Local Privilege Escalation in cifs-utils via Credential File Injection

CVE-2026-104715 is a critical local privilege escalation vulnerability affecting the mount.cifs utility within the cifs-utils package. The vulnerability stems from improper sanitization of the username field within credential files provided via the credentials= mount option. Because mount.cifs copies the username value into the kernel-bound mount option string without filtering for comma separators, an attacker who controls a local credential file can inject arbitrary mount parameters. Since the Linux kernel CIFS parser follows a last-write-wins logic, these injected parameters override earlier, legitimate security settings. By successfully injecting options such as uid=0, forceuid, and file_mode=04755, an attacker can gain a root shell through privileged mount operations performed by administrators, systemd units, or autofs. The vulnerability affects cifs-utils versions 7.0 through 7.6 and bypasses SELinux targeted policies.

Attack Chain

1. Attacker identifies a local credentials file or path that is currently or will be used by mount.cifs with root privileges.
2. Attacker modifies the credentials file, injecting malicious options into the username field (e.g., username=user,uid=0,forceuid,file_mode=04755).
3. A privileged process (e.g., administrator, systemd, or autofs) triggers a mount operation using the compromised credential file.
4. mount.cifs parses the file and concatenates the malicious username string into the kernel mount option buffer.
5. The kernel parses the final mount option string; due to the last-write-wins behavior, the injected uid=0 and permission-altering options override the default system settings.
6. The CIFS share is mounted with the attacker's desired permissions, effectively granting the attacker full control over file ownership and SUID bits on the mounted filesystem.
7. Attacker executes a binary or modifies files on the now-accessible share to gain persistence or escalate to a root shell.

Impact

Successful exploitation results in full local privilege escalation, allowing an unprivileged user to gain root access. This affects any system using cifs-utils for network filesystem management, including enterprise distributions such as Ubuntu 24.04 and RHEL 9.8. The attack is particularly dangerous because it does not require setuid permissions on mount.cifs or the presence of pam_mount, and it remains effective even when SELinux is in enforcing mode.

Recommendation

• Prioritize updating cifs-utils to versions containing the fix provided by the maintainer (Paulo Alcantara) to enforce sanitization of credential file values.
• Audit all fstab entries, systemd .mount units, and administrative scripts that use the credentials= mount option to ensure credential files are stored in directories with restricted write access.
• Restrict write access to any files used as CIFS credentials to the specific user account authorized to use that share, preventing unauthorized injection.
• Monitor system logs for unexpected or unauthorized mount operations, particularly those involving modifications to credential configuration files.