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Checking for special and abnormal files (SUID/SGID, word-writable, hidden, etc)

News Recommended Links      
Thor Findsuid Suidcontrol    
 

What is SUID?

SUID stands for set user id. When a SUID file executed, the process which runs it is granted access to system resources based on the user who owns the file and not the user who created the process. When a file is SUID root it allows a program/script to perform functions that regular users are not allowed to do themselves. Many buffer overflow exploits are the result of SUID programs.

  • SGID

     
     
  • LOCATING SUID and SGID PROGRAMS

     

    Use find to obtain a list of set-UID and set-GID prorgrams installed on a system

    # error : it will rescan every partition 

    for part in \  
    `awk '($4 == "ufs") {print $3 }' /etc/vfstab` 
do  
    find $part \( -perm -04000 -o -perm -02000\) \ 
        -type f -xdev -print 
done

    • :

The proper way of doing this is actully not find by ncheck
    
Checksuid () {

	if [ -x /usr/sbin/ncheck ]; then
	current_suid=`/usr/sbin/ncheck -F ufs -s |wc -l`
	known_suid=276
	if [ ${known_suid} != ${current_suid} ]; then
                echo "Current listing of SUID files does not match the known listing - FAILS CHECK"
               echo "   "
		 exit 1
		else
		echo " Current SUID listing equals known listing - PASSES CHECK"
	fi
	fi
}

    Recommended Links

    In case of broken links please try to use Google search. If you find the page please notify us about new location
    Google     
  • fix-modes

     

    •  

    Thor

    [DISAPPEARED FROM THE WEB] thor.pl keeps tabs on suid and sgid files on your file system. It also keeps track of the checksums of your binaries and the root accounts on the system as well as a few other things. It's a handy script that helps you find possible security risks, or breakins.

    Download: http://www.linuxscripts.com/arc/thor1.0.tar.gz
    Homepage: http://www.linuxscripts.com/

    Findsuid

    findsuid, pcheck Directory of -pub-unix-sec8 This little shell script can be adapted to run from cron in oprder to report setuid and setgid changes (very handy and simple script because main enemy of sysadmins is not hackers, but he himself and his colleagues ;-). The directory contains a lot of other useful for sysadmin scripts ! Here is full INDEX

    Suidcontrol

    23-Aug-98 Suidcontrol-0.1 utility has been released. The suidcontrol is an experimental utility for managing suid/sgid policy under FreeBSD. It actually generated the list and script to check it. In this particular case the idea is not that impressive. http://www.watson.org/fbsd-hardening/suidcontrol.html

     

    Notes:
    • Those pages are written by people for whom English is not a native language. Some amount of grammar and spelling errors should be expected.
    • This is a Spartan WHYFF (We Help You For Free) site. It cannot replace the best teachers and the best books.
    • The site contain some obsolete pages as it develops like a living tree... Some links on older pages are broken. Please try to use Google, Open directory, etc. to find a replacement link (see HOWTO search the WEB for details). We would appreciate if you can mail us a correct link.

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    Open directory
    Research Index
    		 

    News

    Gentoo Linux Documentation -- File Permissions

    6.b. World/Group writable

    Code Listing 1: Finding world-writable files and directories

    		 
    # /usr/bin/find / -type f \( -perm -2 -o -perm -20 \) \ 
   -exec ls -lg {} \; 2>/dev/null >writable.txt
# /usr/bin/find / -type d \( -perm -2 -o -perm -20 \) \ 
   -exec ls -ldg {} \; 2>/dev/null >>writable.txt

    This will create a huge file with permission of all files having either write permission set to the group or everybody. Check the permissions and eliminate world writable files to everyone, by executing /bin/chmod o-w on the files.

    6.c. SUID/SGID files

    Files with the SUID or SGID bit set execute with privileges of the owning user or group and not the user executing the file. Normally these bits are used on files that must run as root in order to do what they do. These files can lead to local root compromises (if they contain security holes). This is dangerous and files with the SUID or SGID bits set should be avoided at any cost. If you do not use these files, use chmod 0 on them or unmerge the package that they came from (check which package they belong to by using equery; if you do not already have it installed simply type emerge gentoolkit). Otherwise just turn the SUID bit off with chmod -s.

    Code Listing 2: Finding setuid files

    		 
    # /usr/bin/find / -type f \( -perm -004000 -o -perm -002000 \) \ 
   -exec ls -lg {} \; 2>/dev/null >suidfiles.txt

    This will create a file containing a list of all the SUID/SGID files.

