CVE-1999-0103 in Echo
Summary
by MITRE
Echo and chargen, or other combinations of UDP services, can be used in tandem to flood the server, a.k.a. UDP bomb or UDP packet storm.
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Analysis
by VulDB Data Team • 04/16/2026
The vulnerability described in CVE-1999-0103 represents a fundamental weakness in network service implementations that enables attackers to exploit UDP-based services for denial-of-service attacks. This vulnerability specifically targets the echo and chargen services, which are part of the Internet Protocol suite and designed to provide simple network testing and debugging capabilities. The echo service responds to UDP packets by echoing them back to the sender, while the chargen service generates and sends character streams to clients. When these services are improperly configured or left running on network systems without adequate security controls, they become prime targets for exploitation.
The technical flaw exploited in this vulnerability stems from the inherent design characteristics of UDP services that do not require connection establishment or authentication. The echo service operates by simply echoing back any data it receives, while chargen generates continuous character streams without requiring any specific client interaction. This design creates a perfect storm for attackers who can leverage these services to create massive network traffic floods. The vulnerability is particularly dangerous because it can be exploited using simple network tools to send small UDP packets that trigger massive responses from vulnerable servers, effectively creating a traffic amplification attack where the response traffic far exceeds the initial packet size.
The operational impact of this vulnerability extends beyond simple service disruption to encompass significant network infrastructure strain and potential system compromise. When attackers utilize the echo and chargen services in conjunction with other UDP-based network services, they can generate what is commonly referred to as a UDP bomb or UDP packet storm. This attack pattern can overwhelm network bandwidth, consume system resources, and potentially cause complete service outages. The vulnerability affects not only individual servers but can also impact entire network segments and even broader internet infrastructure. The attack vectors typically involve sending UDP packets to multiple vulnerable servers simultaneously, creating a distributed denial-of-service scenario that can be difficult to mitigate and trace back to the source.
Network administrators and security professionals should consider implementing comprehensive mitigation strategies that align with industry best practices and standards such as those outlined in the Common Weakness Enumeration (CWE) classification system. This vulnerability falls under CWE-116, which addresses improper encoding or escaping of output, and more specifically relates to CWE-400, which covers uncontrolled resource consumption. The mitigation approaches should include implementing proper firewall rules to block UDP traffic on ports 7 and 19, disabling unnecessary UDP services on network systems, and configuring access control lists to restrict service access. Additionally, the mitigation strategies should align with the MITRE ATT&CK framework's network denial of service tactics, particularly those involving the exploitation of network protocols and services to consume system resources. Organizations should also consider implementing rate limiting mechanisms, traffic monitoring systems, and intrusion detection/prevention systems to detect and block such attacks before they can cause significant damage. Regular vulnerability assessments and network security audits are essential to identify and remediate similar weaknesses in network infrastructure and services.