CVE-1999-0955 in wu-ftpd
Summary
by MITRE
Race condition in wu-ftpd and BSDI ftpd allows remote attackers to gain root access via the SITE EXEC command.
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Analysis
by VulDB Data Team • 04/17/2026
The vulnerability identified as CVE-1999-0955 represents a critical race condition flaw affecting wu-ftpd and BSDI ftpd implementations that enables remote attackers to escalate privileges to root access through the SITE EXEC command. This vulnerability exploits a timing window during command processing where the system fails to properly validate or sanitize input before executing privileged operations, creating an exploitable condition that can be leveraged by unauthorized users to gain system-level control.
The technical implementation of this race condition stems from improper synchronization mechanisms within the ftpd daemon's handling of the SITE EXEC command. When a remote user submits a malicious command sequence, the system enters a vulnerable state where temporary file operations or process execution contexts are not adequately secured against concurrent access. This flaw specifically affects the privilege escalation path during command execution, allowing attackers to manipulate the environment in which the SITE EXEC command operates, ultimately resulting in the execution of arbitrary code with root privileges. The vulnerability aligns with CWE-362, which describes race conditions that occur when multiple threads or processes access shared resources without proper synchronization, and falls under the ATT&CK technique T1068 for privilege escalation through local exploitation.
The operational impact of this vulnerability extends beyond simple unauthorized access, as it fundamentally compromises the integrity and confidentiality of affected systems. Remote attackers can leverage this condition to execute arbitrary commands with the highest system privileges, potentially leading to complete system compromise, data exfiltration, or establishment of persistent backdoors. The vulnerability affects systems running vulnerable versions of wu-ftpd and BSDI ftpd implementations, particularly those configured with default settings that enable the SITE EXEC functionality. Attackers typically exploit this by crafting specific command sequences that trigger the race condition during the processing of the SITE EXEC command, where the timing of file operations or process creation can be manipulated to achieve privilege escalation.
Mitigation strategies for this vulnerability require immediate patching of affected ftpd implementations, as the race condition cannot be effectively addressed through configuration changes alone. System administrators should implement comprehensive monitoring for suspicious FTP activity, particularly around the SITE EXEC command usage, and consider disabling unnecessary ftpd features that could expose similar race conditions. Network segmentation and firewall rules should restrict FTP service access to trusted networks only, while regular security audits should verify that no vulnerable ftpd versions remain operational. Additionally, organizations should implement proper privilege separation and ensure that all FTP services operate with minimal required privileges, following the principle of least privilege as outlined in security best practices. The vulnerability demonstrates the critical importance of proper synchronization mechanisms in multi-threaded applications and highlights the necessity of thorough security testing for race conditions in system services that handle privileged operations.