CVE-1999-0193 in 3com
Summary
by MITRE
Denial of service in Ascend and 3com routers, which can be rebooted by sending a zero length TCP option.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 04/17/2026
The vulnerability described in CVE-1999-0193 represents a significant denial of service weakness affecting Ascend and 3Com network routers from the late 1990s era. This issue stems from improper handling of TCP packet options within the router's network processing stack, specifically when encountering packets containing zero-length TCP options. The flaw exists in the router's TCP/IP implementation where the device fails to properly validate incoming TCP options before processing them, creating an exploitable condition that allows remote attackers to trigger system instability. This vulnerability operates at the network protocol level and demonstrates a classic example of inadequate input validation in network infrastructure devices.
The technical mechanism behind this vulnerability involves the router's failure to properly handle TCP options with zero length fields during the packet processing lifecycle. When a malicious actor sends a TCP packet containing a zero-length option, the router's TCP processing module attempts to parse and handle this malformed option without proper bounds checking or validation. This parsing error causes the router to enter an unstable state where it either crashes or reboots entirely, effectively rendering the network device unavailable to legitimate users. The vulnerability is particularly dangerous because it can be exploited remotely without requiring authentication or special privileges, making it an attractive target for attackers seeking to disrupt network services.
From an operational impact perspective, this vulnerability creates severe service disruption for organizations relying on affected routers, as the reboot process can take several minutes to complete and may result in temporary network outages. The attack vector is straightforward and can be executed using standard network tools, making it accessible to attackers with minimal technical expertise. Network administrators face the challenge of maintaining service availability while dealing with the unpredictable nature of such attacks, as the router may reboot at any time when processing malformed TCP packets. This vulnerability essentially allows attackers to perform a simple but effective denial of service attack that can severely impact business continuity and network reliability.
The vulnerability aligns with CWE-129, which addresses improper validation of length fields, and demonstrates the importance of robust input validation in network protocol implementations. From an ATT&CK framework perspective, this represents a network denial of service technique that falls under the category of service disruption, potentially enabling further attacks by creating network instability that attackers can exploit to gain additional access or cause more severe damage. Organizations should implement network segmentation and access controls to limit exposure, while also ensuring that network infrastructure devices receive timely security updates and patches. The incident highlights the critical need for proper protocol validation in embedded network devices and emphasizes the importance of thorough testing of network protocol implementations before deployment in production environments.
This vulnerability serves as a historical example of how seemingly minor implementation flaws in network protocol handling can result in significant security impacts, particularly in critical infrastructure devices. The lack of proper bounds checking and input validation in the TCP option processing module created a condition where malformed packets could trigger system crashes, demonstrating the importance of defensive programming practices in network security implementations. Modern network security practices emphasize the need for comprehensive protocol validation and robust error handling to prevent similar issues from occurring in contemporary network infrastructure deployments.