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

Overview

Finding IDVersionRule IDIA ControlsSeverity
V-268100ANIX-00-000380SV-268100r1131002_ruleCCI-000135medium
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 on, 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
STIGDate
Anduril NixOS Security Technical Implementation Guide2025-08-19

Related Frameworks

1 mapping across 4 frameworks
SCF0 mappings

No source states a mapping between these directly.

Also reachable in 3 hops (1) derived by us, not asserted by any source

Each of these is a route we composed by chaining published mappings. No publisher states a relationship between the two endpoints.

MON-03.1Sensitive Event Log Information
0.50
NIST 800-530 mappings

No source states a mapping between these directly.

Also reachable in 2 hops (41) derived by us, not asserted by any source

Each of these is a route we composed by chaining published mappings. No publisher states a relationship between the two endpoints.

AC-16Security and Privacy Attributes
1.00
AC-17(1)Monitoring and Control
1.00
AC-2(1)Automated System Account Management
1.00
AC-2(4)Automated Audit Actions
1.00
AC-3(3)Mandatory Access Control
1.00
AC-6(8)Privilege Levels for Code Execution
1.00
AC-6(9)Log Use of Privileged Functions
1.00
AU-10Non-repudiation
1.00
AU-12Audit Record Generation
1.00
AU-12(3)Changes by Authorized Individuals
1.00

+31 more (top 10 by confidence shown)

NIST 800-1710 mappings

No source states a mapping between these directly.

Also reachable in 3 hops (2) derived by us, not asserted by any source

Each of these is a route we composed by chaining published mappings. No publisher states a relationship between the two endpoints.

3.3.1
1.00
3.3.2
1.00
CCI1 mapping
CCI-000135
1.00

Details

Check Text (C-268100r1131002_chk)

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 Text (F-71927r1131001_fix)

Configure NixOS to generate audit records for any attempts to use the "chmod", "fchmod", and "fchmodat" system calls. Add the following Nix code to the NixOS Configuration, usually located in /etc/nixos/configuration.nix or /etc/nixos/flake.nix: 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 and switch to the new NixOS configuration: $ sudo nixos-rebuild switch