CVE-1999-1199 in HTTP Server
Summary
by MITRE
Apache WWW server 1.3.1 and earlier allows remote attackers to cause a denial of service (resource exhaustion) via a large number of MIME headers with the same name, aka the "sioux" vulnerability.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 10/20/2025
The CVE-1999-1199 vulnerability represents a critical resource exhaustion flaw in early versions of the Apache HTTP Server, specifically affecting versions 1.3.1 and earlier. This vulnerability emerged from the server's handling of HTTP request headers, particularly when multiple headers with identical names were present in a single request. The issue was discovered during the early stages of web server security research and became widely known as the "sioux" vulnerability, named after the specific attack pattern that exploited this weakness. The vulnerability was particularly concerning because it could be exploited remotely without requiring authentication or special privileges, making it accessible to any attacker with network access to the affected server.
The technical root cause of this vulnerability lies in how the Apache server processed and stored HTTP headers during request parsing. When the server encountered multiple headers with the same name in a single HTTP request, it would create separate entries for each occurrence without proper deduplication or resource management. This led to exponential memory consumption as the server allocated additional memory for each duplicate header, ultimately exhausting available system resources. The flaw was particularly insidious because it could be triggered with relatively simple HTTP requests containing numerous duplicate headers, causing the server to consume increasing amounts of memory until it became unresponsive or crashed entirely. The vulnerability was classified under CWE-400 as an unspecified resource management error, specifically involving memory exhaustion attacks that target application resource consumption.
The operational impact of CVE-1999-1199 was severe for organizations running affected Apache servers, as it could be exploited to create complete denial of service conditions. Attackers could easily overwhelm target servers by sending HTTP requests containing hundreds or thousands of duplicate headers, causing the server processes to consume all available memory and CPU resources. This resulted in complete service interruption for legitimate users, effectively taking websites offline until the server was manually restarted or the system was rebooted. The vulnerability was particularly dangerous in environments where Apache servers were used for critical business operations, as it could be exploited by malicious actors to disrupt services without requiring any specialized knowledge or tools beyond basic HTTP request construction. The attack vector was simple enough that even novice attackers could successfully exploit the vulnerability, making it a popular choice for service disruption attacks in the late 1990s and early 2000s.
Mitigation strategies for CVE-1999-1199 required immediate action from system administrators and security teams. The most effective solution was to upgrade to Apache HTTP Server version 1.3.2 or later, where the vulnerability was patched through improved header processing and resource management. Organizations should have implemented proper input validation and header limit enforcement on their servers, restricting the number of duplicate headers allowed in HTTP requests. Network-level protections such as rate limiting and request filtering could have provided additional defense-in-depth measures to prevent exploitation attempts. Security monitoring systems should have been configured to detect unusual patterns of header duplication that might indicate attempted exploitation. The vulnerability highlighted the importance of proper resource management in web server implementations and led to improved security practices in HTTP protocol handling across the industry. This vulnerability contributed significantly to the development of more robust HTTP server security standards and influenced the evolution of security frameworks such as those referenced in the MITRE ATT&CK matrix under the category of resource exhaustion attacks targeting web applications. The incident underscored the critical need for proper input sanitization and resource management in server-side applications, establishing best practices that continue to influence web security implementations today.