Successful/unsuccessful uses of the chmod, fchmod, and fchmodat system calls in NixOS must generate an audit record.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-268100	SRG-OS-000042-GPOS-00020	ANIX-00-000380	SV-268100r1039565_rule	2024-10-25	1

Description
Without generating audit records that are specific to the security and mission needs of the organization, it would be difficult to establish, correlate, and investigate the events relating to an incident or identify those responsible for one. Audit records can be generated from various components within the information system (e.g., module or policy filter). The "chmod" system call changes the file mode bits of each given file according to mode, which can be either a symbolic representation of changes to make, or an octal number representing the bit pattern for the new mode bits. The "fchmod" system call is used to change permissions of a file. The "fchmodat" system call is used to change permissions of a file relative to a directory file descriptor. When a user logs in, the AUID is set to the UID of the account that is being authenticated. Daemons are not user sessions and have the loginuid set to "-1". The AUID representation is an unsigned 32-bit integer, which equals "4294967295". The audit system interprets "-1", "4294967295", and "unset" in the same way. The system call rules are loaded into a matching engine that intercepts each syscall made by all programs on the system. Therefore, it is very important to use syscall rules only when absolutely necessary since these affect performance. The more rules, the bigger the performance hit. Performance can be helped, however, by combining syscalls into one rule whenever possible. Satisfies: SRG-OS-000042-GPOS-00020, SRG-OS-000462-GPOS-00206

Description

Without generating audit records that are specific to the security and mission needs of the organization, it would be difficult to establish, correlate, and investigate the events relating to an incident or identify those responsible for one. Audit records can be generated from various components within the information system (e.g., module or policy filter). The "chmod" system call changes the file mode bits of each given file according to mode, which can be either a symbolic representation of changes to make, or an octal number representing the bit pattern for the new mode bits. The "fchmod" system call is used to change permissions of a file. The "fchmodat" system call is used to change permissions of a file relative to a directory file descriptor. When a user logs in, the AUID is set to the UID of the account that is being authenticated. Daemons are not user sessions and have the loginuid set to "-1". The AUID representation is an unsigned 32-bit integer, which equals "4294967295". The audit system interprets "-1", "4294967295", and "unset" in the same way. The system call rules are loaded into a matching engine that intercepts each syscall made by all programs on the system. Therefore, it is very important to use syscall rules only when absolutely necessary since these affect performance. The more rules, the bigger the performance hit. Performance can be helped, however, by combining syscalls into one rule whenever possible. Satisfies: SRG-OS-000042-GPOS-00020, SRG-OS-000462-GPOS-00206

ℹ️ Check
Verify NixOS generates an audit record upon attempts to use the "chmod", "fchmod", and "fchmodat" system calls. Check the auditing rules currently loaded into the audit daemon with the following command: $ sudo auditctl -l \| grep chmod -a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -k perm_mod -a always,exit -F arch=b64 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -k perm_mod If the command does not return an audit rule for "chmod", "fchmod", and "fchmodat, this is a finding. Note: The "-k" allows for specifying an arbitrary identifier. The string following "-k" does not need to match the example output above.

ℹ️ Check

Verify NixOS generates an audit record upon attempts to use the "chmod", "fchmod", and "fchmodat" system calls. Check the auditing rules currently loaded into the audit daemon with the following command: $ sudo auditctl -l | grep chmod -a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -k perm_mod -a always,exit -F arch=b64 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -k perm_mod If the command does not return an audit rule for "chmod", "fchmod", and "fchmodat, this is a finding. Note: The "-k" allows for specifying an arbitrary identifier. The string following "-k" does not need to match the example output above.

✔️ Fix
Configure NixOS to generate audit records for any attempts to use the "chmod", "fchmod", and "fchmodat" system calls. Add or update the "security.audit.rules" configuration in /etc/nixos/configuration.nix to include the following rule: security.audit.rules = [ "-a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -k perm_mod" "-a always,exit -F arch=b64 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -k perm_mod" ]; Rebuild the NixOS configuration with the following command: $ sudo nixos-rebuild switch

✔️ Fix

Configure NixOS to generate audit records for any attempts to use the "chmod", "fchmod", and "fchmodat" system calls. Add or update the "security.audit.rules" configuration in /etc/nixos/configuration.nix to include the following rule: security.audit.rules = [ "-a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -k perm_mod" "-a always,exit -F arch=b64 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -k perm_mod" ]; Rebuild the NixOS configuration with the following command: $ sudo nixos-rebuild switch