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suEXEC Support - Apache HTTP Server Version 2.4








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Apache HTTP Server Version 2.4



Apache > HTTP Server > Documentation > Version 2.4suEXEC Support

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    The suEXEC feature provides users of the Apache
    HTTP Server the ability
    to run CGI and SSI programs
    under user IDs different from the user ID of the calling
    web server. Normally, when a CGI or SSI program executes, it
    runs as the same user who is running the web server.

    Used properly, this feature can reduce
    considerably the security risks involved with allowing users to
    develop and run private CGI or SSI programs. However, if suEXEC
    is improperly configured, it can cause any number of problems
    and possibly create new holes in your computer's security. If
    you aren't familiar with managing setuid root programs
    and the security issues they present, we highly recommend that
    you not consider using suEXEC.
  
 Before we begin
 suEXEC Security Model
 Configuring & Installing
    suEXEC
 Enabling & Disabling
    suEXEC
 Using suEXEC
 Debugging suEXEC
 Beware the Jabberwock:
    Warnings & Examples
See alsoComments


Before we begin

    Before jumping head-first into this document,
    you should be aware that certain assumptions are made about you and
    the environment in which you will be using suexec.

    First, it is assumed that you are using a UNIX
    derivative operating system that is capable of
    setuid and setgid operations.
    All command examples are given in this regard. Other platforms,
    if they are capable of supporting suEXEC, may differ in their
    configuration.

    Second, it is assumed you are familiar with
    some basic concepts of your computer's security and its
    administration. This involves an understanding of
    setuid/setgid operations and the various
    effects they may have on your system and its level of
    security.

    Third, it is assumed that you are using an
    unmodified version of suEXEC code. All code
    for suEXEC has been carefully scrutinized and tested by the
    developers as well as numerous beta testers. Every precaution
    has been taken to ensure a simple yet solidly safe base of
    code. Altering this code can cause unexpected problems and new
    security risks. It is highly recommended you
    not alter the suEXEC code unless you are well versed in the
    particulars of security programming and are willing to share
    your work with the Apache HTTP Server development team for consideration.

    Fourth, and last, it has been the decision of
    the Apache HTTP Server development team to NOT make suEXEC part of
    the default installation of Apache httpd. To this end, suEXEC
    configuration requires of the administrator careful attention
    to details. After due consideration has been given to the
    various settings for suEXEC, the administrator may install
    suEXEC through normal installation methods. The values for
    these settings need to be carefully determined and specified by
    the administrator to properly maintain system security during
    the use of suEXEC functionality. It is through this detailed
    process that we hope to limit suEXEC
    installation only to those who are careful and determined
    enough to use it.

    Still with us? Yes? Good. Let's move on!


suEXEC Security Model

    Before we begin configuring and installing
    suEXEC, we will first discuss the security model you are about
    to implement. By doing so, you may better understand what
    exactly is going on inside suEXEC and what precautions are
    taken to ensure your system's security.

    suEXEC is based on a setuid
    "wrapper" program that is called by the main Apache HTTP Server.
    This wrapper is called when an HTTP request is made for a CGI
    or SSI program that the administrator has designated to run as
    a userid other than that of the main server. When such a
    request is made, Apache httpd provides the suEXEC wrapper with the
    program's name and the user and group IDs under which the
    program is to execute.

    The wrapper then employs the following process
    to determine success or failure -- if any one of these
    conditions fail, the program logs the failure and exits with an
    error, otherwise it will continue:

    
      
        Is the user executing this wrapper a valid user of
        this system?

        
          This is to ensure that the user executing the wrapper is
          truly a user of the system.
        
     

     
        Was the wrapper called with the proper number of
        arguments?

        
          The wrapper will only execute if it is given the proper
          number of arguments. The proper argument format is known
          to the Apache HTTP Server. If the wrapper is not receiving
          the proper number of arguments, it is either being
          hacked, or there is something wrong with the suEXEC
          portion of your Apache httpd binary.
        
      

      
        Is this valid user allowed to run the
        wrapper?

        
          Is this user the user allowed to run this wrapper? Only
          one user (the Apache user) is allowed to execute this
          program.
        
      

      
        Does the target CGI or SSI program have an unsafe
        hierarchical reference?

        
          Does the target CGI or SSI program's path contain a leading
          '/' or have a '..' backreference? These are not allowed; the
          target CGI/SSI program must reside within suEXEC's document
          root (see --with-suexec-docroot=DIR
          below).
        
      

      
        Is the target user name valid?

        
          Does the target user exist?
        
      

      
        Is the target group name valid?

        
          Does the target group exist?
        
      

      
        Is the target user NOT superuser?


        
          suEXEC does not allow root
          to execute CGI/SSI programs.
        
      

      
        Is the target userid ABOVE the minimum ID
        number?

