CVE-1999-1164 is a vulnerability affecting Microsoft Outlook versions 97, 98, and 2000. The issue arises when a remote attacker sends multiple email messages containing identical X-UIDL headers to the targeted Outlook client. The X-UIDL header is used by email clients to uniquely identify messages on the server. When Outlook receives multiple emails with the same X-UIDL, it causes the client to hang, resulting in a denial of service (DoS) condition. This vulnerability does not compromise confidentiality or integrity but impacts availability by rendering the Outlook client unresponsive. The attack vector is network-based (remote), requires no authentication, and can be executed by simply sending crafted emails to the victim's mailbox. The CVSS score of 5.0 (medium severity) reflects the moderate impact and ease of exploitation without authentication. No patches or fixes are available due to the age of the affected software, and no known exploits have been reported in the wild. Given the affected versions are legacy and largely obsolete, the threat is primarily relevant in environments still running these outdated Outlook clients.

For European organizations, the impact of this vulnerability is limited but still relevant in legacy environments. Organizations that continue to use Microsoft Outlook 97, 98, or 2000—often in isolated or legacy systems—may experience service disruption if targeted by this attack. The denial of service could interrupt email communications, affecting business operations and productivity. While modern Outlook versions are not affected, some sectors with legacy dependencies, such as government agencies, financial institutions, or industrial control systems, might still run these older clients. The attack does not lead to data breaches or malware installation but could be used as a nuisance or to disrupt critical communications. Due to the lack of patches, mitigation relies on operational controls rather than software fixes.

1. Upgrade Outlook clients to supported, modern versions that are not vulnerable to this issue. 2. Implement email filtering at the gateway level to detect and block emails with duplicate or suspicious X-UIDL headers. 3. Employ network-level protections such as intrusion detection/prevention systems (IDS/IPS) to identify and block anomalous email traffic patterns. 4. Isolate legacy systems running vulnerable Outlook versions from the internet or untrusted networks to reduce exposure. 5. Educate users and administrators about the risks of using outdated software and encourage migration to supported platforms. 6. Regularly monitor email server logs for unusual patterns that may indicate attempts to exploit this vulnerability.