RHEL 9 must disable the use of user namespaces.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-257816	SRG-OS-000480-GPOS-00227	RHEL-09-213105	SV-257816r1155715_rule	2025-11-24	2

Description
It is detrimental for operating systems to provide, or install by default, functionality exceeding requirements or mission objectives. These unnecessary capabilities or services are often overlooked and therefore, may remain unsecured. They increase the risk to the platform by providing additional attack vectors. The sysctl --system command will load settings from all system configuration files. All configuration files are sorted by their filename in lexicographical order, regardless of the directories in which they reside. If multiple files specify the same option, the entry in the file with the lexicographically latest name will take precedence. Files are read from directories in the following list from top to bottom. Once a file of a given filename is loaded, any file of the same name in subsequent directories is ignored. /etc/sysctl.d/.conf /run/sysctl.d/.conf /usr/local/lib/sysctl.d/.conf /usr/lib/sysctl.d/.conf /lib/sysctl.d/*.conf /etc/sysctl.conf

Description

It is detrimental for operating systems to provide, or install by default, functionality exceeding requirements or mission objectives. These unnecessary capabilities or services are often overlooked and therefore, may remain unsecured. They increase the risk to the platform by providing additional attack vectors. The sysctl --system command will load settings from all system configuration files. All configuration files are sorted by their filename in lexicographical order, regardless of the directories in which they reside. If multiple files specify the same option, the entry in the file with the lexicographically latest name will take precedence. Files are read from directories in the following list from top to bottom. Once a file of a given filename is loaded, any file of the same name in subsequent directories is ignored. /etc/sysctl.d/*.conf /run/sysctl.d/*.conf /usr/local/lib/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /lib/sysctl.d/*.conf /etc/sysctl.conf

ℹ️ Check
Verify RHEL 9 disables the use of user namespaces. Check the status of the "user.max_user_namespaces" parameter with the following command: $ sudo sysctl user.max_user_namespaces user.max_user_namespaces = 0 If "user.max_user_namespaces" is not set to "0" or is missing, this is a finding. If the use of namespaces is operationally required and documented with the information system security manager (ISSM), it is not a finding.

ℹ️ Check

Verify RHEL 9 disables the use of user namespaces. Check the status of the "user.max_user_namespaces" parameter with the following command: $ sudo sysctl user.max_user_namespaces user.max_user_namespaces = 0 If "user.max_user_namespaces" is not set to "0" or is missing, this is a finding. If the use of namespaces is operationally required and documented with the information system security manager (ISSM), it is not a finding.

✔️ Fix
Configure RHEL 9 to disable the use of user namespaces. Create the drop-in if it does not already exist: $ sudo vi /etc/sysctl.d/99-user_max_user_namespaces.conf Add the following line to the file: user.max_user_namespaces = 0 Reload settings from all system configuration files with the following command: $ sudo sysctl --system