CVE-1999-1148 is a vulnerability affecting the FTP service component of Microsoft Internet Information Server (IIS) version 4.0 and earlier. The flaw allows remote attackers to cause a denial of service (DoS) condition by exhausting server resources through the creation of numerous simultaneous passive (PASV) FTP connections. The PASV mode in FTP is used to establish data connections where the server opens a port and waits for the client to connect. By opening many such connections concurrently, an attacker can overwhelm the server's capacity to handle new connections, leading to resource exhaustion and service unavailability. This vulnerability does not compromise confidentiality or integrity but impacts availability, potentially disrupting legitimate FTP services hosted on IIS servers. Exploitation requires no authentication and can be performed remotely over the network. The vulnerability was disclosed in 1999 and has a CVSS v2 base score of 5.0 (medium severity), reflecting its moderate impact and ease of exploitation. Microsoft has released patches addressing this issue, as documented in security bulletin MS98-006. There are no known exploits in the wild currently, but unpatched legacy systems remain at risk. Given the age of the vulnerability, it primarily affects outdated IIS installations that may still be operational in some environments.

For European organizations, the primary impact of this vulnerability is the potential disruption of FTP services hosted on IIS 4.0 or earlier versions. FTP is often used for file transfers in enterprise environments, including for legacy systems or internal applications. A successful DoS attack could interrupt business operations reliant on FTP, causing downtime and potential loss of productivity. While the vulnerability does not allow data theft or modification, the unavailability of FTP services could affect workflows, especially in sectors where legacy systems are still in use, such as manufacturing, logistics, or government agencies. Additionally, organizations with compliance requirements for service availability may face regulatory scrutiny if disruptions occur. The risk is mitigated in modern environments where IIS versions have been updated or replaced, but organizations running legacy infrastructure remain vulnerable. The absence of known active exploits reduces immediate risk, but the ease of exploitation and lack of authentication requirements mean that opportunistic attackers could still leverage this vulnerability if systems are unpatched.

European organizations should first identify any IIS 4.0 or earlier FTP services in their network environment, including legacy systems that may not be actively maintained. Immediate mitigation involves applying the official Microsoft patches referenced in security bulletin MS98-006 to remediate the vulnerability. If patching is not feasible due to system constraints, organizations should consider disabling the FTP service or restricting access to it via network segmentation and firewall rules, limiting exposure to trusted IP addresses only. Monitoring network traffic for unusual spikes in PASV FTP connections can help detect attempted exploitation. Additionally, migrating legacy FTP services to more modern, secure file transfer solutions or protocols (such as SFTP or FTPS) is recommended to reduce reliance on vulnerable legacy software. Regular vulnerability scanning and asset inventory updates will help ensure that outdated IIS versions are identified and remediated promptly. Implementing rate limiting on FTP connections at the network perimeter can also mitigate resource exhaustion attempts.