10 Microsoft IIS/10.0 Vulnerabilities: Critical Exploits You Must Patch Now

If your IIS 10.0 server handles HTTP traffic, CVE-2021-38647 could let attackers execute arbitrary code without authentication. This zero-day flaw exploits the HTTP protocol stack, making it one of the most critical vulnerabilities Microsoft has patched for Windows Server 2016/2019 deployments.

The exploit works by sending malformed HTTP requests, bypassing security controls entirely. Microsoft’s emergency patch (released in September 2021) fixed a memory corruption issue that allowed attackers to escalate privileges or take full control of affected systems. With a CVSS score of 9.8, this flaw demands immediate attention—especially in environments where IIS 10.0 processes untrusted input.

System administrators running IIS 10.0 on Windows Server 2016/2019 should prioritize this patch, as no workarounds exist. Even legacy deployments remain at risk until updated. ⚡

If your shared hosting environment relies on WebDAV for file management, you might be leaving the door wide open for attackers. CVE-2020-16875 exploits a request smuggling flaw in IIS 10.0, letting malicious actors bypass security controls and hijack sessions or poison caches with carefully crafted WebDAV requests.

The vulnerability stems from improper handling of WebDAV headers in HTTP requests, allowing attackers to manipulate how servers parse boundaries between requests. This can lead to session hijacking or cache poisoning, even if other security layers (like WAFs) are in place. Microsoft’s patch for this flaw specifically targets misconfigured IIS modules, making it a prime target for shared hosting providers.

System administrators managing multi-tenant environments or legacy IIS setups should prioritize disabling WebDAV unless absolutely necessary, and apply the latest security updates to block this attack vector. 🔧

If your IIS 10.0 server hosts legacy .NET Framework 4.7.2 applications, you’re at risk of CVE-2019-1132, a memory corruption flaw that can crash your apps—or worse, let attackers execute arbitrary code. Unlike HTTP protocol exploits, this one targets the core runtime itself, making it especially dangerous for high-traffic sites running older .NET stacks.

The vulnerability stems from improper memory handling in ASP.NET Core when processing certain requests, leading to denial-of-service (DoS) or remote code execution (RCE). Microsoft’s patch (KB4499175) fixes the issue, but the real challenge lies in detecting whether your legacy apps are exposed—especially if they rely on IIS 10.0’s integrated pipeline for .NET hosting.

This exploit is a nightmare for enterprises still running .NET Framework 4.7.2 due to compatibility constraints. If you’re stuck on this version, prioritize isolating vulnerable apps behind a WAF or upgrading to a supported runtime. 💻

If your hybrid cloud setup relies on IIS 10.0 bridging legacy systems with modern HTTP/2 traffic, CVE-2021-34473 could be silently compromising your security. Unlike traditional smuggling, this exploit manipulates HTTP/2's multiplexing protocol to hide malicious requests within legitimate streams, bypassing firewalls and WAFs entirely.

The flaw leverages HTTP/2's header compression to craft requests that appear valid to front-end servers but execute differently on back-end systems. Attackers can hijack sessions, steal credentials, or even fixate user sessions to redirect traffic. Microsoft's patch for Windows Server 2016/2019 (KB5005049) specifically addresses this by tightening request validation, but legacy integrations often remain exposed.

System administrators managing hybrid cloud environments—especially those with IIS 10.0 acting as a gateway between on-premises and cloud services—should prioritize this fix. Enabling IIS Request Filtering and disabling HTTP/2 for non-critical paths adds an extra layer of defense. ✨

If your organization relies on Active Directory and IIS 10.0, CVE-2020-0683 could let attackers bypass Kerberos authentication entirely—granting them full access to your web applications without valid credentials. This Schannel spoofing vulnerability exploits weak certificate validation in Windows Server, making it a silent but devastating threat.

The flaw targets the Secure Channel (Schannel) protocol, allowing attackers to spoof authentication tokens and impersonate legitimate users. Microsoft’s patch (KB4551762) fixes the issue, but environments with outdated certificate policies or misconfigured Group Policy remain at risk. The CVSS score of 8.1 underscores its severity, especially for internal-facing IIS applications.

