CVE-2006-5778 in linux-ftpd
Summary
by MITRE
ftpd in linux-ftpd 0.17, and possibly other versions, performs a chdir before setting the UID, which allows local users to bypass intended access restrictions by redirecting their home directory to a restricted directory.
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Analysis
by VulDB Data Team • 04/26/2026
The vulnerability described in CVE-2006-5778 affects the linux-ftpd 0.17 implementation and potentially other versions of the ftpd daemon. This flaw represents a classic privilege escalation issue that exploits the order of operations during user authentication and directory handling. The core problem lies in the sequence of system calls executed by the ftpd service when processing user connections, specifically the improper timing of directory change operations relative to user identity switching.
The technical implementation of this vulnerability stems from the ftpd service executing a chdir system call before performing the necessary uid switching operations. This sequence creates a window of opportunity where local users can manipulate their home directory paths to access restricted areas of the filesystem. When the service changes to a user's home directory before setting the appropriate user privileges, any symbolic links or directory redirects that the user controls can be exploited to gain access to areas of the filesystem that should normally be restricted. This represents a fundamental flaw in the service's privilege management architecture and violates the principle of least privilege.
The operational impact of this vulnerability extends beyond simple access restriction bypass to potentially enable unauthorized file access and modification across the entire filesystem. Attackers can leverage this weakness to read sensitive files, modify system configurations, or even escalate their privileges to root access depending on the system configuration and the specific directories involved. The vulnerability affects systems where the ftpd service runs with elevated privileges and where users have the ability to control their home directory paths through symbolic links or other means of directory redirection. This type of vulnerability is particularly dangerous in multi-user environments where different users may have varying levels of access rights and system privileges.
From a cybersecurity perspective, this vulnerability aligns with CWE-276, which addresses improper privileges, and represents a classic example of privilege escalation through improper access control. The attack vector follows patterns consistent with techniques found in the MITRE ATT&CK framework under privilege escalation tactics, specifically focusing on the abuse of service accounts and improper privilege handling. The vulnerability demonstrates how seemingly minor implementation details in system services can create significant security weaknesses that can be exploited to bypass entire security models.
Mitigation strategies for this vulnerability should include immediate patching of the affected ftpd implementations, ensuring that directory change operations occur after uid switching has been completed. System administrators should also implement proper directory permissions and access controls, regularly audit user home directory configurations, and monitor for unauthorized symbolic link creation. Additionally, the implementation should be reviewed to ensure that all service operations follow proper privilege separation principles, and that no unnecessary directory access occurs before user identity is properly established. Regular security assessments of network services and their privilege handling mechanisms should be conducted to identify similar issues that may exist in other system components.