CVE-2018-1333 in HTTP Server
Summary
by MITRE
By specially crafting HTTP/2 requests, workers would be allocated 60 seconds longer than necessary, leading to worker exhaustion and a denial of service. Fixed in Apache HTTP Server 2.4.34 (Affected 2.4.18-2.4.30,2.4.33).
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Analysis
by VulDB Data Team • 04/29/2023
The vulnerability identified as CVE-2018-1333 represents a significant denial of service weakness within the Apache HTTP Server implementation of the HTTP/2 protocol. This flaw specifically affects versions between 2.4.18 and 2.4.30, as well as version 2.4.33, creating a condition where worker processes receive excessive resource allocation time. The core issue manifests when specially crafted HTTP/2 requests are processed by the server, causing the worker threads to remain active for an additional 60 seconds beyond their normal lifecycle. This behavior directly impacts the server's resource management and operational capacity, as worker threads are essential components responsible for handling incoming client requests and maintaining server responsiveness.
The technical mechanism behind this vulnerability involves the HTTP/2 protocol's handling of stream management and resource allocation within the Apache server's worker model. When processing malformed or specially constructed HTTP/2 requests, the server's internal state management fails to properly terminate worker threads in a timely manner. This results in a gradual depletion of available worker resources, as each affected request causes a worker to remain occupied for an extra minute. The vulnerability operates at the application layer and specifically targets the server's concurrent processing capabilities, where worker threads are designed to handle individual client connections efficiently. The 60-second delay represents a significant resource waste that accumulates with each vulnerable request, ultimately leading to complete worker exhaustion.
The operational impact of CVE-2018-1333 extends beyond simple service disruption to create a cascading failure scenario within the Apache HTTP Server environment. As worker threads become unavailable due to the extended allocation period, legitimate client requests begin to queue or fail entirely, resulting in degraded service availability and potential complete denial of service for the affected server. This vulnerability is particularly dangerous in high-traffic environments where HTTP/2 connections are prevalent, as the resource exhaustion can occur rapidly with multiple concurrent attacks. The impact is amplified by the fact that this affects the core worker model that Apache relies upon for handling concurrent connections, making it a critical weakness that can bring entire web services to their knees under sustained attack conditions.
This vulnerability aligns with CWE-400, which addresses unchecked resource allocation, and demonstrates characteristics consistent with the ATT&CK technique T1499.004 for network denial of service attacks. The flaw represents a classic resource exhaustion attack vector where attacker-controlled input causes the server to allocate resources inefficiently, leading to service unavailability. Organizations affected by this vulnerability should immediately upgrade to Apache HTTP Server version 2.4.34 or later, which contains the necessary patches to properly handle HTTP/2 request processing and prevent the excessive worker allocation behavior. Additionally, implementing rate limiting mechanisms and monitoring for unusual HTTP/2 traffic patterns can provide additional defense in depth. The vulnerability highlights the importance of proper resource management in protocol implementations and serves as a reminder that even seemingly minor protocol handling issues can result in significant security implications.