stealth

stealth_1.46.tar.gz

2005-2007

stealth(1)

stealth_1.46.tar.gz Security Enhancement

2005-2007

NAME

SYNOPSIS

DESCRIPTION

The name of the stealth program is an acronym of:

SSH-based Trust Enforcement Acquired through a Locally Trusted Host.

stealth's main task is to perform file integrity tests. However, the testing itself will leave no sediments on the tested computer. Therefore, stealth has stealthy characteristics. This is considered an important security improving feature.

On the other hand, one should realize that stealth intends to be just another security tool: other security measures like firewalls, portscanners, intrusion detection systems, abolishment of unencrypted protocols, etc. are usually required to improve or promote the security of a group of computers that are connected to the Internet.

stealth uses a policy file to determine the actions to perform. Each policy file is uniquely associated with a host to be tested. This remote host (called the client below) trusts the computer on which stealth runs (hence: a Locally Trusted Host), called the controller. The controller performs tasks (normally file integrity tests) that Enforce the Trust we have in the client computer. Since almost all integrity tests can be run on the client, one controller can control many clients, even if the controller itself uses aged hardware components.

As the controller and the client normally are different computers, the controller must communicate with the client in a secure fashion. This is realized using SSH. So, there's another element of `local trust' involved here: the client should permit the controller to set up a secure SSH connection allowing the controller to access sensitive files and private parts of the client's file system.

It is important to ensure that there is no public access to the controller. All inbound services should be denied. The only access to the controller should be via its console and the controller should be placed in a physically secure location. Sensitive information of clients are stored in the controller, and passwordless access to clients can be obtained from the controller by anyone who gains (root)-access.

The controller itself only needs two kinds of outgoing services: SSH to reach its clients, and some mail transport agent (e.g., sendmail(1)) to forward its outgoing mail to some mail-hub.

Here is what happens when stealth is run using the first synopsis:

• First, the policy file is read. This determines the actions to be performed, and the values of several variables that are used by stealth.
• If the command-line option --keep-alive pidfile is specified, stealth will run as a backgrond process, writing its process id in the file pifile. With --repeat <seconds> the scan will be rerun every <seconds> seconds. The number of seconds until the next rerun will be at least 60. However, using the --rerun pidfile option a background stealth process may always be forced into its next scan. When --keep-alive is specified the scan will be performed just once, whereafter PROGRAM will wait until it is reactivated by another run of stealth, called using the --rerun pidfile command-line option (note that integrity scans are suppressed between a --suppress and a --resume command, see below). Consider specifying --quiet (see below) when --keep-alive is used.
• Then, the controller opens a command shell on the client using ssh(1), and a command shell on the controller itself using sh(1).
• Next, commands defined in the policy file are executed in their order of appearance. Examples are given below. Normally, return values of the programs are tested. Non-zero return values will terminate stealth prematurely. In this case, a message about the reason why stealth terminated is written to the report file (and into the mail message sent by stealth). In some cases (e.g., when the report file could not be written), the message is written to the standard error stream.
• In most cases, integrity tests can be controlled by the find(1) program, calling programs like ls(1), md5sum(1) or its own -printf method to produce file-integrity related statistics. Most of these programs write file names at the end of generated lines. This characteristic is used by an internal routine of stealth to detect changes in the generated output, which could indicate some harmful intent, like an installed root-kit.
• When changes are detected, they are logged on a report file, to which information is always appended. stealth never reduces or rewrites the report file. Whenever information is added to the report file (exceeding a plain time stamp) the appended information is emailed to a configurable email address for further (human) processing. Usually this will be the systems manager of the tested client. stealth follows the `dark cockpit' approach in that no mail is sent when no changes were detected.
• When the --repeat or --rerun options are issued, the report file should not be rotated by, e.g., a log-rotating process, but the report file may safely be rotated between a pair of --suppress and --resume commands.

REPORT FILE ROTATION

• Starting stealth using the option --suppress pidfile will suppress a currently active stealth process. If stealth is actually performing a series of integrity scans when --suppress is issued, the currently executing command is first completed before the --suppress command completes. Once --suppress is active, all scheduled scans are skipped and --rerun is ignored. However, the --resume and --terminate options are still handled.
• Once `stealth --suppress pidfile' has returned, the report file may safely be rotated (using, e.g., logrotate(1)), and a new (empty) report file may optionally be created by the logrotation process.
• Finally, when the log-rotation has been completed, the log-rotation process should issue the command `stealth --resume pidfile'. This will resume a suppressed stealth process, immediately performing the next integrity scan (thus implying --rerun), whereafter stealth will be back in its normal integrity scanning mode (so, resuming repeated scans if originally requested so).

