CVE-1999-0129 is a vulnerability in various versions of the Sendmail mail transfer agent, a widely used software for routing and delivering email on Unix-like systems. This vulnerability allows local users to write to arbitrary files and escalate their privileges by exploiting the handling of .forward or :include: files. Specifically, Sendmail processes these files to determine email forwarding instructions. An attacker with local access can craft a malicious .forward or :include: file to write data to files they should not have permission to modify. This can lead to unauthorized modification of files and gaining group-level permissions, potentially allowing privilege escalation within the system. The vulnerability affects a broad range of Sendmail versions, including legacy releases from 1.0 through 10.20, and various intermediate versions such as 4.x, 5.x, and 8.x branches. The CVSS score assigned is 4.6 (medium severity), reflecting that exploitation requires local access (AV:L), low attack complexity (AC:L), no authentication (Au:N), and impacts confidentiality, integrity, and availability to a partial degree (C:P/I:P/A:P). There is no patch available for this vulnerability, and no known exploits in the wild have been reported. Given the age of this vulnerability (published in 1996), it primarily affects legacy systems that still run these older Sendmail versions. The core risk lies in local users being able to escalate privileges by manipulating mail forwarding files, which could compromise system security and stability.

For European organizations, the impact of this vulnerability depends largely on the presence of legacy Unix or Linux systems running vulnerable Sendmail versions. If such systems are used, especially in critical infrastructure, government, or enterprise environments, local attackers or insiders could exploit this flaw to gain elevated group permissions, potentially leading to unauthorized access to sensitive data, disruption of mail services, or further lateral movement within the network. This could compromise confidentiality and integrity of communications and data. Although remote exploitation is not possible, insider threats or attackers with initial local access could leverage this vulnerability to deepen their foothold. The absence of a patch means organizations must rely on alternative mitigations or system upgrades. Given the age of the vulnerability, modern systems are unlikely to be affected, but legacy systems in use within European organizations, particularly in sectors with long system lifecycles like manufacturing, utilities, or government, remain at risk.

Since no patch is available for CVE-1999-0129, organizations should prioritize upgrading or replacing affected Sendmail versions with modern, supported mail transfer agents that have addressed this vulnerability. If upgrading is not immediately feasible, strict access controls should be enforced to limit local user permissions and prevent unauthorized creation or modification of .forward and :include: files. Monitoring and auditing of mail forwarding files and directories can help detect suspicious changes. Additionally, organizations should implement strong user account management and minimize the number of users with local system access. Employing host-based intrusion detection systems (HIDS) to monitor file integrity and unusual activities related to Sendmail processes can provide early warning of exploitation attempts. Segmentation of critical systems and limiting local access to trusted personnel further reduces risk. Finally, organizations should consider migrating to alternative mail solutions with active security support and regularly review legacy system usage to phase out vulnerable software.