     

    To find all SGID files:
    find / -xdev -type f -perm +g=s -print

    To find all SUID files
    find / -xdev -type f -perm +u=s -print

    To find all Writable dirs:
    find / -xdev -perm +o=w ! \( -type d -perm +o=t \) ! -type l -print

    World writable files problem

    You can use the find command to search for files that are group writable by a particular group, and to print a list of these files. For example, to search for all files that are writable by the group user, you might specify a command in the following form:

    # find / -perm -020 -group user \!   
\( -type l -o -type p -o -type s \) -ls

    If you have NFS, be sure to use the longer version of the command:

    # find / \( -local -o -prune \) -perm -002 -group user \!   
\( -type l -o -type p -o -type s \) -ls

    A more security-conscious site can further generalize this rule:

    Files that begin with a period should not be readable or writable by anyone other than the file's owner and group  (that is, they should be mode 620).

    Use the following form of the find command to search for all files beginning with a period in the /u filesystem that are either group writable or world writable:

    # find /u -perm -2 -o -perm -20 -name .\* -ls

    NOTE: As noted earlier, if you're using NFS, be sure to add the -local or -xdev option to each of the find commands above and run them on each of your servers, or use the fstype/prune options.

     

    Shell script to find all world-writable files and directories on Linux or FreeBSD system

    #!/bin/bash
    # Shell script to find all world-writable files and directories on Linux or
    # FreeBSD system
    #
    # TIP:
    # Set 'umask 002' so that new files created will not be world-writable
    # And use command 'chmod o-w filename' to disable world-wriable file bit
    #
    # Copyright (c) 2005 nixCraft project
    # This script is licensed under GNU GPL version 2.0 or above
    # For more info, please visit:
    #            http://cyberciti.biz/shell_scripting/bmsinstall.php
    # -------------------------------------------------------------------------
    # This script is part of nixCraft shell script collection (NSSC)
    # Visit http://bash.cyberciti.biz/ for more information.
    # -------------------------------------------------------------------------
    SPATH="/usr/local/etc/bashmonscripts"
    INITBMS="$SPATH/defaults.conf"
    [ ! -f $INITBMS ] && exit 1 || . $INITBMS

    [ $# -eq 1 ] && : || die "Usage: $($BASENAME $0) directory" 1

    DIRNAME="$1"
    $FIND $DIRNAME -xdev -perm +o=w ! \( -type d -perm +o=t \) ! -type l -print
     

    find / -perm -2 -a ! -type l
    /* Find files writable by 'others' */     		2005-04-12
    find / -perm -0777 -type d -ls
    /* Find all your writable directories */     		2005-03-22

     

    Hidden Files

    In Unix, "hidden" files are ones with a leading dot, e.g. ".ssh". These don't show up in a normal directory listing (ls), but do with the "all" option (ls -a). Usually, these are used to hide things like login scripts and control files to prevent one's home directory appearing cluttered, but may be used by intruders to hide files.

    Normal Unix commands such as find and ls may be used to locate such files, e.g. 

    find /usr -name '.*' -type d -print
    to find "hidden" files and directories. find may also be used to search by modification time, e.g. 
    find /bin -ctime -365 -print
    will find anything changed in the last year, similarly for /sbin /usr/kvm /usr/local/bin etc.

    In Win95, hidden files are also possible; BackOrifice uses this technique to hide, using a blank icon and blank prefix, ".exe" does not show up in a icon-based directory.

    suid programs

    suid root programs are an especial concern, since they execute with root privilege. If such a program can be fooled into executing an arbitrary command, perhaps by a buffer-overrun exploit, it can be used to create new user IDs or open network holes. On some systems (gnu) find may be used to locate these programs, e.g. 
    find /bin -perm +6000 -print
    or perhaps 
    find /bin -perm +6000 -exec ls -lg {} \;
    On systems where find does not support the perm option, ls may be used, e.g. 
    ls -latg /usr/bin/* /sbin/* /usr/sbin/* /usr/local/bin/* | grep '^...s'
    The point here is to find system files that appear to have been modified since the system was installed, or unauthorized programs, such as an suid shell (which executes a user's every command with root privilege).

    Precautions

    Non-system disk volumes should be mounted nosuid. This is a special concern for Linux users who may have obsolete live systems mounted on CD-ROM.

    Data disks may be mounted noexec. This means that files in these directories cannot be executed.

    Linux users may use the ext2fs utility chattr to make system directories or files "immutable", or create append-only logfiles. An intruder would first have to gain root, then change the filesystem attributes, before creating a file.

    Up to Security Page

     

    A.Daviel
    andrew@vancouver-webpages.com

     

     

     

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    Created: May 16, 1997; Last modified: June 05, 2008