Microsoft IIS Flaw Allows ANY Unauthorized Attacker to Execute Remote Code

 

Unauthenticated RCE via Microsoft IIS can give an attacker full control of the Windows web server and everything behind it.

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TL;DR: A newly disclosed flaw in Microsoft IIS enables unauthenticated remote code execution (RCE) on vulnerable Windows servers. Treat as a perimeter-breaking incident: web shell → credential dump → domain pivot. Patch/mitigate immediately, lock down exposed modules (e.g., WebDAV), and implement strict request filtering + WAF rules.

What’s the risk ?

If your public website, API, extranet, or partner portal runs on Microsoft Internet Information Services (IIS), an attacker on the internet may be able to run commands on your server without logging in. That means they can silently drop a web shell, steal service-account passwords, access databases, encrypt file shares, and move laterally into your Active Directory.

Who should care the most?

• US/EU/UK/AU/IN enterprises hosting customer portals, healthcare apps (HIPAA), fintech/payments (PCI DSS), government workloads, or OT/SCADA gateways proxied via IIS.
• MSP/MSSP environments running shared IIS reverse proxies.
• SaaS/API teams exposing .NET apps or legacy SOAP endpoints.

Likely attack chain

1. Exploit the vulnerable IIS component / handler (often via crafted HTTP verb, header smuggling, or file upload parsing).
2. Persistence by dropping an aspx or ashx web shell to inetpub\wwwroot or temp directories.
3. Privilege escalation using misconfigured App Pools / service accounts; dump machine & domain creds.
4. Lateral movement to SQL Server, file servers, hypervisors, and domain controllers.
5. Impact: data theft (PII/PHI), ransomware, defacement, or supply-chain tampering for downstream customers.

Immediate actions (do these now)

• Patch Windows/IIS via Windows Update or your WSUS/SCCM pipeline. If a fixed KB is available, prioritize internet-facing instances first.
• Disable risky modules you don’t use (WebDAV, IIS FTP, legacy ASP, CGI). Least-functionality beats zero-days.
• Enable Request Filtering: block double-extensions (.aspx;.jpg), executable uploads, and suspicious verbs (PROPFIND, SEARCH, MOVE).
• Front with a WAF (Akamai, Cloudflare, Azure WAF, F5, ModSecurity/OWASP CRS). Turn on RCE & file-upload rulesets; monitor and block.
• Run App Pools as low-privileged identities (no domain admin/service-tier privileges). Use gMSA where possible.
• Turn on detailed logging: IIS logs, Failed Request Tracing (FREB), Windows Event Logs, and PowerShell transcription.
• Network-segment IIS from crown-jewel systems; restrict east-west using firewall/ACLs and just-enough-access.

Detections you can deploy fast

# PowerShell — hunt for suspicious ASPX/ASHX web shells in common paths
Get-ChildItem -Recurse "C:\inetpub\wwwroot","C:\Windows\Temp","C:\ProgramData\Microsoft\IIS" `
  -Include *.aspx,*.ashx,*.asmx -ErrorAction SilentlyContinue |
  Select-Object FullName,Length,LastWriteTime |
  Sort-Object LastWriteTime -Descending | Select-Object -First 200

# Look for anomalous verbs in IIS logs (PROPFIND/MKCOL/SEARCH/TRACE/etc.)
Select-String -Path "C:\inetpub\logs\LogFiles\W3SVC*\u_ex*.log" -Pattern "PROPFIND|SEARCH|MKCOL|TRACE|cmd\.exe|powershell\.exe"

Recommended WAF / URL Rewrite snippets

<requestFiltering>
  <fileExtensions>
    <add fileExtension=".aspx" allowed="false" />
    <add fileExtension=".ashx" allowed="false" />
    <add fileExtension=".exe" allowed="false" />
    <add fileExtension=".dll" allowed="false" />
  </fileExtensions>
</requestFiltering>


<rule name="BlockDoubleExtensions" stopProcessing="true">
  <match url=".*\..+\.(aspx|ashx|asp|php|jsp)$" />
  <action type="CustomResponse" statusCode="403" statusReason="Forbidden" statusDescription="Double extension blocked" />
</rule>

Executive briefing (90 seconds)

• Business risk: Unauthorized RCE on publicly exposed IIS equals full compromise of the web tier and rapid domain takeover.
• Impact: service outage, data breach (GDPR/CCPA), ransomware downtime, regulatory fines, and brand damage.
• Decision: approve emergency maintenance windows; enforce WAF “block” mode; mandate post-patch verification.

DevSecOps hardening checklist

• Bake CIS/ACSC/ANSSI hardening into your golden AMI or VM template for IIS/Windows Server.
• Block dangerous verbs in Azure Front Door/Cloudflare or your L7 gateway.
• Run containerized .NET apps behind Kestrel + reverse proxy with read-only root FS and non-root users.
• CI/CD artifact scanning for web shells and unexpected .aspx payloads.
• Continuous external attack surface monitoring (EASM) to track exposed IIS endpoints and forgotten sub-sites.

SOC response runbook (short)

1. Switch WAF to block on relevant signatures; capture PCAPs if available.
2. Snapshot the VM; preserve C:\inetpub\logs, FREB, Event Logs, and C:\Windows\Temp.
3. Search for suspicious newly written .aspx/.ashx files; quarantine on sight.
4. Rotate application secrets, service-account passwords, and database credentials.
5. Threat hunt for lateral movement (WinRM/SMB/RDP), LSASS dumps, and abnormal Kerberos ticket activity.

Editor’s Picks (Affiliate) — quick wins for defenders

• Kaspersky Endpoint — ransomware & EDR stack
• TurboVPN — protect remote staff on public Wi-Fi
• Rewardful — SaaS referral tracking for security tools
• HideMyName VPN — privacy and geo-fencing bypass

Disclosure: We may earn a commission if you purchase via these links. We only recommend tools we’d use ourselves.

Next Reads

• Supply Chain Security — Guides
• DevSecOps — Pipelines & Policies
• Incident Response — Playbooks

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