cbcvebase.
CVE-2021-34523
published 2021-07-14

CVE-2021-34523: Microsoft Exchange Server Elevation of Privilege Vulnerability

PriorityP197critical9.8CVSS 3.1
AVNACLPRNUINSUCHIHAH
KEVITWEXPLOITRansomwareInitial access
CISA Known Exploited Vulnerabilitydue 2021-11-17
Exploited in the wild
EPSS
99.99%
100.0th percentile
Microsoft Exchange Server Elevation of Privilege 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
processw3wp.exe
pathC:\Windows\debug
pathC:\Windows\System32\AppLocker\winServicePrinter.ps1
ip31.214.157.169
pathC:\Windows\system32\ngrok.exe
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 (CVE-2021-34473/34523/31207) exploitation.
  • Monitor for w3wp.exe (IIS worker process) spawning PowerShell child processes on Exchange servers, which is indicative of webshell-based post-exploitation.
  • Detect webshell activity by monitoring for the exec_code query parameter in HTTP requests to Exchange servers.
  • Hunt for China Chopper webshell activity by detecting w3wp.exe spawning processes with command lines containing patterns like &ipconfig&echo, &whoami&echo, &echo [E], or &echo [S].
  • Detect certutil.exe making outbound connections to public IP addresses on port 8080, which may indicate post-exploitation activity on Exchange servers.
  • Alert on DLL or EXE files written to C:\Users\Public\ by w3wp.exe on Exchange hosts, a post-exploitation staging indicator.
  • Monitor for suspicious file creation in Exchange FrontEnd\HttpProxy directories by w3wp.exe processes, indicating webshell installation.
  • Trace webshell file creation back to MSExchangeMailboxReplication.exe as the parent process, confirming ProxyShell exploitation.
  • BianLian ransomware actors use ProxyShell for initial access; monitor for webshell installation on Exchange servers followed by SMB lateral movement and creation of Domain Admin accounts.
  • ·CVE-2021-34523 is one of three chained ProxyShell vulnerabilities; exploitation requires all three (CVE-2021-34473, CVE-2021-34523, CVE-2021-31207) to achieve full remote code execution. Detections should account for the full chain.
  • ·CVE-2021-34473 and CVE-2021-34523 were patched in July 2021, while CVE-2021-31207 was fixed in May 2021; unpatched Exchange servers remain at risk from the full ProxyShell chain.
  • ·The ProxyNotShell exploit chain (CVE-2022-41040/41082) produces URL request patterns nearly identical to ProxyShell; defenders should verify Exchange server patch level before attributing activity solely to CVE-2021-34523.

CVSS provenance

nvdv3.19.8CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
nvdv2.07.5HIGHAV:N/AC:L/Au:N/C:P/I:P/A:P
vulncheck9.0CRITICAL
cisa9.8CRITICAL
vendor_msrc9.0CRITICAL
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