CVE-1999-1233 in IIS
Summary
by MITRE
IIS 4.0 does not properly restrict access for the initial session request from a user's IP address if the address does not resolve to a DNS domain, aka the "Domain Resolution" vulnerability.
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Analysis
by VulDB Data Team • 04/20/2026
The vulnerability described in CVE-1999-1233 represents a significant access control flaw in Microsoft Internet Information Services version 4.0 that stems from improper handling of IP address resolution during initial session establishment. This weakness specifically affects systems where client IP addresses cannot be resolved to DNS domain names, creating a potential security breach that could allow unauthorized access to web resources. The issue manifests when the IIS server attempts to establish a session with a client whose IP address fails DNS resolution, leading to unpredictable access control behavior that undermines the server's security model.
The technical root cause of this vulnerability lies in the server's session management logic which relies on DNS resolution as part of its access restriction mechanism. When an IP address cannot be resolved to a DNS domain name, the system's default behavior becomes unpredictable, potentially allowing attackers to bypass intended access controls that would normally be enforced based on domain name resolution. This flaw operates at the network layer where the server's authentication and authorization processes become inconsistent when dealing with unresolved IP addresses, creating a potential attack surface that could be exploited by malicious actors.
The operational impact of this vulnerability extends beyond simple unauthorized access, as it can enable attackers to perform session hijacking, privilege escalation, and potentially gain elevated system access depending on the server configuration. Security researchers have noted that this issue particularly affects environments where IIS 4.0 serves as a web server and where access controls are heavily dependent on domain-based restrictions. The vulnerability can be exploited by attackers who manipulate their network requests to trigger the DNS resolution failure condition, potentially gaining access to protected resources that should otherwise be restricted to specific domains or IP ranges.
From a cybersecurity perspective, this vulnerability aligns with CWE-284, which addresses improper access control mechanisms, and can be mapped to ATT&CK technique T1078 for valid accounts and T1566 for credential access through network service manipulation. Organizations running IIS 4.0 servers are particularly vulnerable since this flaw existed in a widely deployed web server platform and affected numerous enterprise environments. The remediation approach requires immediate patching of the IIS 4.0 installation, but administrators should also consider implementing additional network-level controls such as firewall rules that restrict access based on IP address ranges rather than relying solely on DNS-based access controls. System administrators should also review their server configurations to ensure that access controls are not dependent on DNS resolution, and implement proper logging to monitor for unusual access patterns that might indicate exploitation attempts. The vulnerability demonstrates the critical importance of robust access control implementation that does not depend on external resolution services, as these services can fail or be manipulated by attackers to bypass security measures.