CVE-1999-1100 in PIX Private Link
Summary
by MITRE
Cisco PIX Private Link 4.1.6 and earlier does not properly process certain commands in the configuration file, which reduces the effective key length of the DES key to 48 bits instead of 56 bits, which makes it easier for an attacker to find the proper key via a brute force attack.
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Analysis
by VulDB Data Team • 04/20/2026
The vulnerability identified as CVE-1999-1100 affects Cisco PIX Private Link versions 4.1.6 and earlier, representing a critical cryptographic weakness that significantly undermines the security of encrypted communications. This flaw resides within the configuration file processing mechanisms of the PIX firewall software, where specific commands are not properly handled during the key generation process. The vulnerability directly impacts the Data Encryption Standard implementation by reducing the effective key length from the standard 56 bits to only 48 bits, creating a substantial cryptographic weakness that dramatically increases the feasibility of successful brute force attacks against the encryption system.
The technical implementation of this vulnerability stems from improper handling of DES key generation within the PIX configuration parsing logic. When certain commands are processed in the configuration file, the system fails to properly utilize the full 56-bit key space that DES is designed to employ. Instead, the key derivation process truncates the effective key length to 48 bits, which reduces the computational complexity required for cryptanalysis. This reduction represents a fundamental flaw in the cryptographic implementation that aligns with CWE-327, which addresses the use of weak cryptographic algorithms and improper implementation of cryptographic functions. The vulnerability essentially creates a backdoor for attackers by reducing the search space for valid keys from 2^56 to 2^48 possibilities, making brute force attacks approximately 256 times more feasible.
The operational impact of this vulnerability extends beyond simple cryptographic weakness to compromise the integrity and confidentiality of all network communications protected by the affected PIX devices. Attackers can exploit this vulnerability to perform more efficient brute force attacks against DES-encrypted sessions, potentially gaining unauthorized access to sensitive network traffic and compromising the security posture of organizations relying on these firewalls. The reduced key length makes it significantly easier for adversaries to discover valid encryption keys through automated attacks, particularly when combined with modern computing resources and specialized cryptographic tools. This vulnerability directly violates the principles of secure cryptographic implementation and undermines the trust model that organizations place in their network security infrastructure.
Organizations should immediately implement mitigations including upgrading to Cisco PIX Private Link versions that address this vulnerability, as the affected versions no longer receive security updates or support. The recommended approach involves migrating to patched firmware releases that properly implement DES key generation according to industry standards and cryptographic best practices. Additionally, network administrators should consider implementing alternative encryption methods and protocols that do not rely on the vulnerable DES implementation, particularly for sensitive communications. Security monitoring should be enhanced to detect potential exploitation attempts, and organizations should conduct thorough security assessments of their network infrastructure to identify any other systems potentially affected by similar cryptographic weaknesses. The vulnerability demonstrates the critical importance of proper cryptographic implementation and highlights the need for comprehensive security testing of network security appliances before deployment in production environments.