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

If your IIS 10.0 server relies on HTTP/2 for performance, you might be leaving the door open to a sneaky attack: CVE-2021-34473. This exploit weaponizes the protocol’s differences from HTTP/1.1 to smuggle malicious requests past security controls, poisoning caches or executing unauthorized code server-side.

The flaw stems from how IIS handles overlapping or conflicting requests between the two protocols. Attackers craft specially formatted HTTP/2 requests that appear valid to the server but bypass WAFs and other defenses. Microsoft’s patch (via KB5005039) fixes this by enforcing stricter request validation—critical for servers processing sensitive traffic.

This vulnerability is especially risky for environments using HTTP/2 for high-performance APIs or CDN integration. If patching isn’t immediate, consider temporarily disabling HTTP/2 until the fix is applied. ⚡

If your IIS 10.0 server suddenly crashes under heavy traffic without obvious cause, CVE-2021-38647 might be the culprit. This Denial of Service (DoS) flaw exploits how IIS handles 8.3 short file names (like FILENAME~1.TXT), forcing servers to consume excessive CPU and memory until they freeze or reboot.

The exploit works by sending maliciously crafted requests that trigger improper file name resolution, overwhelming the server’s parsing logic. Microsoft classified this as because it affects all IIS 10.0 versions by default, especially high-traffic sites where attackers can trigger crashes with minimal effort. The patch (KB5005039) fixes the underlying file system name resolution logic in the IIS core modules. ⚡

If your IIS 10.0 server has WebDAV enabled, you’re potentially leaving the door wide open for attackers to upload and execute malicious .NET assemblies with full SYSTEM privileges. This isn’t just theoretical—CVE-2020-16875 was actively exploited in the wild before Microsoft released patches, making it one of the most dangerous WebDAV flaws ever documented.

The vulnerability stems from improper validation of WebDAV requests, allowing attackers to bypass authentication and upload arbitrary files—including compiled .NET assemblies—directly to your server. Once executed, these assemblies run with the same elevated permissions as the IIS service, giving attackers full control over the compromised system. Microsoft’s November 2020 patch update (KB4577586) addressed this, but many servers remained unpatched for months, leaving them exposed to ransomware and backdoor attacks.

This exploit is particularly risky for organizations using shared hosting environments or legacy applications relying on WebDAV for file management. If you can’t patch immediately, disable WebDAV via IIS Manager until updates are applied. ⚡

If your IIS servers rely on Active Directory for authentication, CVE-2021-31979 could let attackers bypass Kerberos constraints entirely—escalating privileges without needing valid credentials. This exploit targets the Kerberos protocol implementation in Windows, specifically how IIS handles authentication tokens when integrated with domain environments.

The vulnerability allows attackers to craft malicious Service Principal Names (SPNs) that trick the system into granting unauthorized administrative access. Microsoft rated this as Critical due to its potential for full system compromise, especially in environments where IIS acts as a gateway for internal services. Unlike previous IIS flaws, this one doesn’t rely on misconfigured modules—just a flawed authentication handshake.

This exploit is a nightmare for enterprises using Windows authentication (NTLM/Kerberos) in hybrid or on-premises setups. If your servers handle sensitive data or bridge internal networks, patching this should be top priority. 💻

If your ASP.NET Core apps rely on serialized data—like JSON or XML payloads—you’re at risk from CVE-2021-34523, a flaw that lets attackers execute code remotely by tricking your server into deserializing malicious inputs. Unlike previous IIS exploits targeting protocols or file handling, this one bypasses security by weaponizing trusted data formats.

The vulnerability stems from insecure deserialization in ASP.NET Core’s default configuration, where untrusted data (e.g., user input) is automatically reconstructed into objects. Attackers craft crafted JSON/XML payloads that, when processed, execute arbitrary commands with the server’s privileges. Microsoft’s patch (in .NET 5.0.8+ and .NET Core 3.1.12+) fixes the core issue, but hybrid deployments mixing old and new frameworks remain exposed.

