CVE-2004-0081 in OpenSSL
Summary
by MITRE
OpenSSL 0.9.6 before 0.9.6d does not properly handle unknown message types, which allows remote attackers to cause a denial of service (infinite loop), as demonstrated using the Codenomicon TLS Test Tool.
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Analysis
by VulDB Data Team • 01/22/2025
The vulnerability described in CVE-2004-0081 represents a critical flaw in OpenSSL version 0.9.6 before 0.9.6d that manifests through improper handling of unknown message types within the Transport Layer Security protocol implementation. This vulnerability specifically affects the TLS protocol processing logic where the software fails to correctly validate or reject unexpected message types that may be sent by malicious actors during the SSL/TLS handshake process. The flaw exists at the protocol parsing layer where the system does not adequately check message type identifiers against expected values, leading to a scenario where malformed or unexpected messages can cause the application to enter an infinite loop.
The technical execution of this vulnerability occurs when a remote attacker sends specially crafted TLS messages containing unknown or unexpected message type values to a vulnerable OpenSSL implementation. The OpenSSL library, in its attempt to process these messages, fails to properly validate the message type field and instead falls into a processing loop where it continuously attempts to handle the malformed message without proper termination conditions. This infinite loop consumes system resources and effectively renders the service unavailable to legitimate users, creating a classic denial of service condition. The vulnerability was demonstrated using the Codenomicon TLS Test Tool, which is specifically designed to test TLS implementations and can easily craft the appropriate malformed messages to trigger this behavior.
From an operational impact perspective, this vulnerability creates a severe availability risk for any system relying on OpenSSL 0.9.6 before 0.9.6d for secure communications. The infinite loop condition can cause processes to consume 100% CPU resources, leading to system instability and complete service disruption. Network services such as web servers, email servers, and other SSL/TLS enabled applications become vulnerable to this attack, potentially affecting thousands of users depending on the scope of the affected service. The vulnerability is particularly dangerous because it can be exploited remotely without requiring authentication or specialized privileges, making it an attractive target for automated attacks and DoS campaigns.
The root cause of this vulnerability aligns with CWE-129, which addresses improper validation of input boundaries, and relates to the broader category of protocol implementation flaws. This issue demonstrates poor input validation practices where the software assumes that all received messages will conform to expected formats without proper validation checks. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique related to network denial of service attacks and the T1595.001 technique for network infiltration through protocol manipulation. The vulnerability also reflects the importance of proper error handling and input sanitization in cryptographic libraries, as highlighted in various security standards and best practices for secure coding.
The recommended mitigation strategy involves immediate upgrading to OpenSSL version 0.9.6d or later, which contains the necessary patches to properly handle unknown message types and prevent the infinite loop condition. Organizations should also implement network-level protections such as firewall rules that can detect and block suspicious TLS message patterns, though this approach may not be comprehensive given the subtle nature of the attack. Additionally, monitoring systems should be configured to detect unusual CPU utilization patterns that might indicate exploitation attempts, and regular security audits should verify that all systems are running patched versions of OpenSSL. The vulnerability underscores the critical importance of keeping cryptographic libraries up to date and demonstrates how seemingly minor protocol handling flaws can lead to significant security implications.