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

News SUID Attribute Recommended Links File Permissions Hardening World writable files problem
Finding SUID/SGUID files Findsuid Suidcontrol Humor Etc

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.

In ls listing file permissions for file with  SUID attribute looks like: -rwsr-xr-x

Examples of SUID root programs:

Thomas Akin's Seven Rules for Safe SUID Programming
  1. Do not use SUID shell scripts.
  2. Never use SUID C-shell scripts.
  3. Always manually set your internal field separator (IFS).
  4. Always manually set your PATH and use absolute path names.
  5. Understand how programs you call work, and how they handle arguments.
  6. Do not use temporary files. If you must, don't put them in a publicly writeable area.
  7. Distrust and check all user input and eliminate dangers such as meta-characters.

SUID/SGID files can be a security hazard. To reduce the risks, we have previously already removed the s  bits from root-owned programs that won't absolutely require such privilege, but future and existing files may be set with these s  bits enabled without your notification.

SGID stands for set group id. When looking at files SGID they behave much the same as SUID files, and must be executable for it to have any effect. The SGID bit on a directory means files created in that directory will have their group set to the directory's group.

You can 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 actually by using 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
}

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Old News ;-)

Gentoo Linux Documentation -- File Permissions

  • 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
  •  

    Recommended Links

    Softpanorama hot topic of the month

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    fix-modes

    Casper Dik wrote a tool consiting of a set of scripts which modifies file permissions in an effort to make things more secure. The tool removes group and world write permissions of all files, devices, and directories listed in /var/sadm/install/contents, with the exception of those files listed in exceptions.h, and changes ownership of most files to root.

    NOTE: the script has been tested under Solaris 2.2-8 Operating Environment and will purposely fail on other releases. Casper Dik works for Sun in Holland, but the tool is not officially recognized by Sun so it is distributed from Casper's personal account.

     

    An advantage in using fix-modes is it creates an audit trail and the changes can be undone which is important if you are embarking on an upgrade.

     

    The fix-modes tar-bundle comes with a README with installation instructions.

    Download fix-modes

    Alternate download from CERIAS for fix-modes

    There is a Sun BluePrints[tm] paper discussing fix-modes.

    Fix-modes is incorporated in JumpStart[tm] Architecture and Security Scripts (JASS) toolkit.

    Fix-modes is also part of the Titan OS hardening tool.

    University of Waterloo also has some Bourne Shell scripts for Solaris 2.6, Solaris 7 and Solaris 8 Setuid/Setgid in addition to other Security: How to Documents"

    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

     



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    Created: May 16, 1997; Last modified: August 05, 2013