This is especially critical for legacy ASP.NET Core apps or hybrid environments where older versions coexist with newer ones. If you’re using IIS-hosted APIs or microservices, prioritize this patch—especially if your apps accept unvalidated input from clients. 💻

If your IIS servers integrate with Windows Print Spooler—even indirectly—this zero-day flaw (CVE-2021-1732) could let attackers escalate privileges across your network. Microsoft’s July 2021 patch fixed the core Windows vulnerability, but many admins overlooked how IIS servers with spooler dependencies remain exposed to lateral movement.

The exploit abuses unquoted service paths in Print Spooler, allowing attackers to inject malicious DLLs or commands with elevated permissions. What makes this tricky is that Windows Server 2022 migrations often retain legacy spooler integrations unless explicitly audited. Microsoft’s guidance emphasizes disabling the Print Spooler service if unused, but for IIS environments, prioritizing the KB5005039 patch is critical before upgrading.

This is a must-patch for organizations with hybrid IIS/Windows Server setups or shared print resources. Don’t assume your migration to Server 2022 will auto-fix this—verify spooler dependencies first. ⚡

If your IIS 10.0 servers are running unpatched, the ProxyShell exploit chain (CVE-2021-40449) could turn into your worst nightmare. Unlike standalone vulnerabilities, this attack chains three flaws (CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207) to achieve remote code execution with minimal friction.

Microsoft’s security bulletin highlights how attackers exploit misconfigured Exchange Server environments as a foothold, then pivot to IIS via these chained vulnerabilities. The ASP.NET Core deserialization flaw (CVE-2021-34523) alone lets attackers inject malicious payloads, while the HTTP/2 smuggling (CVE-2021-34473) bypasses security controls. Ransomware groups like Conti have weaponized this chain, demanding payments after encrypting entire server farms.

This exploit chain is a must-patch priority for any organization using IIS 10.0 with Exchange or ASP.NET apps. Apply Microsoft’s July 2021 security updates immediately—delaying risks full system compromise. 💪

Shared hosting providers beware: CVE-2021-38666 lets attackers hijack your web.config files by injecting malicious XML directives. Unlike other IIS exploits, this one doesn’t rely on complex protocol tricks—just file permission gaps in multi-tenant environments where tenants share the same server.

The flaw lets attackers append arbitrary configuration snippets (like <add name="maliciousHandler" ... />) that execute with full IIS privileges. Microsoft’s patch fixes IIS 10.0’s configuration parser, but shared hosts with loose NTFS permissions remain at risk. Even read-only web.config files can be exploited if the server allows write access to the IIS_IUSRS group.

This is especially dangerous for WordPress or PHP-based sites on shared IIS hosts, where attackers could silently add backdoors. Audit your web.config permissions now—this exploit turns misconfigured shared hosting into a remote code execution playground. 💻

If your organization relies on hybrid Active Directory setups with on-premises and cloud components, CVE-2020-0683 could silently expose your IIS servers to LDAP relay attacks. This vulnerability allows attackers to intercept and manipulate unsigned LDAP traffic, bypassing authentication controls entirely.

The flaw stems from IIS’s failure to enforce LDAP signing for connections, enabling attackers to relay credentials across domains. Microsoft’s April 2020 patch (KB4551762) fixed the issue, but many hybrid environments—especially those syncing identities between Azure AD and on-prem AD—remain vulnerable due to misconfigured Group Policy or overlooked updates.

This is a silent threat for enterprises with multi-forest AD setups or those using LDAP for authentication. Prioritize patching and enforce LDAP signing via Group Policy to block relay attacks before they escalate. 💡

If your IIS server relies on the URL Rewrite Module for security policies, you might be leaving a backdoor open. CVE-2021-42278 exploits a flaw that lets attackers bypass rewrite rules entirely, granting access to restricted directories or even executing arbitrary scripts—all without triggering alerts.

The vulnerability stems from improper handling of maliciously crafted URLs that manipulate the module’s parsing logic. Unlike other IIS flaws targeting authentication or deserialization, this one focuses on circumventing security policies rather than exploiting direct code execution paths. Microsoft’s patch (KB5005048) fixes the core issue, but servers still need updated rulesets to prevent false positives.

This exploit is particularly risky for shared hosting providers or enterprises using URL rewrites for access control. If you manage a server with custom rewrite rules, audit your configurations immediately—this bypass could turn a security feature into a vulnerability. 🌟