RHEL-09-213035 - RHEL 9 must enable kernel parameters to enforce discretionary access (DAC) control on symlinks.

Warning! Audit Deprecated

This audit has been deprecated and will be removed in a future update.

Information

DAC is based on the notion that individual users are "owners" of objects and therefore have discretion over who should be authorized to access the object and in which mode (e.g., read or write). Ownership is usually acquired as a consequence of creating the object or via specified ownership assignment. DAC allows the owner to determine who will have access to objects they control. An example of DAC includes user-controlled file permissions.

When discretionary access control policies are implemented, subjects are not constrained with regard to what actions they can take with information for which they have already been granted access. Thus, subjects that have been granted access to information are not prevented from passing (i.e., the subjects have the discretion to pass) the information to other subjects or objects. A subject that is constrained in its operation by Mandatory Access Control policies is still able to operate under the less rigorous constraints of this requirement. Thus, while Mandatory Access Control imposes constraints preventing a subject from passing information to another subject operating at a different sensitivity level, this requirement permits the subject to pass the information to any subject at the same sensitivity level. The policy is bounded by the information system boundary. Once the information is passed outside the control of the information system, additional means may be required to ensure the constraints remain in effect. While the older, more traditional definitions of discretionary access control require identity-based access control, that limitation is not required for this use of discretionary access control.

By enabling the fs.protected_symlinks kernel parameter, symbolic links are permitted to be followed only when outside a sticky world-writable directory, or when the UID of the link and follower match, or when the directory owner matches the symlink's owner. Disallowing such symlinks helps mitigate vulnerabilities based on insecure file system accessed by privileged programs, avoiding an exploitation vector exploiting unsafe use of open() or creat().

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

Satisfies: SRG-OS-000312-GPOS-00122, SRG-OS-000312-GPOS-00123, SRG-OS-000312-GPOS-00124, SRG-OS-000324-GPOS-00125

Solution

Configure RHEL 9 to enable DAC on symlinks with the following:

Create a drop-in if it does not already exist:

$ sudo vi /etc/sysctl.d/99-fs_protected_symlinks.conf

Add the following to the file:
fs.protected_symlinks = 1

Reload settings from all system configuration files with the following command:

$ sudo sysctl --system

Item Details

Audit Name: DISA Red Hat Enterprise Linux 9 STIG v2r7

Plugin: Unix

Control ID: 97f6c073e48aad1d733e421b2f2b9c114c35524088051f107f81ea0b9cef100a

Detections

Analytics

RHEL-09-213035 - RHEL 9 must enable kernel parameters to enforce discretionary access (DAC) control on symlinks.

Warning! Audit Deprecated

Information

Solution

See Also

Item Details