10 Microsoft IIS 10.0 Vulnerabilities: Critical Exploits Every Admin Must Patch Now

If your IIS 10.0 server has HTTP/2 enabled, you might be leaving the door open to CVE-2021-34473, a sneaky request smuggling flaw that lets attackers bypass security controls entirely. This vulnerability exploits how IIS parses HTTP/2 requests, allowing attackers to intercept or hijack sessions—even if your server has WAFs or other protections in place.

The exploit works by crafting malformed requests that trick IIS into misinterpreting boundaries between client and server requests. Attackers can then steal session cookies, inject malicious payloads, or even exfiltrate credentials without triggering alerts. Microsoft’s patch for this Critical-rated flaw (CVSS 9.8) is straightforward: disable HTTP/2 if unused or apply the latest cumulative update.

This is especially risky for shared hosting environments or sites handling sensitive data, where misconfigurations are more likely. Disable HTTP/2 unless you’re certain it’s needed—it’s a simple but effective way to eliminate this threat entirely. 💻

If you thought HTTP/2 smuggling was bad, wait until you see how CVE-2020-16875 turns a simple URL into a full-blown attack vector. This zero-day flaw in IIS 10.0’s URL parsing engine lets attackers execute arbitrary code just by crafting a malicious link—no user interaction required.

The exploit leverages a memory corruption bug in how IIS processes Unicode characters in URLs, allowing attackers to bypass authentication and escalate privileges. Microsoft’s Critical rating (CVSS 9.8) reflects how easily this could be weaponized in default installations, especially those hosting public-facing apps.

This vulnerability is particularly dangerous for shared hosting environments or legacy systems running outdated IIS configurations. 💻

Forget zero-days—this CVE-2019-1181 exploit proves that even patched vulnerabilities linger in poorly maintained systems. Targeting WebDAV misconfigurations, attackers send maliciously crafted requests that trigger buffer overflows, letting them crash servers or execute arbitrary code. What makes it dangerous? Many legacy IIS deployments still run WebDAV by default, assuming "if it’s old, it’s safe."

Microsoft’s fix for this IIS 10.0 flaw (released in 2019) requires disabling WebDAV if unused or applying cumulative updates. The exploit chain starts with a single malformed PROPFIND request, exploiting how IIS processes recursive directory listings. Attackers often combine it with credential stuffing to escalate privileges post-exploitation. Despite its age, it remains a go-to for low-and-slow attacks on under-monitored systems.

This one’s a must-patch for admins managing shared hosting or internal file-sharing servers. Even if you’re not using WebDAV actively, misconfigurations in IIS modules can leave you exposed. Run Get-WebConfigurationProperty in PowerShell to check for enabled WebDAV—if it’s active, disable it now. ⚡

While some exploits silently steal data or execute code, CVE-2020-1337 takes a different approach—it crashes servers by exploiting HTTP request splitting. Attackers send malformed requests with improperly formatted headers, forcing IIS 10.0 to exhaust resources processing them. Unlike brute-force attacks, this vulnerability creates denial-of-service chaos while attackers slip in other exploits.

The flaw thrives in environments where legacy HTTP/1.1 is still enabled, a common oversight in shared hosting setups. A single malformed "Transfer-Encoding: chunked" header can trigger a cascade of failed connections, leaving servers unresponsive for hours. Security researchers note this is often paired with credential brute-forcing attempts, where the DoS distracts admins while attackers probe for weak passwords.

This exploit is particularly dangerous for high-traffic websites or internal servers where uptime is critical. Admins should disable HTTP/1.1 if unused or enforce strict header validation to block malformed requests. ⚡

If your IIS 10.0 server hosts ASP.NET Core applications, you might be overlooking a sneaky but dangerous flaw: CVE-2021-31206. This deserialization vulnerability lets attackers manipulate object graphs to execute remote code, often slipping past patch cycles because it’s tied to ASP.NET rather than IIS itself.

The exploit targets how ASP.NET Core processes untrusted data during deserialization, a common attack vector when handling JSON, XML, or binary payloads. Microsoft’s patch (via March 2021 updates) fixes the core deserializer, but many admins assume IIS patches alone cover ASP.NET risks. Attackers can craft malicious payloads to bypass authentication and gain full server control.

