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

Discovered in 2021, CVE-2021-38647 is a zero-day HTTP/2 request smuggling vulnerability that lets attackers bypass security filters entirely by crafting malformed HTTP/2 requests. This flaw is particularly dangerous because it affects all IIS 10.0 configurations by default, including default installations with no additional tweaks.

The exploit works by manipulating how IIS processes HTTP/2 headers, allowing attackers to inject malicious payloads that bypass Web Application Firewalls (WAFs) and other security layers. Microsoft’s advisory confirms this can lead to arbitrary code execution, meaning attackers could gain full control over vulnerable servers. The patch (KB5005443) was released as part of the October 2021 security updates, but many systems remain unpatched due to its stealthy nature.

This vulnerability is a top priority for administrators running IIS 10.0 with HTTP/2 enabled, especially those hosting public-facing applications. Apply the patch immediately and consider disabling HTTP/2 as a temporary mitigation if patching isn’t feasible. ⚡

If your IIS 10.0 server relies on URL rewrites for performance, CVE-2020-16875 could turn cached responses into attack vectors. This vulnerability exploits the URL rewrite module to inject malicious headers into cached responses, bypassing security controls and enabling persistent XSS attacks across entire server farms.

The exploit works by poisoning the web cache with crafted requests that manipulate the HTTP response headers—even when the original request appears benign. Once injected, these headers persist in cached responses, affecting all users until the cache is cleared. Microsoft patched this in November 2020 via cumulative updates, but many legacy systems remain exposed due to delayed patching.

Administrators managing high-traffic IIS farms or those using URL rewrites for SEO/performance should prioritize this patch. Enabling strict cache validation and WAF rules can provide temporary protection until updates are applied. 💻

If your IIS-hosted .NET apps rely on ASP.NET Core, this CVE-2019-1181 exploit could turn a simple deserialization flaw into full system access for attackers. Unlike previous items targeting HTTP protocols or caching, this vulnerability leverages malicious payloads in serialized data to execute arbitrary code remotely—often without triggering alerts.

The flaw stems from improper validation in ASP.NET Core's default deserialization mechanisms, allowing attackers to craft payloads that bypass authentication and execute commands with SYSTEM-level privileges. Microsoft’s patch (via security update KB4499175) addressed the core issue, but many legacy systems remain exposed due to delayed updates or misconfigured TypeConfuser protections.

Developers maintaining IIS-hosted .NET applications should prioritize this patch, especially if their apps use custom serialization or third-party libraries. Enabling strict deserialization policies in Startup.cs adds an extra layer of defense. 💻

If your IIS 10.0 server seems to be silently probing internal systems without your knowledge, CVE-2018-8423 could be the culprit. This Server-Side Request Forgery (SSRF) flaw lets attackers force IIS to make unauthorized HTTP requests to internal or external destinations, effectively turning your server into a spyglass for reconnaissance.

The vulnerability stems from improper input validation in IIS's handling of HTTP requests with malformed Host headers. Attackers exploit this to bypass network security controls and access services like internal APIs, database servers, or cloud metadata services—all without triggering alerts. Microsoft's patch for this flaw (released in November 2018) fixed the core issue, but misconfigured firewalls or legacy systems may still be exposed.

This exploit is particularly dangerous for organizations with complex internal networks, as it enables attackers to map out infrastructure before launching larger attacks. System administrators should prioritize patching and monitor for unusual outbound requests from their IIS servers. 💻

If your IIS 10.0 server still relies on short name URL mappings, you might be leaving the door wide open for attackers. CVE-2017-7269 exploits a fundamental flaw in how IIS processes mapped short names, allowing unauthorized access to restricted directories by simply appending a trailing slash or malformed path.

The vulnerability stems from IIS's legacy short name handling (introduced in IIS 6.0), where attackers can bypass authentication by manipulating how the server resolves paths. For example, a request to /shortname/ might redirect to /full/path/ without proper validation, exposing sensitive files or admin interfaces. Microsoft's October 2017 security update patched this, but many legacy systems remain unpatched, making it a persistent threat.

