CVE-2004-0113 in HTTP Server
Summary
by MITRE
Memory leak in ssl_engine_io.c for mod_ssl in Apache 2 before 2.0.49 allows remote attackers to cause a denial of service (memory consumption) via plain HTTP requests to the SSL port of an SSL-enabled server.
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Analysis
by VulDB Data Team • 07/13/2025
The vulnerability described in CVE-2004-0113 represents a critical memory management flaw within the Apache HTTP Server's mod_ssl module, specifically in the ssl_engine_io.c component. This issue affects Apache 2.0 versions prior to 2.0.49 and demonstrates how improper resource handling in cryptographic modules can lead to significant security implications. The flaw manifests when the server processes plain HTTP requests directed toward an SSL-enabled port, creating a scenario where memory allocation occurs without proper subsequent deallocation. This memory leak occurs because the mod_ssl module fails to properly clean up allocated memory structures when handling non-SSL requests on SSL ports, leading to progressive memory consumption over time.
The technical implementation of this vulnerability involves the ssl_engine_io.c file's handling of input/output operations within the SSL context. When a plain HTTP request is received on an SSL port, the module attempts to process the request through the SSL engine, but due to insufficient error handling and resource cleanup mechanisms, allocated memory buffers and context structures remain in memory. This behavior creates a memory leak that accumulates with each incoming request, eventually exhausting available system memory. The vulnerability is particularly concerning because it operates at the protocol level where SSL and HTTP protocols intersect, making it difficult to distinguish between legitimate SSL and non-SSL traffic patterns. The flaw can be categorized under CWE-401 as a weakness related to improper handling of memory allocation and deallocation, specifically manifesting as a resource leak that can be exploited to cause denial of service conditions.
The operational impact of this vulnerability extends beyond simple resource exhaustion, as it creates a persistent threat to server availability and system stability. Attackers can repeatedly send plain HTTP requests to SSL ports, causing progressive memory consumption that may eventually lead to complete system crashes or service unavailability. This type of denial of service attack is particularly effective because it requires minimal resources to execute and can be automated to cause sustained degradation of service. The vulnerability affects any Apache 2.0 server configured with SSL enabled, regardless of the actual SSL configuration or certificate used, making it a widespread concern across web server deployments. From an attack perspective, this vulnerability aligns with ATT&CK technique T1499.004, which describes resource exhaustion attacks targeting availability through memory consumption.
The exploitation of CVE-2004-0113 requires minimal technical sophistication, as attackers only need to send HTTP requests to SSL ports, making it accessible to threat actors with basic network connectivity. The memory leak occurs in a predictable pattern, with each request contributing to cumulative memory consumption that grows linearly over time. Organizations running affected Apache versions are particularly vulnerable during high-traffic periods when the memory exhaustion effect compounds rapidly. The vulnerability also highlights the importance of proper input validation and resource management in cryptographic libraries, as the issue stems from inadequate handling of protocol mismatches between HTTP and SSL layers. This flaw underscores the need for comprehensive security testing of SSL/TLS implementations and proper memory management practices in web server components. System administrators should consider implementing rate limiting and monitoring for unusual memory consumption patterns on SSL ports, as well as applying the necessary patches to address the memory leak in mod_ssl. The vulnerability serves as a reminder of how seemingly minor resource management issues in cryptographic modules can have significant operational impacts on server availability and overall system security posture.