RERUN AND TERMINATE

Here is what happens when stealth is run using other synopses:

• When started using the --rerun pidfile command-line option, the stealth process associated with process id file pidfile will perform another scan. This command has no effect following a --suppress command.
• When started using the --terminate pidfile command-line option, the stealth process associated with process id file pidfile is terminated.

OPEN SSH LINK TO CLIENTS

The --keep-alive, --repeat, --rerun, --resume and --suppress options were implemented in such a way that the ssh link to the client remains open, thus minimizing the number of sshd entries caused by PROGRAM in the client's log files.

THE POLICY FILE

The policy file consists of two sets of data: use directives (starting with the keyword USE) and commands. Blank lines and information beyond hash-marks (#) are ignored, while lines following lines terminating in backslashes (\) will be concatenated (en passant removing the backslashes). Initial white space on lines of the policy file is ignored.

DEFINE DIRECTIVES

DEFINE directives may be used to associate longer strings of text with certain symbols. E.g., after DEFINE FINDARGS -xdev -type f -exec /usr/bin/md5sum {} \; the text ${FINDARGS} may be used in USE DIRECTIVES and commands (see below) to use the text associated with the FINDARGS symbol.

Note that DEFINE symbols may be used in the definition of other DEFINE symbols as well. Direct or indirect circular definitions should be avoided, as they are either not or incompletely expanded.

USE DIRECTIVES

The following USE directives may be specified (directives are written in capitals, and should appear exactly as written below: letter casing is preserved). Specifications in angular brackets (like <this>) represent specifications to be given by users of stealth:

• USE BASE <basedirectory>
  BASE defines the directory from where stealth operates. All relative path specifications are interpreted relative to BASE. By default this is the directory where stealth was started.
  BASE is the only directory that must exist when stealth is started. All other non-existing paths are created automatically by stealth.
  Example:
  USE BASE /root/client

• USE DD <dd>
  The DD specification uses /bin/dd as default, and defines the location of the dd(1) program, both on the server and on the client. The bin(1) program is used to copy files between the client and the controller without opening separate ssh-connections. The program specified here is only used by stealth for the PUT and GET commands, described below.
  Example showing the default:
  USE DD /bin/dd

• USE DIFF <diff>
  The DIFF specification uses /usr/bin/diff as default, and defines the location of the diff(1) program on the controller. The diff(1) program is used to compare a formerly created logfile of an integrity check with a newly created logfile.
  Example showing the default:
  USE DIFF /usr/bin/diff

• USE EMAIL <address>
  The EMAIL specification defines the email-address to receive the report of the integrity scan of the client. The `dark cockpit' philosophy is followed here: mail is only sent when a modification is detected.
  Example showing the default (apparently an email address on the controller):
  USE EMAIL root

• USE MAILER <mailer>
  The MAILER specification defines the program that is used to send the mail to the EMAIL-address. Contrary to DIFF and DD and (see below) SH and SSH, MAILER is run as a /bin/sh command, to allow shell-scripts to process the mail too. By default MAILER is defined as /usr/bin/mail(1). MAILER is called with the following arguments:
  ----------------------------------------------------------
  MAILARGS, see below;
  EMAIL, the addressee of the mail.
  ----------------------------------------------------------
  Example showing the default:
  USE MAILER /usr/bin/mail

  As an alternative, the script stealthmail is provided. It offers a convenient filter sorting stealth's output and keeping only lines containing ADDED, MODIFIED, REMOVED and STEALTH. Usually these lines are are the ones system managers are interested in. The report and log files can always be consulted to determine the actual nature of the changes.

• USE MAILARGS <args>
  The MAILARGS specification defines the arguments that are passed to MAILER, followed by the specification of EMAIL. Example showing the default:
  USE MAILARGS -s "STEALTH scan report"
  Note that blanks may be used in the subject specification: use double or single quotes to define elements containing blanks. Use \" to use a double quote in a string that is itself delimted by double quotes, use \' to use a single quote in a string that is itself delimted by single quotes.

