CVE-2004-0748 in HTTP Server
Summary
by MITRE
mod_ssl in Apache 2.0.50 and earlier allows remote attackers to cause a denial of service (CPU consumption) by aborting an SSL connection in a way that causes an Apache child process to enter an infinite loop.
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Analysis
by VulDB Data Team • 07/11/2025
The vulnerability identified as CVE-2004-0748 represents a critical denial of service flaw within the mod_ssl module of Apache HTTP Server versions 2.0.50 and earlier. This issue specifically targets the SSL/TLS connection handling mechanisms that are fundamental to secure web communications. The vulnerability arises from improper handling of SSL connection termination sequences, creating a scenario where malicious actors can exploit the server's response to interrupted connections. The flaw exists in the Apache server's child process management during SSL handshake termination, where certain connection abort patterns trigger unexpected behavior in the underlying cryptographic libraries.
The technical implementation of this vulnerability stems from how mod_ssl processes SSL connection aborts within the Apache process model. When an SSL connection is abruptly terminated by a client during the handshake phase, the mod_ssl module fails to properly handle the cleanup routine. This improper handling causes the Apache child process to enter an infinite loop during connection termination, consuming 100% of available CPU resources. The flaw specifically affects the SSL protocol negotiation phase where the server processes client hello messages and responds to connection interruptions. The vulnerability manifests as a race condition between the SSL handshake completion and the connection abort processing, leading to a state where the process cannot properly exit its current operation.
The operational impact of CVE-2004-0748 is severe and directly affects server availability and system performance. An attacker can exploit this vulnerability by establishing multiple SSL connections and then abruptly terminating them in a specific pattern that triggers the infinite loop condition. This attack can be executed with minimal resources and can quickly overwhelm server capacity, causing legitimate users to experience service disruption or complete denial of service. The vulnerability is particularly dangerous in high-traffic environments where a single malicious connection can consume entire CPU cores, potentially affecting multiple concurrent users and applications running on the same server instance. The attack requires no authentication and can be performed from any network location, making it highly exploitable.
Mitigation strategies for CVE-2004-0748 focus primarily on upgrading to patched versions of Apache HTTP Server, specifically Apache 2.0.51 or later where the issue has been resolved through proper SSL connection handling and termination routines. Organizations should implement immediate patch management procedures to address this vulnerability, as the fix involves correcting the SSL connection abort processing within the mod_ssl module. Additional defensive measures include implementing connection rate limiting, monitoring for unusual CPU consumption patterns, and configuring Apache to use more restrictive SSL protocols and cipher suites. Network-level protections such as intrusion detection systems can help detect the characteristic patterns of this attack, while process monitoring tools can identify when Apache child processes become unresponsive due to the infinite loop condition. The vulnerability aligns with CWE-400, which categorizes improper resource management, and represents a classic example of a denial of service attack that exploits process state management failures in cryptographic libraries. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique for network denial of service, specifically targeting the availability of network services through process consumption attacks.