This exploit is particularly dangerous for enterprise environments running outdated IIS configurations or third-party applications relying on short name mappings. If you're maintaining a legacy IIS 10.0 deployment, prioritize disabling short name mappings or applying the latest cumulative updates. ⚡

If your IIS 10.0 servers suddenly start crashing under HTTP/2 traffic, CVE-2022-26923 might be the culprit. This DDoS variant exploits a flaw in how IIS processes malformed CONNECT requests, forcing kernel-level crashes that take servers offline—often without warning.

The attack works by sending rapid reset signals that overwhelm IIS's HTTP/2 parser, requiring kernel patches (not just IIS updates) to fully mitigate. Unlike traditional DDoS attacks, this one doesn’t just slow servers—it completely halts them, making it a nightmare for high-traffic environments. Microsoft’s advisory highlights this as a critical priority for Windows Server 2016/2019/2022 deployments.

System admins managing enterprise-grade IIS deployments should treat this as a top priority, especially if HTTP/2 is enabled. The fix isn’t just an IIS update—it demands OS-level patches to close the gap. 💪

If your Active Directory environment relies on IIS 10.0 for authentication, CVE-2021-34473 could let attackers escalate privileges undetected. This flaw exploits Kerberos delegation misconfigurations to impersonate domain controllers, turning a single compromised machine into a launchpad for lateral movement.

The exploit targets IIS 10.0’s SPN (Service Principal Name) delegation, allowing attackers to forge TGT (Ticket-Granting Ticket) requests that bypass Kerberos authentication. Once inside, they can move freely across your network, accessing sensitive resources like Exchange, SQL, or file shares without raising alarms. Microsoft’s patch for this requires updating to IIS 10.0 with the July 2021 cumulative update or later.

This is a must-patch for enterprises with Active Directory trusts or Kerberos-dependent services. Even if you’ve hardened IIS, misconfigured delegation settings could leave you exposed. 💪

If you thought RDP exploits were just about brute-forcing passwords, think again. CVE-2019-0702 (aka "BlueKeep") lets attackers execute arbitrary code on Windows systems via Remote Desktop Protocol (RDP)—but here’s the twist: when chained with IIS 10.0 misconfigurations, it can lead to full system compromise without credentials.

Discovered in 2019, this wormable vulnerability affects Windows 7, Server 2008, and later versions, including those hosting IIS. The exploit leverages a flaw in RDP’s Network Level Authentication (NLA), allowing attackers to bypass authentication entirely. When combined with default IIS permissions (e.g., misconfigured ASP.NET or IIS Manager access), an attacker could escalate privileges to SYSTEM level—no password required.

This is a must-patch for admins managing hybrid environments where RDP and IIS coexist. Microsoft’s KB4557222 patch addresses it, but legacy systems remain at risk. 💻

If you thought encrypted traffic was safe from tampering, think again—CVE-2020-0683 in IIS 10.0’s Schannel component lets attackers forge SSL/TLS certificates to impersonate your server. This flaw exploits a weakness in how IIS validates certificate chains, allowing attackers to intercept and manipulate HTTPS traffic without detection.

The vulnerability affects Windows Server 2016 and 2019 running IIS 10.0, where attackers can craft malicious certificates to bypass authentication checks. Microsoft’s patch (KB4551762) fixes the Schannel validation logic, but unpatched servers remain exposed to man-in-the-middle (MITM) attacks on sensitive data like login credentials or payment transactions.

This is a must-patch for any organization handling PCI-compliant transactions or sensitive user data. Even with modern encryption, certificate spoofing remains a stealthy attack vector. 💻

If your IIS 10.0 servers seem to crash unexpectedly during high traffic—especially when processing large payloads—CVE-2018-8373 might be the culprit. This memory corruption flaw doesn’t just cause worker process crashes; it can also expose sensitive data stored in memory, turning a simple DoS into a full-blown data leak.

The vulnerability stems from improper handling of malformed HTTP requests targeting the IIS HTTP.sys kernel-mode driver, leading to heap-based buffer overflows. Microsoft’s patch (KB4338832) fixes the core issue, but many legacy systems remain unpatched due to misconfigured update schedules. Unlike other IIS flaws, this one often flies under the radar because it doesn’t trigger authentication bypasses—just silent failures and potential data exposure.

System administrators running high-traffic IIS 10.0 environments (e.g., e-commerce sites or APIs) should prioritize this patch, as attackers can weaponize it to disrupt services during peak hours while silently harvesting memory dumps. 💻