• USE REPORT <reportfile>
  REPORT defines the name of the reportfile. Information is always appended to this file. For each run of stealth a time marker line is written to the report file. Only when (in addition to the marker line) additional information is appended to the report file the added contents of the report file are mailed to the mail address specified in the USE EMAIL specification.
  Example showing the default:
  USE REPORT report

• USE SH <sh>
  The SH specification uses /bin/sh as default, and defines the command shell used by the controller to execute commands on itself.
  Example showing the default:
  USE SH /bin/sh

• USE SSH <user>
  The SSH specification has no default, and must be specified. Assuming the client trusts the controller (which is, after all, what this program is all about; so this should not be a very strong assumption), preferably the public ssh-identity key of the controller should be placed in the client's root .ssh/authorized_keys file, granting the controller root access to the client. Root access is normally needed to gain access to all directories and files of the client's file system.

  In practice, connecting to a account using the sh(1) shell is preferred. When another shell is already used by that account, one should make sure that that shell doesn't setup its own redirections for standard input and standard output. One way to accomplish that is for force the execution of /bin/sh in the USE SSH specification. Examples:

  
      # root's shell is /bin/sh:
          USE SSH root@client -T -q    
      # root uses another shell
          USE SSH root@client -T -q exec /bin/bash    
      # an alternative:
          USE SSH root@client -T -q exec /bin/bash --noprofile 
          
  

In some installations stealth is used to inspect the computer itself, even though this is not recommended, as it counters one of the main reasons for stealth's existence. In situations where stealth is used to monitor the integrity of the localhost, /bin/bash could be specified with the USE SSH directive. For example:

    # For stealth inspecting localhost:
        USE SSH /bin/bash --noprofile
        

COMMANDS

Following the USE specifications, commands can be specified. The commands are executed in their order of appearance in the policy file. Processing continues until the last command has been processed or until a tested command (see below) returns a non-zero return value.

LABEL COMMANDS

The following LABEL commands are available:

• LABEL <text>
  This defines a text-label which is written to the REPORT file, in front of the output generated by the next CHECK-command. If the next CHECK-command generates no output, the text-label is not written to the REPORT-file. Once a LABEL has been defined, it is used until it is redefined by the next LABEL. Use an empty LABEL specification to suppress the printing of labels.

  The text may contain \n characters (two characters) which are transformed to a newline character.

  Example:
  LABEL Inspecting files in /etc\nIncluding subdirectories
LABEL
  (The former LABEL specification clears the latter label text).

LOCAL COMMANDS

The following LOCAL commands are available to be executed on the controller:

• LOCAL <command>
  Execute command on the controller, using the SH command shell. The command must succeed (i.e., must return a zero exit value).
  Example:
  LOCAL scp rootsh@client:/usr/bin/md5sum /tmp
  This command will copy the client's md5sum(1) program to the controller.

• LOCAL NOTEST <command>
  Execute command on the controller, using the SH command shell. The command may or may not succeed.
  Example:
  LOCAL NOTEST mkdir /tmp/subdir
  This command will create /tmp/subdir on the controller. The command will fail if the directory cannot be created, but this will not terminate stealth.

• LOCAL CHECK [LOG =] <logfile> <command>
  Execute command on the controller, using the SH command shell. The command must succeed. The output of this command is compared to the output of this command generated during the previous run of stealth. The phrase LOG = is optional. Any differences are written to REPORT. If differences were found, the existing logfile name is renamed to logfile.YYMMDD-HHMMSS, with YYMMDD-HHMMSS the datetime-stamp at the time stealth was run.

  Note that eventually many logfile.YYMMDD-HHMMSS files could be created: It is up to the controller's systems manager to decide what to do with old datetime-stamped logfiles.

  The logfile specifications may use relative and absolute paths. When relative paths are used, these paths are relative to BASE. When the directories implied by the logfile specifications do not yet exist, they are created first.

  Example:
  LOCAL CHECK LOG = local/md5sum md5sum /tmp/md5sum
  This command will check the MD5 sum of the /tmp/md5sum program. The resulting output is saved at BASE/local/md5sum. The program must succeed (i.e., md5sum must return a zero exit-value).

• LOCAL NOTEST CHECK <logfile> <command>
  Execute command on the controller, using the SH command shell. The command may or may not succeed. Otherwise, the program acts identically as the LOCAL CHECK ... command, discussed previously.

  Example:
  LOCAL NOTEST CHECK LOG=local/md5sum md5sum /tmp/md5sum
  This command will check the MD5 sum of the /tmp/md5sum program. The resulting output is saved at BASE/local/md5sum. The program must succeed (i.e., md5sum must return a zero exit-value).

