CRypted Admin SHell

An SSH alternative featuring:

• 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 blinding against timing and packet-size infoleak attacks
• 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
• SNI hiding mode

Build

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 Makefile.android or Makefile.android.aarch64 to reflect your NDK and BoringSSL install and use these. The build was tested with android-ndk-r17b.

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.

OpenSSL3 builds

The OpenSSL 3 API is quite different from the OpenSSL-1.0 and 1.1 API. In order to make use of OpenSSL 3, you have to edit Makefile and newdh to reflect your path setup for your OpenSSL install. Invoking newdh is mandatory, unlike for the 1.x builds. After that you just do make and everything should be the same as with 1.x.

Run

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

crypted admin shell (C) 2021 Sebastian Krahmer https://github.com/stealth/crash

./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] [-S SNI]
	 [-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 to appear in system PID space (must be last arg!)
	 -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 ./serverkey.pub
	 -t -- watch triggerfile for certain message (-m) before connect/listen
	 -m -- wait with connect/listen until message in file (-t) is seen
         -S -- SNI to hide behind

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. This way, from TCP view client and server role may be reversed, while still having crashd as the shell server. If -w is used it forks itself as [nfsd] and tries to wrap around its pid to be somewhere around the system daemons. As -w is overwriting main()'s argv array, it must appear last in the option list, otherwise option processing will not work correctly.

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

src $ ./crashd -U -p 2222

and

src $ ./crashc -v -K none -i authkey.priv -H 127.0.0.1 -p 2222 -l $USER

Key setup

Unless you want to use SNI-hiding (see section below), you can type straight ahead:

$ 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 serverkey.pub

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

$ openssl x509 -in serverkey.pub -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).

Unless you use SNI hiding (see section below), 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 authkey.pub

Copy authkey.pub 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.

Hostkeys

By default, crashc will compare server hostkeys to the local key cache that is found inside the .crash/ subdir of CWD. You may override the path of the cache folder by using the -K switch. For example by using -K ~/.crash/, you use the folder inside your home directory. If the pathname does not end with a slash, it is treated as a filename instead of a directory. If a cache directory is used instead of an filename, each hostkey is expected to be found inside the folder as of the name HK_$HOST:$PORT where $HOST is the -H argument and $PORT the -p argument. If using -v the server hostkey will be printed on stdout and may be pasted to the cache folder into the HK_$HOST:$PORT file.

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.

CGI

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:

http://127.0.0.1/cgi-bin/crashd?-K=/path/to/serverkey.pem&    \
      -C=/path/to/pubkey.x509&-p=1234&-A=/tmp/.crash/authorized_keys&-U=1&-a=1

which invokes crashd on the host 127.0.0.1 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 serverkey.pub>>crashd
$ cat serverkey.priv>>crashd
$ cat authkey.pub>>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 'http://127.0.0.1/cgi-bin/crashd?-A=self&-U=1&-e=1&-a=1'

-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

Traffic analysis mitigation has two goals. First, to make it hard to find out actual typing sequences and potential info leaks about whats being typed inside an encrypted, interactive channel. Second, to make it hard for a (semi-)global observer to track connections streams across packet mixes or hubs.

Completely mitigating traffic analysis for a capable (global) observer is very hard. It would require many crash users so to sink all individual packets in a swarm and make it impossible to find patterns that could be used to track individual users across packet mixes. It would also require a fixed packet size for all packets as well as a constant delay between the sends to make all connections look equal. Even then, theres still the problem of the overall amount of traffic sent that may be measured and used to track individuals. As having constant size and delays would make the connection feel slow or even unusable, crash lets you choose between traffic policies which are controlled by -R <value>. Value is an integer with the following meaning:

• 0: disable all padding of payloads and don't inject random traffic. The pure feeling!
• 1: pad payload to the next 256, 512, 1024 or 1420 byte boundary, no injects (default)
• 2: pad payload to the next 256, 512, 1024 or 1420 byte boundary, random injects client side
• 3: pad payload to the next 256, 512, 1024 or 1420 byte boundary, random injects with server responses
• 4: pad payload to 1420 byte boundary, no injects
• 5: pad payload to 1420 byte boundary, random injects client side
• 6: pad payload to 1420 byte boundary, random injects with server responses

1420 is an often seen TCP MSS. The values were chosen in a way so that sent data fits most likely into a single packet. Note however that these are the packet sizes (plus the TLS record size) as it is passed to the TCP stack. TCP will decide itself how it will send the segements. There is no way to enforce 'TCP packet sizes', but this does not matter as the deps to the actual payload size is already blurred.

