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基于 SOC 职业分类
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| name | cis-ubuntu2004-v300-5-4-3-3 |
| description | Ensure default user umask is configured |
| category | cis-iam |
| version | 3.0.0 |
| author | cyberstrike-official |
| tags | ["cis","ubuntu","linux","ubuntu-20.04","users","authentication"] |
| cis_id | 5.4.3.3 |
| cis_benchmark | CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0 |
| tech_stack | ["ubuntu","linux"] |
| cwe_ids | [] |
| chains_with | [] |
| prerequisites | [] |
| severity_boost | {} |
The user file-creation mode mask (umask) is used to determine the file permission for newly created directories and files. In Linux, the default permissions for any newly created directory is 0777 (rwxrwxrwx), and for any newly created file it is 0666 (rw-rw-rw-). The umask modifies the default Linux permissions by restricting (masking) these permissions. The umask is not simply subtracted, but is processed bitwise. Bits set in the umask are cleared in the resulting file mode.
umask can be set with either Octal or Symbolic values:
Octal (Numeric) Value - Represented by either three or four digits. ie umask 0027 or umask 027. If a four digit umask is used, the first digit is ignored. The remaining three digits effect the resulting permissions for user, group, and world/other respectively.Symbolic Value - Represented by a comma separated list for User u, group g, and world/other o. The permissions listed are not masked by umask. ie a umask set by umask u=rwx,g=rx,o= is the Symbolic equivalent of the Octal umask 027. This umask would set a newly created directory with file mode drwxr-x--- and a newly created file with file mode rw-r-----.The default umask can be set to use the pam_umask module or in a System Wide Shell Configuration File. The user creating the directories or files has the discretion of changing the permissions via the chmod command, or choosing a different default umask by adding the umask command into a User Shell Configuration File, (.bash_profile or .bashrc), in their home directory.
Setting the default umask:
pam_umask module:
/etc/login.defsumaskumask=<mask> value in the /etc/login.defs file is interpreted as OctalUSERGROUPS_ENAB to yes in /etc/login.defs (default):
umask group bits to be the same as owner bits. (examples: 022 -> 002, 077 -> 007) for non-root users, if the uid is the same as gid, and username is the same as the <primary group name>System Wide Shell Configuration File:
/etc/profile - used to set system wide environmental variables on users shells. The variables are sometimes the same ones that are in the .bash_profile, however this file is used to set an initial PATH or PS1 for all shell users of the system. is only executed for interactive login shells, or shells executed with the --login parameter./etc/profile.d - /etc/profile will execute the scripts within /etc/profile.d/*.sh. It is recommended to place your configuration in a shell script within /etc/profile.d to set your own system wide environmental variables./etc/bashrc - System wide version of .bashrc. In Fedora derived distributions, etc/bashrc also invokes /etc/profile.d/*.sh if non-login shell, but redirects output to /dev/null if non-interactive. Is only executed for interactive shells or if BASH_ENV is set to /etc/bashrc.User Shell Configuration Files:
~/.bash_profile - Is executed to configure your shell before the initial command prompt. Is only read by login shells.~/.bashrc - Is executed for interactive shells. only read by a shell that's both interactive and non-loginumask is set by order of precedence. If umask is set in multiple locations, this order of precedence will determine the system's default umask.
Order of precedence:
/etc/profile.d/ ending in .sh - This will override any other system-wide umask setting/etc/profilepam_umask.so module in /etc/pam.d/postlogin/etc/login.defs/etc/default/loginSetting a secure default value for umask ensures that users make a conscious choice about their file permissions. A permissive umask value could result in directories or files with excessive permissions that can be read and/or written to by unauthorized users.