Note that the scp(1) command can be used to copy files between the client and the controller, using a local command. This, however, is discouraged, as a separate ssh(1)-connection is required for each separate scp(1) command. This subtlety was brought to the author's attention by Hopko Meijerink (h.meijering@rc.rug.nl). Using scp(1) results in several additional entries showing sshd(1) connections in the client's logfiles, which in turn may provide hints to a hacker that the client is intensively monitored. In order to copy files between the client and the controller, the GET and PUT commands (described below) may be used, which use the existing ssh(1) connection. In general, LOCAL commands should not be used to establish additional ssh(1) connections to a client.

REMOTE COMMANDS

Remote commands are commands executed on the client using the SSH shell. These commands are executed using the standard PATH set for the SSH shell. However, it is advised to specify the full pathname to the programs to be executed, to prevent ``trojan approaches'' where a trojan horse is installed in an `earlier' directory of the PATH-specification than the intended program.

Two special remote commands are GET and PUT, which can be used to copy files between the client and the controller. Internally, GET and PUT use the DD use-specification. If a non-default specification is used, one should ensure that the alternate program accepts dd(1)'s if=, of=, bs= and count= options. With GET the options bs=, count= and of= are used, with PUT the options bs=, count= and if= are used. Normally there should be no need to alter the default DD specification.

The GET command may be used as follows:

• GET <client-path> <local-path>
  Copy the file indicated by client-path at the client to local-path at the controller. client-path must be the full path of an existing file on the client, local-path may either be a local directory, in which case the client's file name is used, or another file name may be specified, in which case the client's file is copied to the specified local filename. If the local file already exists, it is overwritten by the copy-procedure.

  Example:
  GET /usr/bin/md5sum /tmp
  The program /usr/bin/md5sum, available at the client, is copied to the controller's /tmp directory. If the copying fails for some reason, any subsequent commands are skipped, and stealth terminates.

• GET NOTEST <client-path> <local-path>
  Copy the file indicated by client-path at the client to local-path at the controller. client-path must be the full path of an existing file on the client, local-path may either be a local directory, in which case the client's file name is used, or another file name may be specified, in which case the client's file is copied to the specified local filename. If the local file already exists, it is overwritten by the copy-procedure.

  Example:
  GET NOTEST /usr/bin/md5sum /tmp
  The program /usr/bin/md5sum, available at the client, is copied to the controller's /tmp directory. Remaining commands in the policy file are executed, even if the copying process wasn't successful.

The PUT command may be used as follows:

• PUT <local-path> <remote-path>
  Copy the file indicated by local-path at the controller to remote-path at the client. The argument local-path must be the full path of an existing file on the controller. The argument remote-path must be the full path to a file on the client. If the remote file already exists, it is overwritten by PUT.

  Example:
  PUT /tmp/md5sum /usr/bin/md5sum
  The program /tmp/md5sum, available at the controller, is copied to the client as usr/bin/md5sum. If the copying fails for some reason, any subsequent commands are skipped, and stealth terminates.

• PUT NOTEST <local-path> <remote-path>
  Copy the file indicated by local-path at the controller to remote-path at the client. The argument local-path must be the full path of an existing file on the controller. The argument remote-path must be the full path to a file on the client. If the remote file already exists, it is overwritten by PUT.

  Example:
  PUT NOTEST /tmp/md5sum /usr/bin/md5sum
  Copy the file indicated by local-path at the controller to remote-path at the client. The argument local-path must be the full path of an existing file on the controller. The argument remote-path must be the full path to a file on the client. If the remote file already exists, it is overwritten by PUT. Remaining commands in the policy file are executed, even if the copying process wasn't successful.

Plain commands can be executed on the client computer by merely specifying them. Of course, this means that programs on the client called, e.g., LABEL, LOCAL or USE, cannot be executed, since these names are interpreted otherwise by stealth. I don't think that represents much of a problem, though....

The following commands are available to be executed on the client:

• <command>
  Execute command on the client, using the SSH command shell. The command must succeed (i.e., must return a zero exit value). However, any output generated by the the command is ignored.
  Example:
  /usr/bin/find /tmp -type f -exec /bin/rm {} \;
  This command will remove all ordinary files in and below the client's /tmp directory.

