CVE-2016-1546 in HTTP Server
Summary
by MITRE
The Apache HTTP Server 2.4.17 and 2.4.18, when mod_http2 is enabled, does not limit the number of simultaneous stream workers for a single HTTP/2 connection, which allows remote attackers to cause a denial of service (stream-processing outage) via modified flow-control windows.
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Analysis
by VulDB Data Team • 10/14/2024
The vulnerability identified as CVE-2016-1546 represents a critical denial of service flaw in the Apache HTTP Server version 2.4.17 and 2.4.18 when the mod_http2 module is enabled. This issue stems from inadequate resource management within the HTTP/2 implementation, specifically concerning the handling of stream workers that process concurrent requests within a single connection. The flaw allows remote attackers to manipulate flow-control windows in a manner that triggers excessive resource consumption, ultimately leading to service disruption.
The technical root cause of this vulnerability lies in the improper limitation of simultaneous stream workers per HTTP/2 connection. In HTTP/2 protocol implementation, flow-control windows regulate how much data can be transmitted before acknowledgment is required. When mod_http2 fails to enforce adequate limits on concurrent stream processing, malicious actors can exploit this by sending crafted requests that manipulate these flow-control parameters. The vulnerability specifically targets the connection-level resource management where multiple streams can be processed simultaneously, creating a scenario where an attacker can exhaust available worker threads or memory resources.
This vulnerability operates within the context of the HTTP/2 protocol's inherent complexity and the Apache server's implementation of the mod_http2 module. The flaw enables attackers to cause a stream-processing outage by modifying flow-control windows in ways that overwhelm the server's capacity to handle concurrent streams. The impact manifests as a denial of service condition where legitimate users cannot access the service due to resource exhaustion. The vulnerability is particularly dangerous because HTTP/2 connections can support multiple concurrent streams, and without proper limits, a single malicious connection can consume resources that should be available for legitimate traffic.
From a cybersecurity perspective, this vulnerability aligns with CWE-400, which addresses Uncontrolled Resource Consumption, and maps to ATT&CK technique T1499.004 for Network Denial of Service. The exploitability of this flaw demonstrates the importance of proper resource management in protocol implementations and highlights how seemingly minor implementation gaps can lead to significant service disruption. The vulnerability affects systems where Apache HTTP Server is configured to support HTTP/2 protocol, particularly in environments where the mod_http2 module is enabled for handling modern web traffic.
The operational impact of CVE-2016-1546 extends beyond simple service unavailability to potentially affect business continuity and customer experience. Organizations relying on Apache servers with HTTP/2 support face the risk of sustained denial of service attacks that can be difficult to distinguish from legitimate traffic patterns. The vulnerability requires specific conditions to be exploited, namely the presence of mod_http2 module and HTTP/2 protocol support, but once triggered, it can effectively render the affected service unavailable to legitimate users. Mitigation strategies include updating to patched versions of Apache HTTP Server, implementing connection and stream limiting configurations, and deploying network-level protections to detect and block anomalous flow-control window modifications. The vulnerability underscores the critical need for thorough security testing of protocol implementations and proper resource management in web server configurations.