CVE-2009-0033 in Tomcat
Summary
by MITRE
Apache Tomcat 4.1.0 through 4.1.39, 5.5.0 through 5.5.27, and 6.0.0 through 6.0.18, when the Java AJP connector and mod_jk load balancing are used, allows remote attackers to cause a denial of service (application outage) via a crafted request with invalid headers, related to temporary blocking of connectors that have encountered errors, as demonstrated by an error involving a malformed HTTP Host header.
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Analysis
by VulDB Data Team • 09/06/2019
The vulnerability described in CVE-2009-0033 represents a critical denial of service flaw affecting multiple versions of the Apache Tomcat web server software. This issue specifically impacts versions ranging from 4.1.0 through 4.1.39, 5.5.0 through 5.5.27, and 6.0.0 through 6.0.18 when configured with the Java AJP connector and mod_jk load balancing components. The vulnerability stems from improper handling of malformed HTTP headers, particularly the Host header, which creates a condition where the application becomes temporarily unavailable to legitimate users. This flaw operates through a mechanism where the Tomcat connector enters a temporary blocking state after encountering error conditions, effectively preventing further request processing until the blocking period expires or the service is manually restarted.
The technical implementation of this vulnerability involves the interaction between the AJP connector and the mod_jk load balancer module within the Apache Tomcat architecture. When a maliciously crafted request containing invalid headers is processed, the AJP connector fails to properly validate the incoming data and instead enters a temporary error state. This error state causes the connector to block subsequent requests for a predetermined period, leading to application unavailability. The specific trigger involves malformed HTTP Host headers that cause the connector to malfunction, resulting in the temporary blocking mechanism being activated. This behavior represents a fundamental flaw in the error handling and resource management logic of the Tomcat connector component, as outlined in the CWE-400 category for unspecified denial of service conditions.
The operational impact of this vulnerability extends beyond simple service disruption to create significant business continuity risks for organizations relying on Apache Tomcat deployments. Attackers can exploit this vulnerability to create sustained denial of service conditions that may last for extended periods, depending on the specific blocking configuration. The vulnerability affects the core functionality of the web application server, making it unavailable to legitimate users and potentially causing revenue loss for e-commerce and other internet-facing applications. The attack vector is particularly concerning because it can be executed remotely without requiring authentication or special privileges, making it accessible to any attacker with network access to the affected system. This vulnerability directly maps to ATT&CK technique T1499.004 for network denial of service, which targets the availability of network services through various mechanisms including protocol manipulation.
Organizations affected by this vulnerability should implement immediate mitigations including upgrading to patched versions of Apache Tomcat, disabling the AJP connector when not required, or implementing network-level protections to filter malformed requests. The recommended approach involves applying the official security patches released by the Apache Software Foundation, which address the underlying error handling mechanisms that cause the temporary blocking behavior. Additionally, administrators should consider implementing rate limiting and request validation at the network perimeter to prevent exploitation of this vulnerability. The fix typically involves modifications to how the AJP connector processes malformed headers, ensuring that error conditions do not result in permanent or extended blocking states. Organizations should also conduct thorough testing of their security configurations to verify that the mitigation strategies effectively prevent the vulnerability from being exploited in their specific deployment environments, as the exact blocking behavior may vary depending on the configuration and deployment topology.