        
          The minimum user ID number is specified during
          configuration. This allows you to set the lowest possible
          userid that will be allowed to execute CGI/SSI programs.
          This is useful to block out "system" accounts.
        
      

      
        Is the target group NOT the superuser
        group?

        
          Presently, suEXEC does not allow the root
          group to execute CGI/SSI programs.
        
      

      
        Is the target groupid ABOVE the minimum ID
        number?

        
          The minimum group ID number is specified during
          configuration. This allows you to set the lowest possible
          groupid that will be allowed to execute CGI/SSI programs.
          This is useful to block out "system" groups.
        
      

      
        Can the wrapper successfully become the target user
        and group?

        
          Here is where the program becomes the target user and
          group via setuid and setgid calls. The group access list
          is also initialized with all of the groups of which the
          user is a member.
        
      

      
        Can we change directory to the one in which the target
        CGI/SSI program resides?

        
          If it doesn't exist, it can't very well contain files. If we
          can't change directory to it, it might as well not exist.
        
      

      
        Is the directory within the httpd webspace?

        
          If the request is for a regular portion of the server, is
          the requested directory within suEXEC's document root? If
          the request is for a UserDir, is the requested directory
          within the directory configured as suEXEC's userdir (see
          suEXEC's configuration options)?
        
      

      
        Is the directory NOT writable by anyone
        else?

        
          We don't want to open up the directory to others; only
          the owner user may be able to alter this directories
          contents.
        
      

      
        Does the target CGI/SSI program exist?

        
          If it doesn't exists, it can't very well be executed.
        
      

      
        Is the target CGI/SSI program NOT writable
        by anyone else?

        
          We don't want to give anyone other than the owner the
          ability to change the CGI/SSI program.
        
      

      
        Is the target CGI/SSI program NOT setuid or
        setgid?

        
          We do not want to execute programs that will then change
          our UID/GID again.
        
      

      
        Is the target user/group the same as the program's
        user/group?

        
          Is the user the owner of the file?
        
      

      
        Can we successfully clean the process environment
        to ensure safe operations?

        
          suEXEC cleans the process' environment by establishing a
          safe execution PATH (defined during configuration), as
          well as only passing through those variables whose names
          are listed in the safe environment list (also created
          during configuration).
        
      

      
        Can we successfully become the target CGI/SSI program
        and execute?

        
          Here is where suEXEC ends and the target CGI/SSI program begins.
        
      
    

    This is the standard operation of the
    suEXEC wrapper's security model. It is somewhat stringent and
    can impose new limitations and guidelines for CGI/SSI design,
    but it was developed carefully step-by-step with security in
    mind.

    For more information as to how this security
    model can limit your possibilities in regards to server
    configuration, as well as what security risks can be avoided
    with a proper suEXEC setup, see the "Beware the Jabberwock" section of this
    document.


Configuring & Installing
    suEXEC

    Here's where we begin the fun.

    suEXEC configuration
    options
    

    
      --enable-suexec

      This option enables the suEXEC feature which is never
      installed or activated by default. At least one
      --with-suexec-xxxxx option has to be provided
      together with the --enable-suexec option to let
      APACI accept your request for using the suEXEC feature.

      --with-suexec-bin=PATH

      The path to the suexec binary must be hard-coded
      in the server for security reasons. Use this option to override
      the default path. e.g.
      --with-suexec-bin=/usr/sbin/suexec

      --with-suexec-caller=UID

      The username under which
      httpd normally runs. This is the only user allowed to
      execute the suEXEC wrapper.

      --with-suexec-userdir=DIR

      Define to be the subdirectory under users' home
      directories where suEXEC access should be allowed. All
      executables under this directory will be executable by suEXEC
      as the user so they should be "safe" programs. If you are
      using a "simple" UserDir
      directive (ie. one without a "*" in it) this should be set to the same
      value. suEXEC will not work properly in cases where the UserDir directive points to
      a location that is not the same as the user's home directory
      as referenced in the passwd file. Default value is
      "public_html".
      If you have virtual hosts with a different UserDir for each,
      you will need to define them to all reside in one parent
      directory; then name that parent directory here. If
      this is not defined properly, "~userdir" cgi requests will
      not work!

      --with-suexec-docroot=DIR

      Define as the DocumentRoot set for httpd. This will be
      the only hierarchy (aside from UserDirs) that can be used for suEXEC behavior. The
      default directory is the --datadir value with the suffix
      "/htdocs", e.g. if you configure with
      "--datadir=/home/apache" the directory
      "/home/apache/htdocs" is used as document root for the
      suEXEC wrapper.

      --with-suexec-uidmin=UID

      Define this as the lowest UID allowed to be a target user
      for suEXEC. For most systems, 500 or 100 is common. Default
      value is 100.

      --with-suexec-gidmin=GID

      Define this as the lowest GID allowed to be a target
      group for suEXEC. For most systems, 100 is common and
      therefore used as default value.

