CVE-1999-0453 in Cisco
Summary
by MITRE
An attacker can identify a CISCO device by sending a SYN packet to port 1999, which is for the Cisco Discovery Protocol (CDP).
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Analysis
by VulDB Data Team • 04/17/2026
The vulnerability described in CVE-1999-0453 represents a significant information disclosure issue within Cisco networking equipment that persists even decades after its initial identification. This flaw specifically targets the Cisco Discovery Protocol implementation running on port 1999, which serves as the designated communication channel for CDP traffic. The vulnerability exploits a fundamental design weakness where the protocol does not adequately protect against reconnaissance attempts, allowing unauthorized parties to determine the presence of Cisco devices within a network topology simply by sending a single SYN packet to the well-known port 1999. This behavior directly violates the principle of least information disclosure and creates a trivial attack vector for network reconnaissance activities.
The technical nature of this vulnerability stems from the implementation of the Cisco Discovery Protocol itself, which operates at the data link layer and is designed to provide information about neighboring Cisco devices. When a SYN packet is sent to port 1999, the device responds with a CDP advertisement packet that contains detailed information about the device including its model, software version, capabilities, and other identifying characteristics. This response occurs without any authentication or access control mechanisms, making it extremely easy for attackers to enumerate Cisco devices within their network. The vulnerability is classified as a weakness in protocol implementation and can be categorized under CWE-200, which deals with information exposure, and more specifically under CWE-444, which addresses improper handling of CDP packets. From an attack perspective, this vulnerability aligns with techniques described in the MITRE ATT&CK framework under the reconnaissance phase, particularly T1590 for reconnaissance using network scanning and T1592 for reconnaissance using network service scanning.
The operational impact of this vulnerability extends far beyond simple information disclosure, as it provides attackers with critical intelligence for subsequent exploitation phases. Once an attacker identifies the presence of Cisco devices, they can leverage the disclosed information to tailor more sophisticated attacks targeting specific device models, software versions, or known vulnerabilities within those devices. The vulnerability essentially acts as a free reconnaissance tool for threat actors, eliminating the need for more complex scanning techniques to identify Cisco infrastructure. This information can be particularly valuable for attackers planning targeted attacks against specific device types, as different Cisco products may have varying levels of security and different sets of vulnerabilities. The exposure of device information also enables attackers to conduct more effective social engineering campaigns, as they can use the disclosed information to craft more convincing phishing or impersonation attacks.
Mitigation strategies for CVE-1999-0453 should address both the immediate exposure and the underlying protocol design issues. Network administrators should implement firewall rules to block incoming traffic on port 1999 from unauthorized sources, effectively preventing the CDP protocol from responding to external reconnaissance attempts. However, this approach requires careful network planning as CDP is often essential for internal network management and device discovery. The recommended solution involves configuring Cisco devices to disable CDP when it is not required for network operations, or implementing access control lists that restrict CDP responses to trusted network segments only. Organizations should also consider implementing network segmentation to isolate critical infrastructure from less secure network zones where external scanning might occur. Additionally, regular network monitoring should be implemented to detect unusual patterns of CDP traffic that might indicate reconnaissance activities. From a compliance perspective, this vulnerability highlights the importance of following security best practices outlined in standards such as NIST SP 800-53 and ISO 27001, which emphasize the need for proper network access controls and information protection measures. The vulnerability also underscores the critical importance of regularly updating network device configurations and implementing defense-in-depth strategies that reduce the attack surface available to potential adversaries.