crypted admin shell: SSH-like strong crypto remote admin shell for Linux, BSD, Android, Solaris and OSX

Related tags

CLI ssh encryption pty

CRypted Admin SHell

  • IPv6 ready
  • lightweight, straight forward and extensible protocol using TLS 1.2+ as transport layer
  • man-in-the-middle safe due to its authentication mechanism which involves the servers host key into the auth process
  • built-in traffic analysis mitigation for timing and packet sizes
  • not relying on any system auth frameworks such as PAM
  • can be entirely run as user, no need to setup config files
  • passive/active connects on both ends with most flexible local/remote port binding possibilities
  • easy to port to embedded systems such as routers
  • quiet/hidden mode for secret administration and take-back functionality for owned boxes
  • trigger-mode via syslog, mail or other files if requested
  • emergency mode to extract all necessary key files from the running binary
  • may be started as a CGI with all above functionality, command switches passed via query-string
  • integrated tcp-wrapper-like D/DoS protection
  • intentionally not passing local $ENV to remote to avoid info leaks
  • supports Perfect Forward Secrecy via DH Kex
  • can forward TCP and UDP sockets to remote
  • SOCKS4 and SOCKS5 support to forward browser sessions to remote


You need to have a reasonable version of OpenSSL installed. Inside the cloned git repo:

$ make -C src

On BSD systems you need to install GNU make and type gmake instead.

If you have a particular OpenSSL or LibreSSL setup, check the Makefile and set the appropriate $SSL variable. crash builds also nicely with LibreSSL and BoringSSL.

For embedded systems, please see at the end of this document.

For Android, edit the or to reflect your NDK and BoringSSL install and use these.

crash was successfully tested on Linux, FreeBSD, NetBSD, OpenSolaris, OSX and Android.

After that, to generate the required server and authentication keys:

$ make -C src keys

or see further instructions in this document. If you want to use ephemeral keying (aka PFS), invoke

$ cd src; ./newdh

before make in order to generate DH parameters before the build.


crash does not need any config or option files to run. Its easy and straight forward to use. Anything can be enabled/disabled by runtime switches:

[email protected] ~> ./crashd -h

crypted admin shell (C) 2021 Sebastian Krahmer

./crashd: invalid option -- 'h'

Usage:	./crashd [-U] [-q] [-a] [-6] [-w] [-H host] [-p port] [-A auth keys]
	 [-k server key-file] [-c server X509 certificate] [-P port]
	 [-t trigger-file] [-m trigger message] [-e] [-g good IPs]

	 -a -- always login if authenticated, despite false/nologin shells
	 -U -- run as user (e.g. turn off setuid() calls) if invoked as such
	 -e -- extract key and certfile from the binary itself (no -k/-c needed)
	 -q -- quiet mode, turns off logging and utmp entries
	 -6 -- use IPv6 rather than IPv4
	 -w -- wrap around PID's so crashd appears in system PID space
	 -H -- host to connect to; if omitted: passive connect (default)
	 -p -- port to connect/listen to; default is 2222
	 -P -- local port used in active connects (default is no bind)
	 -g -- file containing list of good IP/IP6's in D/DoS case (default off)
	 -A -- authorized-key file for users if starts with '/'; folder inside ~
	       containing authorized_keys file otherwise; 'self' means to use
	       blob-extraction (see -e); default is .crash
	 -k -- servers key file; default is ./serverkey.priv
	 -c -- X509 certificate-file that belongs to serverkey (-k);
	       default is ./
	 -t -- watch triggerfile for certain message (-m) before connect/listen
	 -m -- wait with connect/listen until message in file (-t) is seen

Most of it is pretty self-explaining. crashd can run as user. -U lets crashd skip setuid() calls, effectively being able to run as user. In this case, it only accepts logins to that user then by checking login name's uid against current euid. Both, crashc and crashd can use active and passive connects. Whenever a host-argument -H is given, it uses active connect to this host and the belonging port -p. If -H is given, it also accepts -P which specifies the local port it has to bind to before doing active connect. Without -H it will listen for incoming connects. If -w is used it forks itself as [nfsd] and tries to wrap around its pid to be somewhere around the system daemons.

