CVE-2000-0226 in IIS
Summary
by MITRE
IIS 4.0 allows attackers to cause a denial of service by requesting a large buffer in a POST or PUT command which consumes memory, aka the "Chunked Transfer Encoding Buffer Overflow Vulnerability."
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Analysis
by VulDB Data Team • 10/18/2025
The CVE-2000-0226 vulnerability represents a critical denial of service flaw in Microsoft Internet Information Services version 4.0 that exploits the chunked transfer encoding mechanism used in http protocols. This vulnerability specifically targets the way IIS 4.0 handles incoming data streams during POST and PUT operations, creating a buffer overflow condition that can be leveraged by malicious actors to consume excessive system memory resources. The flaw operates through a carefully crafted request that includes a large buffer specification within the chunked transfer encoding header, causing the web server to allocate insufficient memory for processing the incoming data. This type of vulnerability falls under the CWE-122 category of insufficient resource allocation, where the system fails to properly validate or limit the size of buffer allocations. The vulnerability is particularly dangerous because it can be exploited without authentication, allowing remote attackers to disrupt services simply by sending malformed requests to the affected web server.
The technical implementation of this vulnerability involves the exploitation of how IIS 4.0 processes chunked transfer encoding data streams. When a client sends a POST or PUT request with chunked encoding, the server must parse the chunk size indicators and allocate memory accordingly for each chunk. The flaw occurs when the attacker sends a chunk size that appears legitimate but is actually designed to trigger excessive memory allocation. The server's buffer management code fails to properly validate the chunk size parameters, leading to allocation of memory that far exceeds what is actually needed for processing the request. This results in memory exhaustion on the server, causing the system to become unresponsive or crash entirely. The vulnerability specifically affects the HTTP.sys kernel driver component of IIS 4.0, which handles the core protocol processing functions. This aligns with ATT&CK technique T1499.004 which covers network denial of service attacks through resource exhaustion.
The operational impact of CVE-2000-0226 extends beyond simple service disruption to potentially compromise the entire web server infrastructure. When exploited successfully, the vulnerability can cause the affected IIS server to consume all available memory resources, leading to system instability and complete denial of service for legitimate users. The attack can be executed rapidly and with minimal resources, making it particularly effective as a low-cost disruption method. Organizations running IIS 4.0 servers are vulnerable to both automated scanning attacks and targeted exploitation attempts, as the vulnerability affects the core HTTP processing capabilities of the web server. The memory consumption pattern typical of this exploit makes it difficult to distinguish from legitimate high-volume traffic, complicating detection and mitigation efforts. This vulnerability particularly affects organizations that rely heavily on web-based applications and services, as the disruption can impact business operations and customer access to critical online services.
Mitigation strategies for CVE-2000-0226 focus on both immediate defensive measures and long-term architectural improvements. The most effective immediate solution involves applying the official Microsoft security patch that addresses the buffer overflow in the HTTP.sys driver component. Organizations should also implement network-level protections such as rate limiting and connection throttling to prevent excessive resource consumption from single clients. Input validation mechanisms should be strengthened to reject malformed chunked encoding requests before they reach the vulnerable processing code. Network segmentation and firewall rules can help limit the exposure of vulnerable IIS 4.0 servers to external threats. Additionally, implementing monitoring solutions that track memory usage patterns and unusual resource consumption can provide early detection of exploitation attempts. The vulnerability highlights the importance of proper buffer management and resource validation in web server implementations, aligning with security best practices outlined in NIST SP 800-128 and other cybersecurity frameworks. Organizations should also consider migrating away from unsupported software versions to newer IIS releases that have addressed these fundamental security flaws.