nfpm has incorrect default permissions

Summary

When building packages directly from source control, file permissions on the checked-in files are not maintained.

Details

When building packages directly from source control, file permissions on the checked-in files are not maintained. When nfpm packaged the files (without extra config for enforcing its own permissions) files could go out with bad permissions (chmod 666 or 777).

PoC

Create a default nfpm structure.

Within the test folder, create 3 files named chmod-XXX.sh. Each script has file
permissions set corresponding with their file names (chmod-777.sh = chmod 777). Below each
file and permissions can be seen.

$ ls -lart test 
total 24
-rwxrwxrwx   1 user  group   11 May 19 13:15 chmod-777.sh
-rw-rw-rw-   1 user  group   11 May 19 13:16 chmod-666.sh
drwxr-xr-x   5 user  group  160 May 19 13:19 .
-rw-rw-r--   1 user  group   11 May 19 13:19 chmod-664.sh
drwxr-xr-x  10 user  group  320 May 19 13:29 ..

Below is the snippet nfpm configuration file of the contents of the package. The test folder
and files has no extra config for enforcing permissions.

contents:
- src: foo-binary
  dst: /usr/bin/bar
- src: bar-config.conf
  dst: /etc/foo-binary/bar-config.conf
  type: config
- src: test
  dst: /etc/test/scripts

The next step is to create a deb package.

$ nfpm package -p deb # Create dep package
using deb packager...
created package: foo_1.0.0_arm64.deb

When on a Ubuntu VM, install the foo package which was created

$ sudo dpkg -i foo_1.0.0_arm64.deb # Installing deb package within Ubuntu
Selecting previously unselected package foo.
(Reading database ... 67540 files and directories currently installed.)
Preparing to unpack foo_1.0.0_arm64.deb ...
Unpacking foo (1.0.0) ...
Setting up foo (1.0.0) ...

Looking at /etc/test/scripts and viewing the permissions. Permissions are passed exactly the same as the source.

$ ls -lart /etc/test/scripts
total 20
-rwxrwxrwx 1 root root   11 May 22 12:15 chmod-777.sh
-rw-rw-rw- 1 root root   11 May 22 12:16 chmod-666.sh
-rw-rw-r-- 1 root root   11 May 22 12:19 chmod-664.sh
drwxr-xr-x 3 root root 4096 May 22 13:00 ..
drwxr-xr-x 2 root root 4096 May 22 13:00 .

Solution

To prevent world-writable files from making it into the packages, add the ability to override the default permissions of packaged files using a umask config option in the packaging spec file. This feature in nfpm would allow applying a global umask across any files being packaged, therefore, with the correct configuration, preventing world-writable files without needing to list permissions on each and every file in the package

Impact

Vulnerability is https://cwe.mitre.org/data/definitions/276.html
https://www.first.org/cvss/calculator/3.0#CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

Anyone using nfpm for creating packages and not checking/setting file permissions before packaging could result in bad permissions for files/folders.