Capabilities

Theory

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 granted to processes. This way the full set of privileges is reduced, decreasing the risks of exploitation

Capabilities Sets

Conceptually capabilities are maintained in sets, which are represented as bit masks. For all running processes capability information is maintained per thread; for binaries in the file system it’s stored in extended attributes. Thread capability sets are copied on fork() and specially transformed on execve()

CapEff

CapEff is the effective capability set represents all capabilities the process is using at the moment (this is the actual set of capabilities that the kernel uses for permission checks). For file capabilities the effective set is in fact a single bit indicating whether the capabilities of the permitted set will be moved to the effective set upon running a binary. This makes it possible for binaries that are not capability-aware to make use of file capabilities without issuing special system calls.

CapPrm

CapInh

CapBnd

CapAmb

Practice

Processes Capabilities

Enumerate

We can can find the capabilities of a process as follow

#List current process capabilities
cat /proc/self/status | grep Cap
cat /proc/$$/status | grep Cap
capsh --print

#List capabilities of <PID> process
cat /proc/<PID>/status | grep Cap

Using the capsh utility we can decode them into the capabilities name.

capsh --decode=0000003fffffffff
0x0000003fffffffff=cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,37

Binaries Capabilities

Enumerate

Binaries can have capabilities that can be used while executing. We can search binaries with capabilities as follow

getcap -r / 2>/dev/null

Exploit

Having the capability =ep means the binary has all the capabilities

Setcap with SUID/SUDO

Enumerate

If you found the setcap binary with the SUID bit or with SUDO permissions, you can obtain root access.

#SUID
$ find / -type f -perm -4000 2>/dev/null
/usr/sbin/setcap

#SUDO
$ sudo -l
    (root) NOPASSWD: /usr/sbin/setcap /home/<user>/*

Exploit

Interesting Capabilities

CAP_SYS_ADMIN

Desc

CAP_SYS_ADMIN is largely a catchall capability, it can easily lead to additional capabilities or full root (typically access to all capabilities). CAP_SYS_ADMIN is required to perform a range of administrative operations, which is difficult to drop from containers if privileged operations are performed within the container.

Retaining this capability is often necessary for containers which mimic entire systems versus individual application containers which can be more restrictive. Among other things this allows to mount devices or abuse release_agent to escape from the container.

Exploit - Python

CAP_SYS_PTRACE

Desc

CAP_SYS_PTRACE allows to use ptrace(2) and recently introduced cross memory attach system calls such as process_vm_readv(2) and process_vm_writev(2).

If this capability is granted and the ptrace(2) system call itself is not blocked by a seccomp filter, this will allow an attacker to bypass other seccomp restrictions, see PoC for bypassing seccomp if ptrace is allowed.

Exploit - Python

Exploit - Gdb

CAP_SYS_MODULE

Desc

CAP_SYS_MODULE allows the process to load and unload arbitrary kernel modules (init_module(2), finit_module(2) and delete_module(2) system calls).

This could lead to trivial privilege escalation and ring-0 compromise. The kernel can be modified at will, subverting all system security, Linux Security Modules, and container systems.

This means that you can insert/remove kernel modules in/from the kernel of the host machine.

Exploit - Python

Exploit - kmod

CAP_DAC_READ_SEARCH

Desc

CAP_DAC_READ_SEARCH allows a process to bypass file read, and directory read and execute permissions. While this was designed to be used for searching or reading files, it also grants the process permission to invoke open_by_handle_at(2).

Any process with the capability CAP_DAC_READ_SEARCH can use open_by_handle_at(2) to gain access to any file, even files outside their mount namespace. The handle passed into open_by_handle_at(2) is intended to be an opaque identifier retrieved using name_to_handle_at(2). However, this handle contains sensitive and tamperable information, such as inode numbers. This was first shown to be an issue in Docker containers by Sebastian Krahmer with shocker exploit.

This means that you can bypass can bypass file read permission checks and directory read/execute permission checks.

Exploit - Python

Exploit - Tar

CAP_DAC_OVERRIDE

Desc

CAP_DAC_OVERRIDE allows to ignore the permission bits of files. With this capability, you can modify any file like passwd, sudoers or shadow to obtain root access.

This mean that you can bypass write permission checks on any file, so you can write any file.

Exploit - Python

Exploit Vim

CAP_CHOWN

Desc

CAP_CHOWN allow us to make arbitrary changes to file UIDs and GIDs.

This means that it's possible to change the ownership of any file.

Exploit - Python

Exploit - Ruby

CAP_FOWNER

Desc

CAP_CHOWN allow us to bypass permission checks on operations that normally require the filesystem UID of the process to match the UID of the file. excluding those operations covered by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH. Additionally it allow us to set inode flags, ACLs on arbitrary files, ignore the directory sticky bit on file deletion.

