libcap-ng

The libcap-ng library should make programming with POSIX capabilities easier. The library has some utilities to help you analyse a system for apps that may have too much privileges.

The included utilities are designed to let admins and developers spot apps from various ways that may be running with too much privilege. For example, any investigation should start with network facing apps since they would be prime targets for intrusion. The netcap program will check all running apps that have listening socket and display the results. Sample output from netcap:

ppid  pid   acct       command          type port  capabilities
1     2295  root       nasd             tcp  8000  full
2323  2383  root       dnsmasq          tcp  53    net_admin, net_raw +
1     2286  root       sshd             tcp  22    full
1     2365  root       cupsd            tcp  631   full
1     2286  root       sshd             tcp6 22    full
1     2365  root       cupsd            tcp6 631   full
2323  2383  root       dnsmasq          udp  53    net_admin, net_raw +
2323  2383  root       dnsmasq          udp  67    net_admin, net_raw +
1     2365  root       cupsd            udp  631   full

After checking the networking apps, you should check all running apps with pscap. If you are a developer and have to give your application CAP_DAC_OVERRIDE, you must be accessing files for which you have no permission to access. This typically can be resolved by having membership in the correct groups. Try to avoid needing CAP_DAC_OVERRIDE...you may as well be root if you need it.

Some application developers have chosen to use file system base capabilities rather than be setuid root and have to drop capabilities. Libcap-ng provides filecap to recursively search directories and show you which ones have capabilities and exactly what those are.

C Examples

As an application developer, there are probably 6 use cases that you are interested in: drop all capabilities, keep one capability, keep several capabilities, check if you have any capabilities at all, check for certain capabilities, and retain capabilities across a uid change.

1. Drop all capabilities

   capng_clear(CAPNG_SELECT_BOTH);
   capng_apply(CAPNG_SELECT_BOTH);
   

2. Keep one capability

   capng_clear(CAPNG_SELECT_BOTH);
   capng_update(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED, CAP_CHOWN);
   capng_apply(CAPNG_SELECT_BOTH);
   

3. Keep several capabilities

   capng_clear(CAPNG_SELECT_BOTH);
   capng_updatev(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED, CAP_SETUID, CAP_SETGID, -1);
   capng_apply(CAPNG_SELECT_BOTH);
   

4. Check if you have any capabilities

   if (capng_have_capabilities(CAPNG_SELECT_CAPS) > CAPNG_NONE)
       do_something();
   

5. Check for a specific capability

   if (capng_have_capability(CAPNG_EFFECTIVE, CAP_CHOWN))
       do_something();
   

6. Retain capabilities across a uid change

   capng_clear(CAPNG_SELECT_BOTH);
   capng_update(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED, CAP_CHOWN);
   if (capng_change_id(99, 99, CAPNG_DROP_SUPP_GRP | CAPNG_CLEAR_BOUNDING))
       error();
   

Now, isn't that a lot simpler? Note that the last example takes about 60 lines of code using the older capabilities library. As of the 0.6 release, there is a m4 macro file to help adding libcap-ng to your autotools config system. In configure.ac, add LIBCAP_NG_PATH. Then in Makefile.am locate the apps that link to libcap-ng, add $(CAPNG_LDADD) to their LDADD entries. And lastly, surround the optional capabilities code with #ifdef HAVE_LIBCAP_NG.

Python

Libcap-ng 0.6 and later has python bindings. (Only python3 is supported from 0.8.4 onward.) You simply add 'import capng' in your script. Here are the same examples as above in python:

1. Drop all capabilities

   capng.capng_clear(capng.CAPNG_SELECT_BOTH)
   capng.capng_apply(capng.CAPNG_SELECT_BOTH)
   

2. Keep one capability

   capng.capng_clear(capng.CAPNG_SELECT_BOTH)
   capng.capng_update(capng.CAPNG_ADD, capng.CAPNG_EFFECTIVE|capng.CAPNG_PERMITTED, capng.CAP_CHOWN)
   capng.capng_apply(capng.CAPNG_SELECT_BOTH)
   

3. Keep several capabilities

   capng.capng_clear(capng.CAPNG_SELECT_BOTH)
   capng.capng_updatev(capng.CAPNG_ADD, capng.CAPNG_EFFECTIVE|capng.CAPNG_PERMITTED, capng.CAP_SETUID, capng.CAP_SETGID, -1)
   capng.capng_apply(capng.CAPNG_SELECT_BOTH)
   

4. Check if you have any capabilities

   if capng.capng_have_capabilities(capng.CAPNG_SELECT_CAPS) > capng.CAPNG_NONE:
       do_something()
   

5. Check for a specific capability

   if capng.capng_have_capability(capng.CAPNG_EFFECTIVE, capng.CAP_CHOWN):
       do_something()
   

6. Retain capabilities across a uid change

   capng.capng_clear(capng.CAPNG_SELECT_BOTH)
   capng.capng_update(capng.CAPNG_ADD, capng.CAPNG_EFFECTIVE|capng.CAPNG_PERMITTED, capng.CAP_CHOWN)
   if capng.capng_change_id(99, 99, capng.CAPNG_DROP_SUPP_GRP | capng.CAPNG_CLEAR_BOUNDING) < 0:
       error()
   

The one caveat is that printing capabilities from python does not work. But you can still manipulate capabilities, though.

Ambient Capabilities

Ambient capabilities arrived in the 4.3 Linux kernel. Ambient capabilities allow a privileged process to bestow capabilities to a child process. This is how systemd grants capabilities to a daemon running in a service account. The problem with ambient capabilities is they are inherited forever. Every process exec'ed from the original service also has the capabilities. This is a security issue.

To find and fix this, you can run the pscap program and grep for '@'. The '@' symbol denotes processes that have ambient capabilities. For example:

# pscap | grep @
1     1655  systemd-oom  systemd-oomd        dac_override, kill @ +
1     1656  systemd-resolve  systemd-resolve     net_raw @ +

To fix this, libcap-ng 0.8.3 and later ships libdrop_ambient.so.0. It is designed to be used with LD_PRELOAD. It has a constructor function that forces the dropping of ambient capabilities. By the time the application starts, it has both effective and ambient capabilities - meaning is safe to drop ambient capabilities very early. You can either link it to an application run as a systemd service (using ld), or create a wrapper script that then starts the daemon.

Building

After cloning libcap-ng, run:

cd libcap-ng
./autogen.sh
./configure
make
make install

If you want python bindings, add that option to the configure command. There is also a spec file to use if you are on a rpm based distribution. To do that, run "make dist" instead of make in the above instructions. Then use the resulting tar file with the spec file.

NOTE: to distributions

There is a "make check" target. It only works if the available kernel headers roughly match the build root kernel. Iow, if you have a chroot build system that is using a much older kernel, the macros in the kernel header files will describe functionality that does not exist in the build root. The capng_init function will probe the kernel and decide we can only do v1 rather than v3 capabilities instead of what the kernel headers said was possible. If that is your case, just don't do the "make check" as part of the build process. This problem should go away as build roots eventually switch to the 5.0 or later kernels.

Reporting

Report any bugs in this package to: https://github.com/stevegrubb/libcap-ng/issue