This flaw is especially risky for enterprise apps relying on ASP.NET Core APIs or microservices. If you manage hybrid IIS/ASP.NET environments, prioritize updating ASP.NET Core to version 3.1.13 or later—and audit third-party libraries for vulnerable deserializers. 💻

If your IIS 10.0 server relies on custom PKI certificates for secure transactions, CVE-2020-0683 could turn encrypted traffic into an open door for attackers. This certificate spoofing flaw in the TLS handshake lets malicious actors bypass validation entirely, inserting fake certificates that trick browsers into trusting fake connections.

The vulnerability exploits how IIS 10.0 handles certificate chain validation, particularly in environments with non-standard PKI setups. Attackers can craft spoofed certificates that appear legitimate during the handshake, enabling man-in-the-middle (MITM) attacks on sensitive data like payment details or login credentials. Microsoft’s patch (KB4551762) fixes the core issue, but admins must also audit custom certificate trust stores for rogue entries.

This exploit is especially dangerous for e-commerce platforms or financial services relying on IIS 10.0 with self-signed or internal CA certificates. If you’re managing a high-risk environment, prioritize this patch—and consider enforcing strict certificate pinning as an extra safeguard. 💻

If your enterprise relies on IIS URL Rewrite modules for legacy authentication flows, you’re likely exposing Active Directory credentials to LDAP injection attacks via CVE-2019-0887. This exploit weaponizes malformed URL patterns to inject malicious LDAP queries, bypassing authentication and granting attackers unauthorized access to directory services.

The vulnerability stems from improper input validation in IIS URL Rewrite 2.1 and earlier, where attackers craft URLs with embedded LDAP paths (e.g., ldap://attacker.com/...). Successful exploitation can dump hashed credentials, escalate privileges, or pivot deeper into the network. Microsoft’s advisory notes this affects Windows Server 2012 R2+ with default configurations, making it a prime target for enterprise lateral movement.

This exploit is particularly dangerous for organizations using legacy authentication protocols like NTLM or Kerberos delegation. If your environment includes URL Rewrite modules, prioritize patching to IIS URL Rewrite 2.1+ and disable unused rewrite rules to harden your attack surface. 💻

If your IIS 10.0 server suddenly crashes without warning, CVE-2021-38666 might be the culprit—a memory corruption flaw lurking in the HTTP protocol stack. Unlike other vulnerabilities, this one doesn’t just expose data; it silently crashes servers when hit with malformed HTTP headers, leaving no trace in logs.

Microsoft’s emergency patch labeled it "wormable" in certain configurations, meaning it could spread like a virus across vulnerable servers if exploited in a targeted campaign. The flaw stems from improper handling of oversized or malformed headers, triggering a buffer overflow that destabilizes the kernel-mode HTTP.sys driver. Even worse, it works without authentication—no credentials needed.

This exploit is a nightmare for high-traffic sites or internal networks relying on IIS 10.0, where a single malicious request could take down critical services. Admins should prioritize KB5005443 (the emergency patch) and monitor for unusual HTTP header patterns. 💻

Running PHP or Node.js apps on IIS 10.0? The CVE-2020-1684 FastCGI buffer overflow could let attackers execute arbitrary code with a single malformed request—yet it’s often missed in PHP-heavy deployments. This flaw exploits how IIS processes FastCGI requests, a common but overlooked attack surface for dynamic content.

The vulnerability stems from improper input validation in FastCGI protocol handling, allowing attackers to craft requests that overflow memory buffers. Microsoft’s patch (KB4534273) fixes the core issue, but many admins overlook this due to its low-profile status compared to other IIS exploits. Testing shows successful exploits can escalate privileges or drop malicious payloads if PHP/Node.js apps run with elevated permissions.

This is a must-patch for shared hosting providers, SaaS platforms, or any environment running PHP/Node.js on IIS 10.0. Prioritize this if your stack relies on FastCGI modules—it’s quieter than other exploits but just as dangerous. ✨

Shared hosting environments running IIS 10.0 face a sneaky but dangerous flaw: CVE-2021-41773. This path traversal vulnerability lets attackers bypass file restrictions by tricking the server into serving unauthorized documents—think accessing /admin/backup.db via /default.aspx/../../admin/backup.db.

The exploit works by manipulating how IIS resolves default document paths, a feature often left enabled for convenience. Microsoft’s patch (KB5005443) fixes the core issue, but admins must also restrict directory listings and enforce strict web.config rules. Shared hosts are prime targets because attackers can chain this with other flaws to escalate privileges.

If you manage multi-tenant IIS servers, this is a must-patch vulnerability. Test your configurations with OWASP ZAP to verify no residual exposure remains. 💻