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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchProxyShell is a three-vulnerability attack chain against on-premises Microsoft Exchange Server. An unauthenticated attacker who chains CVE-2021-34473, CVE-2021-34523 and CVE-2021-31207 can reach command execution as SYSTEM. Patching is urgent, but a server exploited before it was updated must also be treated as potentially compromised.
What ProxyShell is
ProxyShell abuses Exchange’s Autodiscover and PowerShell components. The vulnerabilities are individually serious; chained together, they provide a path from an unauthenticated internet request to arbitrary command execution with the highest local privilege.
| Vulnerability | Role in the chain | Result |
|---|---|---|
| CVE-2021-34473 | Pre-authentication path confusion and access-control bypass involving Autodiscover | Lets an attacker reach an otherwise protected Exchange backend URL |
| CVE-2021-34523 | Privilege escalation in the Exchange PowerShell backend | Provides the permissions needed for the next stage |
| CVE-2021-31207 | Post-authentication arbitrary file write | Enables remote code execution, including execution as SYSTEM when the chain is completed |
The Canadian Centre for Cyber Security reported ongoing scanning and exploitation of unpatched Exchange servers. The Irish National Cyber Security Centre described the combined effect as unauthenticated remote execution of arbitrary commands as SYSTEM.
Which Exchange servers are exposed?
The affected product is self-hosted Microsoft Exchange Server. Ireland’s September 2021 alert identified Exchange Server 2013, 2016 and 2019 installations that had not received the May 2021 cumulative update KB5003435. A build number or product label alone does not establish safety: administrators must inventory every internet-facing server and verify its cumulative and security-update level against Microsoft’s current support guidance.
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Exchange versions that have reached the end of support also carry operational and security risk beyond ProxyShell. If an installation cannot receive supported security updates, plan a migration or replacement rather than treating one patch as a permanent fix.
What the historical 40% figure means
| Statistic | Scope and date | How to interpret it |
|---|---|---|
| Circa 40% of internet-facing Microsoft Exchange servers | Potentially vulnerable in Ireland, estimated by Ireland’s National Cyber Security Centre in September 2021 | A historical, Ireland-specific estimate; it is not a current global vulnerability or victim count |
Does ProxyShell affect Microsoft 365?
ProxyShell targets on-premises Exchange Server. In its alert, CISA said the vulnerabilities were not known to affect Exchange Online or Microsoft 365 cloud email services at that time. Organizations running hybrid environments should still investigate the on-premises servers, connectors, identities and administrative accounts that bridge into cloud services.
What attackers can do after exploitation
Initial command execution is only the beginning. Official Microsoft and CISA guidance describes attackers using Exchange compromise to:
- Install persistent web shells for remote administration.
- Read or export mailboxes and access files on the server.
- Steal credentials and abuse privileged accounts.
- Move laterally to other systems on the network.
- Download or launch additional malware and payloads.
Microsoft specifically warned that malicious actors used ProxyShell to place web shells in Exchange. CISA’s guidance says that discovering exploitation activity should trigger an assumption of network-identity compromise and formal incident-response procedures.
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Why applying the patch may not be enough
A security update closes the vulnerable code path; it does not automatically remove a web shell, reverse a new account, restore stolen credentials or undo lateral movement. If exploitation could have occurred before patching, preserve evidence and handle the server as an incident rather than as a routine update.
- Isolate the affected host or restrict its network access while maintaining the evidence needed for investigation.
- Reset exposed credentials, beginning with privileged, service and Exchange-related accounts, from a known-clean system.
- Determine whether attackers accessed mailboxes, files, credentials or other hosts.
- Look for persistence and additional payloads before returning the server to normal service.
Response plan for a suspected ProxyShell attack
-
Inventory every exposed Exchange server
Identify all internet-facing Exchange 2013, 2016 and 2019 systems, their roles, hostnames, IP addresses, cumulative updates and security updates. Include load-balanced nodes and disaster-recovery systems that may be reachable from the internet.