• NOTEST <command>
  Execute command on the client, using the SSH command shell. The command may or may not succeed.
  Example:
  NOTEST /usr/bin/find /tmp -type f -exec /bin/rm {} \;
  Same as the previous command, but this time the exit value of /usr/bin/find is not interpreted.

• CHECK [LOG =] <logfile> <command>
  Execute command on the client, using the SSH command shell. The phrase LOG = is optional. The command must succeed. The output of this command is compared to the output of this command generated during the previous run of stealth. Any differences are written to REPORT. If differences were found, the existing logfile name is renamed to logfile.YYMMDD-HHMMSS, with YYMMDD-HHMMSS the datetime-stamp at the time stealth was run.

  Note that the command is executed on the client, but the logfile is kept on the controller. This command represents the core of the method implemented by stealth: there will be no residues of the actions performed by stealth on the client computers.

  Several examples (note the use of the backslash as line continuation characters):

  CHECK LOG = remote/ls.root \
/usr/bin/find / \
-xdev -perm +6111 -type f -exec /bin/ls -l {} \;

  All suid/gid/executable files on the same device as the root-directory (/) on the client computer are listed with their permissions, owner and size information. The resulting listing is written on the file BASE/remote/ls.root.

  CHECK remote/md5.root \
/usr/bin/find / \
-xdev -perm +6111 -type f -exec /usr/bin/md5sum {} \;

  The MD5 checksums of all suid/gid/executable files on the same device as the root-directory (/) on the client computer are determined. The resulting listing is written on the file BASE/remote/md5.root.

• NOTEST CHECK [LOG =] <logfile> <command>
  Execute command on the client, using the SSH command shell. The phrase LOG = is optional. The command may or may not succeed. Otherwise, the program acts identically as the CHECK ... command, discussed previously.

  Example:
  NOTEST CHECK LOG = remote/md5.root \
/usr/bin/find / \
-xdev -perm +6111 -type f -exec /usr/bin/md5sum {} \;

  The MD5 checksums of all suid/gid/executable files on the same device as the root-directory (/) on the client computer are determined. The resulting listing is written on the file BASE/remote/md5.root. stealth will not terminate if the /usr/bin/find program returns a non-zero exit value.

OPTIONS

Long options are given immediately following the short-option equivalents, if available. Either can be used.

• -d --debug: Write debug messages to std error;
• -c --parse-config-file: Process the config file, no further action,
  report the results to std output;
• -e --echo-commands: echo commands to std error when they are processed (implied by -d);
• -i --random-interval <interval>[m]>: start the scan a random interval of <interval> seconds (or minutes if an `m' is appended to <interval>) following the delay specified at --repeat (see below). This option is ignored unless --repeat is provided as well.
• -n --no-child-processes: No child processes are executed: child actions
  are faked to be OK.
• -o --only-stdout: Scan report is written to stdout. No mail is sent.
  (implied by -d);
• -q --quiet: Suppress progress messages written to stderr;
• -r --run-command <nr>: Only run command <nr> (natural number). Command numbers are shown by stealth -c;
• -v --version: Display version information and exit;
• --keep-alive pidfile: Keep running as a daemon, wake up at interrupts.
• --repeat <seconds>: keep running as a daemon, wake up at interrupts or after <seconds> seconds. The interval will be at least 60 seconds. To this interval a random delay may be added (see --random-interval).
• --rerun pidfile: restart the scan of a currently active stealth process;
• --resume pidfile: resume a suppressed stealth process, implying --rerun;
• --suppress pidfile: suppress a currently active stealth process. All scheduled scans following --suppress are skipped, --rerun is ignored, but --resume and --terminate (see below) may be issued;
• --terminate pidfile: terminate a currently active stealth process;
• --usage: Display help information and exit;
• --help: Display help information and exit;
• pidfile: file containing the process id of a stealth process;
• policy: path to the policyfile;

DEPLOYMENT SUMMARY

• Install stealth (e.g., use dpkg(1) to install the .deb file);
• Construct one or more policy files;
• Automate running stealth using cron(1) (possibly calling stealthcron);
• Set up automated log-file rotation, using, e.g., stealthcleanup and logrotate(1), defining one or more /etc/logrotate.d/stealth... configuration files.

FILES

/usr/share/doc/stealth/;
the policy file;
files under the BASE directory as defined in the policy file;
the report file as defined by the policy's USE REPORT directive.

SEE ALSO

DIAGNOSTICS

BUGS

COPYRIGHT

ORGANIZATION

AUTHOR