If you live in a country with restrictive egress filtering, it may be helpful to test how long connections can survive. Note that due to -4 and -U which allows to proxy TCP and UDP (DNS) to a remote site, crash may be used as a shadowsocks alternative that requires basically no setup and just needs a user-shell behind egress.

If you think that all of this is paranoia, go get some product sheets for devices that detect and classify SSH traffic by behavioral analysis.

Hiding by SNI

By default, the crashd whill show a banner upon connect to tell the peer major and minor version numbers. Censorship countries might block addresses wich show banners they dislike. To combat this, crash allows for a TLS-only mode that is indistinguishable from a HTTPS session. Just start crashd with -S and give a semi-secret name (Server Name Indicator, SNI). Only clients that also use the correct -S parameter will reach the gate for authentication at all. Other TLS sessions will just be rejected. Note that the SNI travels the network in plain-text and that -S is not meant for authentication. The only reason for SNI hiding is to hide the crash banner from probing/crawling. You may also use SNI proxies such as sshttp to hide crash even deeper and to forward all non-correct SNI connects to some web-site. This way you may hide your server behind neutral web-sites from agressively probing/blocking censors.

In order for probing to not reveal that you are running crash by checking the X509 certificate details, you should use reasonable values for Country Name, City etc. when asked for it during the make keys process. For instance it would make no sense to setup a pro-regime web-site to hide behind and enter anti-regime values for the X509 specific namings.

File up/download

Although there is nothing like sftp for crash, it may be used for file up/downloads.

In order to upload a file:

~ $ crashc -H host -i authkey.priv -l root -c 'dd of=/path/on/remote status=progress' < local.file

Or to download a file:

~ $ crashc -H host -i authkey.priv -l root -c 'dd if=/path/on/remote status=progress' > local.file

Note that in the download case you must not specify the -v switch since this would add the verbose output to the local.file. For -c commands, crash will forward stdout and stderr separated to the local tty's fd 1 and 2, so above commands add a nice progress bar during the xfer.

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:

stealth@linux ~> ./crashc -l root -H 192.168.2.1 -K .crash/HK_127.0.0.1\:2222 -v  \
                          -i privkey.pem -c '/bin/sh -i'
crash: starting crypted administration shell
crash: connecting to 192.168.2.1:2222 ...
Enter PEM pass phrase:
crash:{
-----BEGIN PUBLIC KEY-----
MIICIjANBgkqhkiG9w0BAQEFAAOCAg8AMIICCgKCAgEA4PKEDfrj1v1RlK307j1o
JX9aUuadp3KjGqs2gpoxYTuYf8oBh+1qhwbsH5wC1oQ4HerlIAfLFfUL0+yjy1BO
jNW/4sCo3QSxByRC9DORmfADrxJYkWjyC+cgdXCRaudYBLsz4oIDbg6B8Gtzma8c
ifFPUuSfEoLMRlQOSBIr15WzjS9/wIJfU+D0maJAFRySvVhcRQEbQZ5uuhLnmf4R
zEEiVV+HU79AfhuaKjs1gnTCZwj/dUmsD2zNjHuU6bZpjiQlQdkXLKe2wTvvYFQL
pbmx/XEapoW+tes9pbGgkFIs0jQLz/xYB07luUy46teVAVyB70R7l/WDyFpocPwW
seLNINDUKTvN6vo4UJD9tg1xhClim+P8Jnkyh5X5MqSdXwYC4N/FQsGs223jtxY2
XvQNPBR06d8HJu66GCDSTy7jKKARQ3c3Odq87JDOquUntR1pg26Dj3lOHWVQpuof
7jZ6vLyivpnh4JcdBgGTkzC+Ua0VE1tlokl58+FjhkIbeijD6FK+xcUNaylRo9YM
TrHjBnCi9FIb85WfBo1pj+basKAM85+BkFX3sLC+HR62CL1qJKue7qDBMJbK4INx
ZvoDpdJyBRZXLx+lGisasT3zgNDjJAjSWYn5bY1FfTMBZbvhZyN2/5Uxe85VicUV
8NS0w2TCghjgA0B+hiln1mMCAwEAAQ==
-----END PUBLIC KEY-----
crash:}
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
192.168.3.1
192.168.2.0
10.0.0.0
fe80:216::1234
# 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.