System administrators managing Active Directory-integrated environments should prioritize this patch, particularly if relying on legacy certificate authorities. Enabling Certificate Revocation List (CRL) checks adds an extra layer of defense. 💻

If your IIS 10.0 server integrates with a Windows Print Spooler, you might be exposing more than just print jobs. CVE-2021-1732—a critical flaw in the Print Spooler service—can be weaponized when misconfigured IIS 10.0 environments allow attackers to escalate privileges and move laterally into web applications. This isn’t just a Windows issue; it’s a gateway for IIS-specific attacks.

The vulnerability stems from how the Print Spooler handles untrusted input, allowing attackers to execute arbitrary code with SYSTEM-level privileges. When IIS 10.0 shares resources or relies on Print Spooler for shared services (like document conversion), the attack surface expands. Microsoft’s patch (KB5006670) fixes the core flaw, but IIS-specific hardening—like disabling unnecessary print-related modules—is critical to prevent pivot attacks. ⚡

If your IIS 10.0 server shares SMB resources with internal systems, you’re at risk of SMBGhost (CVE-2020-0796), a Server-Side Request Forgery (SSRF) flaw that lets attackers map your entire network topology or steal data from misconfigured file shares.

This vulnerability exploits the SMBv3 protocol in Windows Server 2019/2016 when IIS 10.0 acts as a gateway. Attackers send crafted SMB packets to force the server into revealing internal IPs or exfiltrating files via unauthenticated SMB connections. Microsoft’s patch (KB4551762) fixes the flaw, but legacy systems often miss updates, leaving them exposed to lateral movement attacks.

System admins managing hybrid cloud environments or shared file servers should prioritize this patch, as SSRF attacks can bypass firewalls by targeting internal resources. 💻

If your IIS 10.0 servers rely on CDNs or HTTP/2, you’re at risk of a stealthy attack: CVE-2021-31166 exploits Host header manipulation to smuggle malicious requests past WAFs, bypassing even the most robust defenses. Unlike other smuggling flaws, this one targets the HTTP/2 protocol, making it harder to detect with traditional monitoring tools.

The vulnerability lets attackers poison caches, hijack sessions, or execute arbitrary code by crafting conflicting Host headers in HTTP/2 requests. Microsoft’s patch (KB5005049) fixes the core issue, but CDN-integrated deployments—especially those using Cloudflare, Akamai, or Fastly—require additional WAF rule updates to block exploitation attempts. The attack surface expands when HTTP/2 is enabled, as many admins assume their WAFs already cover these risks.

This flaw is a nightmare for e-commerce platforms using CDNs to offload traffic, as attackers can redirect users to malicious sites or steal session cookies without triggering alerts. If you’re running IIS 10.0 with HTTP/2, prioritize this patch—especially if your WAF lacks HTTP/2-specific protections. ✨

If your Active Directory environment relies on IIS 10.0, CVE-2020-1350 (Zerologon) could silently hand attackers domain admin privileges—without a trace. Unlike other exploits targeting IIS directly, this flaw weaponizes the Netlogon protocol, making it a stealthy gateway for lateral movement across your entire network.

The vulnerability stems from a weak cryptographic handshake in Netlogon, allowing attackers to forge authentication tokens with zero authentication. Microsoft’s patch (KB4557222) fixes the core issue, but legacy systems or misconfigured Active Directory setups remain at risk. This exploit has been observed in real-world attacks, often used to pivot from compromised workstations to domain controllers hosting IIS 10.0.

System administrators managing hybrid cloud or on-premises AD environments should prioritize this patch, especially if Netlogon signing is disabled or LDAP signing is misconfigured. 💻

If your IIS 10.0 server runs the DNS Server role, you’re at risk from CVE-2021-26855, a critical flaw that lets attackers execute arbitrary code via malformed DNS responses. Unlike other IIS vulnerabilities targeting HTTP or SMB, this exploit weaponizes DNS traffic—often overlooked in security audits.

The vulnerability stems from improper input validation in the Windows DNS Server service, allowing attackers to craft malicious DNS responses that trigger remote code execution. Microsoft assigned it a CVSS score of 9.8, making it one of the most severe DNS-related flaws ever discovered. Hybrid cloud environments with split DNS zones (e.g., on-premises DNS resolving cloud resources) are especially vulnerable, as attackers can pivot between trusted zones.

Administrators should prioritize applying KB5000865 (Windows Server 2019) or KB5000866 (Windows Server 2016) and disable unnecessary DNS recursion if external exposure is unavoidable. 💻