CVE-2003-0512 in IOS
Summary
by MITRE
Cisco IOS 12.2 and earlier generates a "% Login invalid" message instead of prompting for a password when an invalid username is provided, which allows remote attackers to identify valid usernames on the system and conduct brute force password guessing, as reported for the Aironet Bridge.
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Analysis
by VulDB Data Team • 08/12/2025
This vulnerability in Cisco IOS version 12.2 and earlier represents a critical information disclosure flaw that fundamentally undermines authentication security mechanisms. The issue manifests when an attacker provides an invalid username to the system, causing it to respond with a generic "% Login invalid" message rather than prompting for a password authentication. This behavior creates a clear distinction between valid and invalid usernames, as the system provides different responses for each case. The vulnerability specifically affects Cisco Aironet Bridge implementations and demonstrates how improper error handling can expose system internals to malicious actors. From a cybersecurity perspective, this flaw directly violates the principle of least information disclosure, where systems should provide minimal information about their internal state to prevent attackers from gaining insights into system configuration.
The technical implementation of this vulnerability stems from the authentication protocol design within Cisco IOS, where the system fails to maintain consistent response behavior regardless of user credentials validity. When a valid username is entered, the system proceeds to prompt for a password, but when an invalid username is provided, it immediately terminates the authentication attempt with an error message. This inconsistent response pattern creates a timing and behavioral differential that attackers can exploit through systematic username enumeration. The flaw operates at the application layer of the network stack, specifically within the authentication subsystem that handles user login requests. According to CWE classification, this vulnerability maps to CWE-200, Information Exposure, as it provides attackers with information about valid usernames that should remain confidential. The vulnerability also aligns with ATT&CK technique T1078.004, Valid Accounts, as it enables adversaries to gather credential information through legitimate authentication pathways.
The operational impact of this vulnerability extends beyond simple username enumeration, as it provides attackers with a foundation for conducting successful brute force password guessing attacks. Once valid usernames are identified through this method, attackers can systematically attempt password guessing against these accounts, dramatically reducing the search space for successful credential compromise. The vulnerability affects network infrastructure components that rely on IOS authentication, particularly wireless access points and bridge devices that may be exposed to external network traffic. The exposure of valid usernames creates a significant risk for organizations that rely on strong authentication practices, as it undermines the effectiveness of password-based security controls. Network administrators face the challenge of identifying and remediating this issue across multiple devices, as the vulnerability exists in widely deployed IOS versions. The impact is particularly severe in environments where network devices are accessible from untrusted networks, as the information disclosure can occur without requiring any special privileges or advanced attack techniques.
Mitigation strategies for this vulnerability require both immediate and long-term approaches to address the fundamental authentication design flaw. The primary recommendation involves upgrading affected Cisco IOS versions to patches that implement consistent error handling for authentication attempts, ensuring that all username validation scenarios produce identical responses. Organizations should also implement account lockout mechanisms and rate limiting to prevent automated brute force attacks from succeeding even if valid usernames are identified. Network segmentation and access control measures can help reduce the attack surface by limiting external access to authentication points. The implementation of multi-factor authentication provides additional defense layers that can protect against credential compromise even when username information is exposed. Regular security audits should verify that authentication systems maintain consistent behavior and do not inadvertently disclose system information. Network administrators must also establish monitoring procedures to detect unusual authentication patterns that may indicate username enumeration attempts. Cisco recommends applying the appropriate security patches and following best practices for network device hardening as outlined in their security advisory documentation.