This means that it's possible to change the permission of any file.

Exploit - Python

Exploit - Ruby

CAP_SETUID

Desc

CAP_SETUID allow us to make arbitrary manipulations of process UIDs (setuid(2), setreuid(2), setresuid(2), setfsuid(2));

This means that it's possible to set the effective user id of the created process.

Exploit - Python

Exploit - Ruby

Exploit - Perl

Exploit - PHP

CAP_SETGID

Desc

CAP_SETGID allow us to make arbitrary manipulations of process GIDs and supplementary GID list.

This means that it's possible to set the effective group id of the created process.

Exploit - Python

Exploit - Ruby

Exploit - Perl

Exploit - PHP

CAP_SETFCAP

Desc

CAP_SETFCAP allow us to set arbitrary capabilities on a file.

This means that it's possible to set capabilities on files and processes

Exploit - Python

CAP_SYS_RAWIO

Desc

CAP_SYS_RAWIO provides a number of sensitive operations including access to /dev/mem, /dev/kmem or /proc/kcore, modify mmap_min_addr, access ioperm(2) and iopl(2) system calls, and various disk commands. The FIBMAP ioctl(2) is also enabled via this capability, which has caused issues in the past. As per the man page, this also allows the holder to descriptively perform a range of device-specific operations on other devices.

This can be useful for privilege escalation and Docker breakout.

CAP_KILL

Desc

CAP_KILL allow us to bypass permission checks for sending signals (see kill(2)). This includes use of the ioctl(2) KDSIGACCEPT operation.

This means that it's possible to kill any process.

Exploit - Python

CAP_NET_BIND_SERVICE

Desc

CAP_NET_BIND_SERVICE allow us to Bind a socket to Internet domain privileged ports (port numbers less than 1024).

This means that it's possible to listen in any port (even in privileged ones). You cannot escalate privileges directly with this capability.

Exploit - Python

CAP_NET_RAW

Desc

CAP_NET_RAW allows a process to be able to create RAW and PACKET socket types for the available network namespaces. This allows arbitrary packet generation and transmission through the exposed network interfaces. In many cases this interface will be a virtual Ethernet device which may allow for a malicious or compromised container to spoof packets at various network layers. A malicious process or compromised container with this capability may inject into upstream bridge, exploit routing between containers, bypass network access controls, and otherwise tamper with host networking if a firewall is not in place to limit the packet types and contents. Finally, this capability allows the process to bind to any address within the available namespaces. This capability is often retained by privileged containers to allow ping to function by using RAW sockets to create ICMP requests from a container.

This means that it's possible to sniff traffic. You cannot escalate privileges directly with this capability.

Exploit - Python

Exploit - Tcpdump

CAP_NET_ADMIN + CAP_NET_RAW

Desc

CAP_NET_ADMIN allows the capability holder to modify the exposed network namespaces' firewall, routing tables, socket permissions, network interface configuration and other related settings on exposed network interfaces. This also provides the ability to enable promiscuous mode for the attached network interfaces and potentially sniff across namespaces.

Exploit - Python

CAP_LINUX_IMMUTABLE

Desc

CAP_LINUX_IMMUTABLE allow us to set the FS_APPEND_FL and FS_IMMUTABLE_FL inode flags

If you find that a file is immutable and python has this capability, you can remove the immutable attribute and make the file modifiable

Exploit - Python

CAP_SYS_CHROOT

Desc

CAP_SYS_CHROOT permits the use of the chroot(2) system call. This may allow escaping of any chroot(2) environment, using known weaknesses and escapes.

Exploit - Python

CAP_SETPCAP

Desc

CAP_SETCAP is a Linux capability that allows a process to modify the capability sets of another process. It grants the ability to add or remove capabilities from the effective, inheritable, and permitted capability sets of other processes. However, there are certain restrictions on how this capability can be used.

A process with CAP_SETPCAP can only grant or remove capabilities that are in its own permitted capability set. In other words, a process cannot grant a capability to another process if it does not have that capability itself. This restriction prevents a process from elevating the privileges of another process beyond its own level of privilege.

Moreover, in recent kernel versions, the CAP_SETPCAP capability has been further restricted. It no longer allows a process to arbitrarily modify the capability sets of other processes. Instead, it only allows a process to lower the capabilities in its own permitted capability set or the permitted capability set of its descendants. This change was introduced to reduce potential security risks associated with the capability.

References

Linux Capabilities - HackTricksbook.hacktricks.xyz

Understanding Capabilities in LinuxTinker, Tamper, Alter, Fry