-
Patch supported systems
Apply Microsoft’s latest security updates for the installed, supported Exchange build. Verify installation and reboot requirements on every node; updating only one server in a pool leaves the others exposed.
-
Contain systems with evidence of exploitation
Isolate a suspected server and start incident response. Do not assume that a successful update proves the host was clean. Record the time of isolation and preserve relevant disk, memory and log evidence according to your response procedures.
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Review web, Exchange and endpoint telemetry
Examine IIS, ECP, OWA and Exchange logs for unusual requests, encoded parameters, unexpected administrative actions and activity outside normal working patterns. Correlate those events with Microsoft Defender and AMSI alerts, process creation, outbound connections and mailbox-export activity.
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Hunt for web shells and modified files
Search Exchange web directories for newly created or altered ASPX files, then compare each file with a known-good baseline. Microsoft’s guidance calls out suspicious ASPX files created by
MSExchangeMailboxReplication.exe.$roots = @( "C:Program FilesMicrosoftExchange ServerV15FrontEndHttpProxy", "C:Program FilesMicrosoftExchange ServerV15ClientAccess" ) Get-ChildItem -Path $roots -Filter *.aspx -Recurse -File | Select-Object FullName, CreationTimeUtc, LastWriteTimeUtc, LengthUse the actual Exchange installation directories if they differ. Treat an unexpected file as a lead for forensic analysis, not as proof by itself. CISA malware-analysis reporting includes YARA rules that can help organizations extend this hunt.
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Investigate identity and lateral movement
Review sign-ins, privilege changes, new services, scheduled tasks, remote-management sessions and authentication from the Exchange host to other systems. Reset or decommission credentials that may have been exposed, and inspect connected cloud or hybrid identities.
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Document the timeline and recovery decision
Establish whether suspicious activity predates patching, what data and hosts were reachable, which credentials were reset, and why the server was either rebuilt or returned to service. Continue heightened monitoring after remediation.
How to check specifically for an Exchange web shell
Start with file-system anomalies
Build a list of ASPX files under Exchange web roots and compare paths, hashes, owners, timestamps and content with a verified clean server or approved installation media. Pay particular attention to files that appeared shortly before or after suspicious requests and files written by an Exchange worker process.
Correlate files with requests and processes
A web shell is more convincing when its creation aligns with IIS or Exchange requests, a new child process, unusual outbound traffic or a Defender/AMSI detection. Review the full event window rather than relying on a single timestamp.
Use detection content, then validate manually
Microsoft’s 2025 security guidance describes AMSI and Defender detections for possible IIS web shells, suspicious Exchange process execution and possible exploitation. Apply those detections and CISA YARA content where appropriate, then have an analyst validate matches before deleting files or rebuilding systems.
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Patch in place or rebuild?
| Situation | Preferred response | Reason |
|---|---|---|
| No evidence of exploitation and complete, trustworthy telemetry | Patch every node, verify the update and continue enhanced monitoring | The vulnerable entry point can be closed while normal operations continue |
| Suspicious requests or files, but scope is still being determined | Isolate the server and conduct forensic investigation before declaring recovery | Patch status does not answer whether persistence or credential theft occurred |
| Confirmed web shell, credential theft, lateral movement or inadequate evidence | Follow incident-response guidance; rebuild or replace the host when integrity cannot be established | A clean rebuild is often safer than trusting an extensively modified system |
The decision depends on patch coverage and support status, evidence of pre-patch compromise, the depth of IIS/Exchange/Defender telemetry, credential and identity containment, available forensic expertise and whether monitoring can continue after remediation.
What is known—and what is not
ProxyShell remains relevant because exploitation continued long after fixes were released, according to Microsoft’s 2025 security reporting. The available official material establishes ongoing scanning, exploitation techniques and defensive actions, but it does not establish a 2026 global victim count, exploitation rate or number of vulnerable servers. Do not use the 2021 Ireland estimate as a present-day worldwide measure.
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