CVE-2003-1230 in FreeBSD
Summary
by MITRE
The implementation of SYN cookies (syncookies) in FreeBSD 4.5 through 5.0-RELEASE-p3 uses only 32-bit internal keys when generating syncookies, which makes it easier for remote attackers to conduct brute force ISN guessing attacks and spoof legitimate traffic.
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Analysis
by VulDB Data Team • 09/02/2019
The vulnerability described in CVE-2003-1230 represents a critical weakness in the network stack implementation of FreeBSD operating systems version 4.5 through 5.0-RELEASE-p3. This issue specifically targets the SYN cookies mechanism, which serves as a fundamental security feature designed to protect against SYN flood denial of service attacks by implementing a cryptographic technique that allows the server to maintain stateless connections. The SYN cookies mechanism works by generating a unique sequence number for each connection attempt using a secret key and connection parameters, enabling the server to verify legitimate connection requests without storing connection state information in memory. However, the implementation in these FreeBSD versions suffers from a severe cryptographic weakness that fundamentally undermines its security effectiveness.
The technical flaw lies in the 32-bit internal key size used within the SYN cookies implementation, which significantly reduces the entropy available for generating secure sequence numbers. This limited key space creates a vulnerable attack surface that allows remote adversaries to perform brute force guessing attacks against the sequence number generation algorithm. With only 32 bits of key space, the cryptographic strength becomes insufficient to resist systematic guessing attempts, making it feasible for attackers to predict or enumerate valid sequence numbers used in legitimate connection establishment. The vulnerability stems from the fact that modern cryptographic requirements demand much larger key sizes to maintain security against computational attacks, and a 32-bit key provides only 2^32 possible combinations, which is easily exhaustible through brute force methods. This weakness directly violates the principles of secure cryptographic implementation as outlined in industry standards such as NIST SP 800-57 and CWE-326, which emphasize the importance of adequate key lengths for cryptographic functions.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to encompass more sophisticated attack vectors that can compromise connection integrity and potentially enable man-in-the-middle attacks. Attackers can leverage the predictable nature of the 32-bit key generation to forge legitimate-looking connection requests, allowing them to bypass the SYN cookies protection mechanism entirely. This creates opportunities for attackers to spoof legitimate traffic, impersonate legitimate users, or establish unauthorized connections to services protected by the vulnerable FreeBSD implementation. The attack surface includes any network service running on affected FreeBSD systems that relies on TCP connection establishment, potentially affecting web servers, mail servers, database servers, and other critical infrastructure components. The vulnerability represents a classic case of insufficient entropy in cryptographic implementations, aligning with ATT&CK technique T1071.004 for application layer protocol manipulation and potentially enabling credential theft or session hijacking attacks.
The recommended mitigation strategies for this vulnerability involve immediate system upgrades to FreeBSD versions that address the cryptographic weakness in the SYN cookies implementation. Administrators should upgrade to FreeBSD 5.0-RELEASE-p4 or later versions where the implementation was corrected to use stronger cryptographic keys. Additionally, system administrators can implement alternative security measures such as increasing the TCP SYN backlog, implementing rate limiting for connection attempts, and deploying intrusion detection systems to monitor for suspicious connection patterns. The fix typically involves expanding the internal key size used in the SYN cookies generation algorithm to provide sufficient entropy for cryptographic security, aligning with industry best practices for cryptographic key management. Organizations should also consider implementing network-level protections such as TCP SYN cookies enforcement at the network level, firewall rules to limit connection attempts from suspicious sources, and monitoring for unusual connection patterns that may indicate active exploitation attempts. This vulnerability highlights the critical importance of proper cryptographic implementation and the necessity of adhering to established security standards and guidelines for network security mechanisms.