Run the following to verify the default user umask is set to 027(octal) or u=rwx,g=rx,o= (Symbolic) to enforce newly created directories' permissions to be 750 (drwxr-x---), and newly created file's permissions be 640 (rw-r-----), or more restrictive:
#!/usr/bin/env bash
{
l_output="" l_output2=""
file_umask_chk()
{
if grep -Psiq -- '^\h*umask\h+(0?[0-7][2-7]7|u=(rwx]{0,3}),g=([rx]{0,2}),o=)\h*#.*?$' "$l_file"; then
l_output="$l_output\n - umask is set correctly in \"$l_file\""
elif grep -Psiq -- '^\h*umask\h+(([0-7][0-7][01][0-7]\b|[0-7][0-7][0-7][0-6]\b)|([0-7][01][0-7]\b|[0-7][0-7][0-6]\b)|(u=[rwx]{1,3},)?((g=[rx]?[rx]?w[rx]?[rx]?\b)(,o=[rwx]{1,3})?|((g=[wrx]{1,3},)?o=[wrx]{1,3}\b)))' "$l_file"; then
l_output2="$l_output2\n - umask is incorrectly set in \"$l_file\""
fi
}
while IFS= read -r -d $'\0' l_file; do
file_umask_chk
done < <(find /etc/profile.d/ -type f -name '*.sh' -print0)
[ -z "$l_output" ] && l_file="/etc/profile" && file_umask_chk
[ -z "$l_output" ] && l_file="/etc/bashrc" && file_umask_chk
[ -z "$l_output" ] && l_file="/etc/bash.bashrc" && file_umask_chk
[ -z "$l_output" ] && l_file="/etc/pam.d/postlogin"
[ -z "$l_output" ] && l_file="/etc/login.defs" && file_umask_chk
[ -z "$l_output" ] && l_file="/etc/default/login" && file_umask_chk
if [ -z "$l_output" ]; then
if grep -Psiq '^\h*session\h+[^\#\n\r]+\h+pam_umask\.so\h+([^\#\n\r]+\h+)?umask=(0?[0-7][2-7]7)\b' "$l_file"; then
l_output1="$l_output1\n - umask is set correctly in \"$l_file\""
elif grep -Psiq '^\h*session\h+[^\#\n\r]+\h+pam_umask\.so\h+([^\#\n\r]+\h+)?umask=([0-7][0-7][01][0-7]\b)|([0-7][01][0-7]\b)' "$l_file"; then
l_output2="$l_output2\n - umask is incorrectly set in \"$l_file\""
fi
fi
[ -z "$l_output" ] && l_file="/etc/login.defs" && file_umask_chk
[ -z "$l_output" ] && l_file="/etc/default/login" && file_umask_chk
[[ -z "$l_output" && -z "$l_output2" ]] && l_output2="$l_output2\n - umask is not set"
if [ -z "$l_output2" ]; then
echo -e "\n- Audit Result:\n ** PASS **\n - * Correctly configured * :\n$l_output\n"
else
echo -e "\n- Audit Result:\n ** FAIL **\n - * Reasons for audit failure * :\n$l_output2"
[ -n "$l_output" ] && echo -e "\n- * Correctly configured * :\n$l_output\n"
fi
}
- Audit Result:
** PASS **
Configure the default user umask according to your organization's security policy. The recommended value is 027 or more restrictive.
The umask can be configured in one of the following locations (in order of precedence):
/etc/profile.d/ ending in .sh/etc/profilepam_umask.so module in /etc/pam.d/postlogin/etc/login.defs/etc/default/loginSystem default umask
None specified.
v8 - 3.3 Configure Data Access Control Lists: Configure data access control lists based on a user's need to know. Apply data access control lists, also known as access permissions, to local and remote file systems, databases, and applications. (IG 1, IG 2, IG 3)
v7 - 14.6 Protect Information through Access Control Lists: Protect all information stored on systems with file system, network share, claims, application, or database specific access control lists. These controls will enforce the principle that only authorized individuals should have access to the information based on their need to access the information as a part of their responsibilities. (IG 1, IG 2, IG 3)
MITRE ATT&CK Mappings: T1083 - Tactics: TA0007