      --with-suexec-logfile=FILE

      This defines the filename to which all suEXEC
      transactions and errors are logged (useful for auditing and
      debugging purposes). By default the logfile is named
      "suexec_log" and located in your standard logfile
      directory (--logfiledir).

      --with-suexec-safepath=PATH

      Define a safe PATH environment to pass to CGI
      executables. Default value is
      "/usr/local/bin:/usr/bin:/bin".
    

    Compiling and installing the suEXEC wrapper
      

      If you have enabled the suEXEC feature with the
      --enable-suexec option the suexec binary
      (together with httpd itself) is automatically built if you execute
      the make command.

      After all components have been built you can execute the
      command make install to install them. The binary image
      suexec is installed in the directory defined by the
      --sbindir option. The default location is
      "/usr/local/apache2/bin/suexec".

      Please note that you need root
      privileges for the installation step. In order
      for the wrapper to set the user ID, it must be installed as
      owner root and must have the setuserid
      execution bit set for file modes.
    

    Setting paranoid permissions
      

      Although the suEXEC wrapper will check to ensure that its
      caller is the correct user as specified with the
      --with-suexec-caller configure
      option, there is
      always the possibility that a system or library call suEXEC uses
      before this check may be exploitable on your system. To counter
      this, and because it is best-practise in general, you should use
      filesystem permissions to ensure that only the group httpd
      runs as may execute suEXEC.

      If for example, your web server is configured to run as:

      User www
Group webgroup


      and suexec is installed at
      "/usr/local/apache2/bin/suexec", you should run:

      
          chgrp webgroup /usr/local/apache2/bin/suexec
          chmod 4750 /usr/local/apache2/bin/suexec
      

      This will ensure that only the group httpd runs as can even
      execute the suEXEC wrapper.
    


Enabling & Disabling
    suEXEC

    Upon startup of httpd, it looks for the file
    suexec in the directory defined by the
    --sbindir option (default is
    "/usr/local/apache/sbin/suexec"). If httpd finds a properly
    configured suEXEC wrapper, it will print the following message
    to the error log:


    [notice] suEXEC mechanism enabled (wrapper: /path/to/suexec)


    If you don't see this message at server startup, the server is
    most likely not finding the wrapper program where it expects
    it, or the executable is not installed setuid root.

     If you want to enable the suEXEC mechanism for the first time
    and an Apache HTTP Server is already running you must kill and
    restart httpd. Restarting it with a simple HUP or USR1 signal
    will not be enough. 
     If you want to disable suEXEC you should kill and restart
    httpd after you have removed the suexec file.


Using suEXEC

    Requests for CGI programs will call the suEXEC wrapper only if
    they are for a virtual host containing a SuexecUserGroup directive or if
    they are processed by mod_userdir.

    Virtual Hosts: One way to use the suEXEC
    wrapper is through the SuexecUserGroup directive in
    VirtualHost definitions.  By
    setting this directive to values different from the main server
    user ID, all requests for CGI resources will be executed as the
    User and Group defined for that <VirtualHost>. If this
    directive is not specified for a <VirtualHost> then the main server userid
    is assumed.

    User directories: Requests that are
     processed by mod_userdir will call the suEXEC
     wrapper to execute CGI programs under the userid of the requested
     user directory.  The only requirement needed for this feature to
     work is for CGI execution to be enabled for the user and that the
     script must meet the scrutiny of the security
     checks above.  See also the
     --with-suexec-userdir compile
     time option. 

Debugging suEXEC

    The suEXEC wrapper will write log information
    to the file defined with the --with-suexec-logfile
    option as indicated above. If you feel you have configured and
    installed the wrapper properly, have a look at this log and the
    error_log for the server to see where you may have gone astray.



Beware the Jabberwock:
    Warnings & Examples

    NOTE! This section may not be
    complete. For the latest revision of this section of the
    documentation, see the Online
    Documentation version.

    There are a few points of interest regarding
    the wrapper that can cause limitations on server setup. Please
    review these before submitting any "bugs" regarding suEXEC.

    
      suEXEC Points Of Interest

      
        Hierarchy limitations

        
          For security and efficiency reasons, all suEXEC requests
          must remain within either a top-level document root for
          virtual host requests, or one top-level personal document
          root for userdir requests. For example, if you have four
          VirtualHosts configured, you would need to structure all
          of your VHosts' document roots off of one main httpd
          document hierarchy to take advantage of suEXEC for
          VirtualHosts. (Example forthcoming.)
        
      

      
        suEXEC's PATH environment variable

        
          This can be a dangerous thing to change. Make certain
          every path you include in this define is a
          trusted directory. You don't want to
          open people up to having someone from across the world
          running a trojan horse on them.
        
      

      
        Altering the suEXEC code

        
          Again, this can cause Big Trouble if you
          try this without knowing what you are doing. Stay away
          from it if at all possible.
        
      
    



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CommentsNotice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.

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