CVE-2018-17189 in HTTP Server
Summary
by MITRE
In Apache HTTP server versions 2.4.37 and prior, by sending request bodies in a slow loris way to plain resources, the h2 stream for that request unnecessarily occupied a server thread cleaning up that incoming data. This affects only HTTP/2 (mod_http2) connections.
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Analysis
by VulDB Data Team • 07/04/2023
The vulnerability described in CVE-2018-17189 represents a significant denial of service weakness within the Apache HTTP Server software family, specifically impacting versions up to and including 2.4.37. This flaw manifests through a sophisticated attack vector that exploits the HTTP/2 protocol implementation known as mod_http2. The issue occurs when malicious actors send request bodies using a slow loris technique against plain HTTP resources, which causes the server to maintain unnecessary thread occupation during the cleanup process of incoming data. This behavior creates a resource exhaustion scenario that severely impacts server performance and availability. The vulnerability operates at the application layer and specifically targets the HTTP/2 processing capabilities of the server, making it particularly dangerous for high-traffic web environments where resource management is critical.
The technical root cause of this vulnerability lies in how the mod_http2 module handles incoming request data streams during the cleanup phase of HTTP/2 connections. When a slow loris attack is initiated against plain resources, the server's thread management system becomes inefficiently occupied with cleaning up the malformed or deliberately slow data streams. This creates a scenario where legitimate requests cannot be processed effectively due to thread exhaustion, as the server allocates resources to manage the cleanup of these artificially prolonged connections. The flaw demonstrates poor resource management practices within the HTTP/2 implementation, where the server fails to properly terminate or abandon these resource-intensive cleanup operations in a timely manner. This behavior is classified under CWE-400 as an Uncontrolled Resource Consumption vulnerability, specifically manifesting as a DoS condition that affects system availability.
The operational impact of this vulnerability extends beyond simple service disruption to encompass significant performance degradation and resource waste within affected Apache servers. When exploited, the vulnerability can cause servers to become unresponsive to legitimate requests, effectively rendering them unavailable to users. The resource consumption patterns are particularly concerning because they affect server threads that are critical for handling concurrent connections, leading to cascading failures that can impact entire web applications. Attackers can maintain these resource-consuming connections for extended periods without requiring substantial bandwidth, making the attack particularly efficient and difficult to detect. The vulnerability affects only HTTP/2 connections, which means that servers using HTTP/1.1 or other protocols remain unaffected, but those with mod_http2 enabled are at risk. This creates a scenario where organizations must either disable HTTP/2 support or apply immediate patches to prevent exploitation, as the attack does not require authentication or special privileges to be effective.
Mitigation strategies for CVE-2018-17189 focus primarily on updating to patched versions of the Apache HTTP Server, specifically versions 2.4.38 and later, which contain fixes addressing the thread management issues in mod_http2. Organizations should also implement connection rate limiting and request timeout configurations to reduce the effectiveness of slow loris attacks against their servers. Network-level protections such as load balancer configurations and firewall rules can help detect and block suspicious connection patterns that resemble slow loris attacks. Additionally, monitoring systems should be configured to track thread utilization and connection counts to identify potential exploitation attempts. The vulnerability's characteristics align with ATT&CK technique T1499.001, which involves resource exhaustion attacks targeting availability. Implementing proper HTTP/2 configuration parameters and limiting the number of concurrent streams can further reduce the attack surface. Organizations should also consider implementing intrusion detection systems that can identify patterns consistent with slow loris attacks and automatically trigger defensive measures to protect their Apache server infrastructure from exploitation.