CVE-2021-23240 in sudo
Summary
by MITRE • 01/12/2021
selinux_edit_copy_tfiles in sudoedit in Sudo before 1.9.5 allows a local unprivileged user to gain file ownership and escalate privileges by replacing a temporary file with a symlink to an arbitrary file target. This affects SELinux RBAC support in permissive mode. Machines without SELinux are not vulnerable.
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Analysis
by VulDB Data Team • 05/16/2025
The vulnerability identified as CVE-2021-23240 represents a critical privilege escalation flaw within the sudoedit utility of Sudo versions prior to 1.9.5. This issue specifically targets systems running SELinux in permissive mode where the sudoedit command executes with the selinux_edit_copy_tfiles context. The vulnerability stems from improper handling of temporary files during the sudoedit operation, creating a race condition that allows local unprivileged users to manipulate file ownership and gain elevated privileges.
The technical implementation of this flaw involves the manipulation of temporary files used by sudoedit during the editing process. When a user invokes sudoedit, the system creates temporary files that are subsequently copied to their final destination. In the vulnerable versions, these temporary files are not properly secured against symbolic link attacks. An attacker can create a symbolic link in the temporary file location that points to a target file they wish to compromise, then execute sudoedit which will overwrite the target file with the contents of the temporary file, effectively changing the ownership and permissions of the target file. This behavior directly violates the principle of least privilege and enables unauthorized access to system resources.
The operational impact of this vulnerability is significant as it allows local unprivileged users to escalate their privileges to root level access on affected systems. The attack requires no special privileges or network access, making it particularly dangerous in multi-user environments where users may have legitimate sudoedit access for administrative tasks. The vulnerability specifically affects SELinux RBAC support in permissive mode, meaning that systems with SELinux disabled or in enforcing mode are not vulnerable to this particular exploit. This selective vulnerability profile makes the attack surface more targeted but also more dangerous when present, as it can be exploited in environments where SELinux is configured to allow certain operations while maintaining permissive mode for specific contexts.
This vulnerability maps directly to CWE-377 and CWE-378, which address insecure temporary file creation and improper temporary file handling respectively. The attack pattern aligns with TTPs described in the MITRE ATT&CK framework under privilege escalation techniques, specifically focusing on local privilege escalation through file system manipulation. The flaw represents a classic race condition vulnerability where the timing of file operations creates an exploitable window for malicious activity. Organizations should immediately update their Sudo installations to version 1.9.5 or later to mitigate this vulnerability, as the fix addresses the improper temporary file handling by implementing proper file ownership verification and secure temporary file creation mechanisms.
The security implications extend beyond simple privilege escalation, as successful exploitation can lead to complete system compromise. Attackers can leverage this vulnerability to modify critical system files, install backdoors, or establish persistent access to the compromised system. The permissive mode aspect of SELinux makes this particularly concerning, as it allows the execution of potentially malicious operations that would otherwise be restricted in enforcing mode. System administrators should also consider implementing additional monitoring and logging of sudoedit operations to detect potential exploitation attempts and maintain audit trails of privilege escalation activities.