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



You need to have a quite current version of openssl installed. Then, just

$ make

for Linux, and

$ make -f Makefile.bsd

for BSD etc. For embedded systems please see at the end of this document. crash was successfully tested on Linux (openSUSE 10.3, 11.1, 12.3, 13.1), FreeBSD 7.2, OpenSolaris and OSX 10.8. It probably runs on a lot of other Linux and BSD's but I was not able to test it there yet.

To generate server and authentication keys:

$ make keys

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

$ ./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:

stealth@linux ~> ./crashd -h
./crashd: invalid option -- h

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

                 -a -- always login (if authenticated), even with a /bin/false default shell
                 -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/IP to connect to; if omitted it uses 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 files for users if starts with '/'; subdir in users ~
                       containing authorized_keys file otherwise; 'self' means to use
                       blob-extraction (see -e); default is .crash
                 -k -- servers key file; default is ./serverkey.pem
                 -c -- X509 certificate-file that belongs to serverkey (-k); default is ./pubkey.x509
                 -t -- watch a 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 makes it skipping apropriate setuid() calls, effectively being able to run as user. 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 connect. 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. If -w is used it forks itself as [nfsd] and tries to wrap around its PID to be somewhere around the system daemons. This can take a while!

Key setup

The easiest way is to just

$ make keys

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

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

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

$ openssl x509 -in pubkey.x509 -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. Rather it compares the public key it obtained from the server with the key it has in its local key-store belonging to that server (similar to SSH). That way, crashc cannot fall into the common X509 traps like a lot of web browsers do. The server key is not encrypted since crashd is usually started via init scripts. Rather, the key file must have proper permissions so only apropriate users can read it (mode 0600). You can, if you like, also encrypt the server key but then you have to enter a passphrase whenever crashd is started.

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

$ openssl genrsa -out privkey.pem -aes256 4096
$ openssl rsa -in privkey.pem -pubout -out pubkey.pem

Copy pubkey.pem to ~/.crash/authorized_keys on the remote box, and use privkey.pem for the crashc -i argument.

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 $port then a keyfile of form .crash/HK_$host:$port is looked up unless you specify a path to a known keyfile.


crashd automatically detects whether it has been invoked as a CGI by a webserver 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 dont 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 webserver is running as). For pentesting or embedded/emergency systems, 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 first:

$ cat serverkey.pem>>crashd
$ cat pubkey.x509>>crashd

If you give -A self instead of a valid authentication directory or file, it also extracts the user-key used for authentication from its binary. It has to be appended in the same manner as above. This is useful in pentests where you cannot upload arbitrary amount of files or 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.

cross-build for speedport/fritzbox DSL routers

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/tarball and the MIPS toolchain. I assume you are familar 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 doesnt strip off the embedded keys if using -e.

Then, inside userapps/opensource/ directory, unpack the crash tarball, 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:

stealth@linux ~> ./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.