For testing, when you did make keys (next section), you can just run

src $ ./crashd -U -p 2222


src $ ./crashc -v -K none -i authkey.priv -H -p 2222 -l $USER

Key setup

The easiest way is to just

$ make -C src keys

But you can also do it by hand. To generate a X509 certificate containing the server key:

$ umask 066
$ openssl genrsa -out serverkey.priv 4096
$ openssl req -new -x509 -nodes -sha1 -key serverkey.priv -out

To extract the public key in a form crashc can use it as a hostkey for comparison:

$ openssl x509 -in -pubkey -noout > HK_127.0.0.1

So you have HK_127.0.0.1 as the known-hosts keyfile for this host. As an alternative, you can use crashc with -v upon connect to extract the pubkey. But note that this might already be a key presented to you during an attack. So only do that if you know that the connection is not tampered with (e.g. single user on localhost).

The values you enter for Country-Name, Email, CN etc. do not matter since crashc is not validating the X509. It just compares the public key value it obtained from the server with the key it has in its local key-store belonging to that server (similar to SSH). The server key is not encrypted since crashd is usually started via init scripts. Instead, the key file must have proper permissions so only appropriate users can read it (mode 0600). You can, if you like, also encrypt the server key but then you have to enter a pass-phrase whenever crashd is started.

To generate a public/private RSA keypair for your authentication:

$ openssl genrsa -out authkey.priv -aes256 4096
$ openssl rsa -in authkey.priv -pubout -out

Copy to ~/.crash/authorized_keys on the remote box, and use authkey.priv for the crashc -i argument. Note, that upon authentication you will be asked for the pass-phrase to unlock your private key that is stored locally. The pass-phrase will not travel the network.

Auth-Key sizes larger than 7500 bit must not be used; the tokens do not fit into the auth handshake otherwise.

crashc is using the .crash/ subdir by default to check for already seen server keys. If you connect to a host via -H $host -p $port, a keyfile of form .crash/HK_$host:$port is looked up unless you specify an absolute path to a known keyfile.


By default, crashc will compare server hostkeys to the local key cache that is found inside ~/.crash. You may override the path of the cache folder or an absolute filename by using the -K switch. Hostkey checking may be suppressed by using -K none.

The crash auth protocol incorporates the server host key when signing authentication requests. This way its not strictly necessary to check server host keys as you know it from SSH password authentication. Two things have to be considered if host-key checks are suppressed with -K none though:

  • The user-name will potentially leak to a MiM server

This is not an issue if you use a system user-name such as root.

  • The MiM could sort of phish you, by showing you a fake-shell where you think it belongs to your real server. This could be used to wait for su and similar commands and to record sensitive information as you type on the MiM shell.

To conquer this, you have to make sure you are indeed on your real shell when you see the prompt. This can be achieved by echoing a secret token to the tty upon login, for example via one of the .profile or .bashrc files. As the MiM cannot know this token, you can be sure you have a confidential and untampered session when you see this token upon login; even if you omit the host-key check.


crashd automatically detects whether it has been invoked as a CGI by a web-server by checking QUERY_STRING environment variable. It parses and converts the query-string into arguments it understands. It does not translate %2F etc characters! They should not be needed, since spaces, '(' and other weird characters do not make sense when calling crashd. Arguments that don't have a parameter such as -U have to be given =1 argument to enable it, such as in:    \

which invokes crashd on the host as user (probably "wwwrun" or whatever the web-server is running as). For pen-testing or in emergency case, crashd has the -e option. If -e is used, it extracts the server key-file and the X509 certificate from the ELF binary file, which have to be appended before using -e:

$ cat>>crashd
$ cat serverkey.priv>>crashd
$ cat>>crashd

If you give -A self instead of a valid authentication directory or file, crashd also extracts the user-key used for authentication from its binary.

This is useful in pen-tests where you cannot upload arbitrary amount of files or you do not know the exact pathname of the upload storage:

$ curl ''

-a is needed since most likely the wwwrun user has a /bin/false shell, which -a ignores. crashd is using mkstemp() to store the key files temporarily, with mode 0444 (world readable) since it needs to access authentication files as user. So be warned that, if you have users, they may read the private key used during SSL handshake. After all, its just an emergency mode. Stripping the crashd binary is not possible after appending the keys, or they will get lost. Back-connect etc. also work in CGI mode as well. If using that, client should use -K switch to tell client which key to use to authenticate the server.

