CVE-1999-0682 is a vulnerability found in Microsoft Exchange Server 5.5, an email server product widely used in the late 1990s and early 2000s. The vulnerability allows a remote attacker to relay email messages through the server by exploiting the way Exchange 5.5 handles encapsulated SMTP addresses. Specifically, even if anti-relaying features are enabled, the server can be tricked into forwarding emails from unauthorized sources. This effectively turns the vulnerable Exchange server into an open relay, which attackers can abuse to send spam or malicious emails without authentication. The vulnerability does not directly compromise confidentiality or integrity of the server or its data but impacts availability and reputation by enabling spam relay. The CVSS score of 5.0 (medium severity) reflects that the attack vector is network-based, requires no authentication, and is easy to exploit, but the impact is limited to availability (email relay abuse) without direct data compromise. A patch addressing this issue was released by Microsoft in 1999 (MS99-027), which should be applied to remediate the vulnerability. No known exploits in the wild have been reported recently, likely due to the age of the product and its declining usage.

For European organizations, the primary impact of this vulnerability lies in the potential misuse of their Exchange 5.5 servers as spam relays. This can lead to blacklisting of the organization's email domains and IP addresses, damaging their email reputation and causing legitimate emails to be blocked or filtered as spam. Additionally, the abuse of the mail server for spam distribution can consume network and server resources, potentially degrading email service availability. While the vulnerability does not allow direct data theft or system compromise, the reputational damage and operational disruption can be significant, especially for organizations relying heavily on email communications. Given that Exchange 5.5 is an outdated product, most European organizations should have migrated to newer versions or alternative platforms; however, legacy systems may still exist in some environments, particularly in sectors with slower IT modernization cycles.

Organizations should ensure that Microsoft Exchange Server 5.5 is either fully patched with the MS99-027 security update or, preferably, replaced with a supported and updated email server platform. Specific mitigation steps include: 1) Applying the official Microsoft patch to fix the SMTP relay vulnerability. 2) Disabling or restricting SMTP relay permissions to only trusted IP addresses or authenticated users. 3) Implementing network-level controls such as firewalls or SMTP gateways to prevent unauthorized relay attempts. 4) Monitoring mail server logs for unusual relay activity or spikes in outbound email traffic. 5) Considering migration away from Exchange 5.5 to modern, supported email solutions that include enhanced security features and ongoing vendor support. 6) Employing anti-spam and email reputation services to detect and mitigate abuse if legacy systems remain in use.