0858.01015.02.0858.010
msrcmicrosoft_exchange_server_2013_cumulative_update_23
msrcmicrosoft_exchange_server_2016_cumulative_update_19
msrcmicrosoft_exchange_server_2016_cumulative_update_20
msrcmicrosoft_exchange_server_2019_cumulative_update_8
msrcmicrosoft_exchange_server_2019_cumulative_update_9

Detection & IOCsextracted from sources · hover to see the quote

ip5.255.100.242
filenamelibeay32.dll
pathc:/inetpub/wwwroot/aspnet_client/{5-random-digit}.aspx
url/jquery-3.5.1.slim.min.js
pathC:\Windows\debug
processw3wp.exe
commandrundll32.exe ... ASN1_OBJECT_create
pathC:\Windows\System32\AppLocker\winServicePrinter.ps1
urlhttp://31.214.157.169:80/ngrok.exe
ip31.214.157.169
sigma
filter event_type = STORY and actor_process_image_name = "certutil.exe" and actor_process_image_command_line contains ":8080"
sigma
filter event_type = FILE and (event_sub_type = ENUM.FILE_WRITE or event_sub_type = ENUM.FILE_CREATE_NEW) and action_file_extension in ("exe", "dll") and action_file_path contains "C:\Users\Public"
sigma
filter event_type = ENUM.PROCESS and actor_process_image_name = "w3wp.exe" and (action_process_image_command_line contains "&ipconfig&echo" or action_process_image_command_line contains "&quesr&echo" or action_process_image_command_line contains "&whoami&echo" or action_process_image_command_line contains "&c:&echo" or action_process_image_command_line contains "&cd&echo" or action_process_image_command_line contains "&echo [E]" or action_process_image_command_line contains "&echo [S]")
sigma
filter event_type = ENUM.FILE and actor_process_image_name = "w3wp.exe" and action_file_path contains "FrontEnd\HttpProxy" and actor_process_command_line contains "MSExchange"
  • Look for ASPX webshell files dropped in the aspnet_client directory on Exchange servers, a hallmark of ProxyShell exploitation (CVE-2021-34473 chain).
  • Monitor IIS (w3wp.exe) spawning PowerShell child processes on Exchange servers as a post-exploitation indicator.
  • Detect web shell activity via exec_code query parameter in HTTP requests to Exchange servers.
  • Hunt for China Chopper webshell command patterns spawned from w3wp.exe, including &ipconfig&echo, &whoami&echo, &echo [E], and &echo [S].
  • Alert on certutil.exe making outbound connections on port 8080 to public IP addresses, a post-exploitation indicator on Exchange servers.
  • Detect EXE/DLL file writes to C:\Users\Public\ on Exchange hosts (where w3wp.exe is present) as a post-exploitation indicator.
  • Monitor for suspicious file creation in Exchange FrontEnd\HttpProxy directories by w3wp.exe processes running MSExchange.
  • Trace webshell file creation back to MSExchangeMailboxReplication.exe as the parent process, confirming ProxyShell exploitation.
  • Detect Ngrok tunneling on ports 3389 and 443 on Exchange servers as a post-ProxyShell lateral movement indicator.
  • Alert on rdpclip.exe execution followed by lsass.exe memory access as an indicator of credential dumping after ProxyShell-enabled RDP lateral movement.
  • ·CVE-2021-34473 is part of the ProxyShell chain and requires chaining with CVE-2021-34523 and CVE-2021-31207 for full remote code execution; exploitation of all three together enables arbitrary file write (webshell upload) on Exchange.
  • ·The Cobalt Strike stager used after ProxyShell exploitation mimics legitimate jQuery traffic; HTTP GET requests to /jquery-3.5.1.slim.min.js should not be assumed benign on Exchange-adjacent hosts.
  • ·The tampered libeay32.dll used post-ProxyShell carries a non-valid certificate spoofing Zoom (issued by GoDaddy); certificate validation alone is insufficient to detect this malicious DLL.

CVSS provenance

nvdv3.19.8CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
nvdv2.010.0CRITICALAV:N/AC:L/Au:N/C:C/I:C/A:C
vulncheck9.1CRITICAL
cisa9.8CRITICAL
vendor_msrc9.1CRITICAL
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