TCP and UDP port forward

crash uses the same network engine as psc. Therefore you may use the same -U and -T parameters as known from psc and which are similar to those of OpenSSH's -L parameter. It will bind to lport and will forward connections to [ip]:rport, initiating the connection from the remote host. The same works for UDP packets, which is not possible with SSH.

SOCKS4 and SOCKS5 support

crash also supports forwarding of TCP connections via SOCKS4 (-4 port) and SOCKS5 (-5 port). This sets up port as SOCKS port for TCP connections, so for instance you can browse remote networks via crashc sessions without the need to open any other connection during a pen-test. For chrome, SOCKS4 must be used, as the crash SOCKS implementation does not support resolving domain names on their own. Instead, it requires IPv4 or IPv6 addresses to be passed along. Since chrome will set the SOCKS5 protocol address type always to domain name (0x03) - even if an IP address is entered in the address bar - SOCKS5 is not usable with chrome. But you can use chrome with SOCKS4, since this protocol only supports IPv4 addresses, not domain names.

Mitigating traffic analysis

If you invoke crashc with -R, it will insert random internal ping packets into the stream. Packet sizes are always one of 256, 512, 1024 or 1500 bytes in order to make it very hard for a global observer to track back connections across a shell mix by monitoring traffic for unique packet size sequences.

cross-build for speedport/fritzbox DSL routers

This section is just here for historical reasons. It's from the early 2000's when hacking DSL modems was a widespread hobby.

In Germany, these devices are quite common and support open build environment/SDK since the firmware is based on Linux and available in source. Download/unpack the SDK/tar-ball and the MIPS tool-chain. I assume you are familiar with building router images for these devices. The openssl package for the speedport W-500V I tested has version 0.9.7 and I had to build it by hand before, so the references to the libssl.a inside crash Makefile are valid. On the system I tested (500V) I had to make it static, since dynamic openssl library requires libdl which was not on the device. Building a firmware image often calls strip on the user binaries, so take care it doesn't strip off the embedded keys if using -e.

Then, inside userapps/opensource/ directory, unpack the crash tar-ball, edit the Makefile to match "true" at the first ifeq() and then you are able to just "make crashd" to build crashd for the DSL router.

You can now put it into the firmware image, make it loaded at boot and you are able to admin your DSL box from remote via full protected crypto shell and without stupid web interface.

Here's how I connect to a 500V DSL modem running crashd inside:

[email protected] ~> ./crashc -l root -H -K .crash/HK_127.0.0.1\:2222 -v  \
                          -i privkey.pem -c '/bin/sh -i'
crash: starting crypted administration shell
crash: connecting to ...
Enter PEM pass phrase:
-----END PUBLIC KEY-----
Using fallback suid method
Using fallback suid method

BusyBox v1.00 (2008.06.03-09:17+0000) Built-in shell (msh)
Enter 'help' for a list of built-in commands.

# ps aux
  206 root       1712 S   httpd
  210 root        232 S   smdog 
  520 root       1160 S   /bin/crashd -A /var/pubkey.pem 
  546 root       1464 S   /bin/crashd -A /var/pubkey.pem 
  561 root        352 S   /bin/sh -c /bin/sh -i 
  562 root        396 S   /bin/sh -i 
  715 root        340 R   ps aux 

Keep in mind that on embedded systems, UNIX98 pty's are often not available and there are only a limited number of BSD pty's (2 or so) so you cant login a hundred times simultaneously.

DoS mitigation

crashd includes some sort of D/DoS protection. Only one connection per second is allowed per IP, except if the IP is listed (or the network it belongs to) in a good-IP file given with -g at startup. Per default no good IPs are assigned. Network-address-goodness only works with IPv4 yet. A simple good-IP file may look like this:

# sample good-IP file
# end of file

Together with the interval timer for hanging un-authenticated connections this allows to have no more than 12 'hanging' crashd's at the same time, still allowing you to login if you are listed in good-IPs and your underlying TCP/IP stack is not already trashed.

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