The Hacker's Encyclopedia 1998

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This information is important to most people who are considering setting up an anonymous FTP site with their Linux system. It was written by Chistopher Klaus and is posted to the comp.security.misc news group. I am submitting this to the doc directory of sunsite (or so I hope that will be its home) because many people will check there first before looking elsewhere. From: cklaus@anshar.shadow.net (Christopher Klaus) Subject: computer-security/anonymous-ftp FAQ Keywords: security anonymous ftp Lines: 575 Reply-To: cklaus@shadow.net Organization: ISS, Inc. Date: Tue, 26 Jul 1994 23:10:55 GMT Archive-name: computer-security/anonymous-ftp-faq Post-Frequency: monthly Last-modified: 1994/7/24 Version: 1.4 How to set up a Secure Anonymous FTP Site The following is a FAQ on setting up a secure FTP Site. FTP sites are known for much abuse by transferring illegal files. They also open many oppurtunities for intruders to gain access via misconfigured setups. And lastly many versions of ftp servers have had security holes. This FAQ is intended to clean up this abuse by allowing administrators to go through this check list of steps to make sure their FTP is correctly configured and that they are running the most current ftp daemon. This is organized in the following fashion, I am breaking into several parts as follows: Part 1 - General Description of Setting up an "anonymous" ftp server. Part 2 - Setting up a chrooted Secure Anonymous ftp server. Part 3 - OS Specific needed information and suggestions. Part 4 - Where to get other FTP daemons Part 5 - How to Know if your Anonymous FTP Server is secure Part 6 - Archie Part 7 - Acknowledgements. Part 1 - General Description of Setting up an "anonymous" ftp server. How do I setup "anonymous" ftp securely? PLEASE READ ALL NOTES AND WARNINGS!!! 1) Create the user ftp in /etc/passwd. Use a misc group. The user's home directory will be ~ftp where ~ftp is the root you wish anonymous users to see. Creating this user turns on anonymous ftp. Use an invalid password and user shell for better security. The entry in the passwd file should look something like: ftp:*:400:400:Anonymous FTP:/home/ftp:/bin/true 2) Create the home directory ~ftp. Make the directory owned by root (NOT ftp) with the same group as ftp. Thus, owner permissions are for root and group permissions are for the anonymous users. Set the permissions for ~ftp to 555 (read, nowrite, execute). *** Some MAN pages recommend making the ~ftp directory owned by ftp. *** This is a big NO-NO, if you want any type of security on your system. 3) Create the directory ~ftp/bin. This directory is owned by root (group e.g. wheel) with permissions 111 (noread, nowrite, execute). 4) Copy the program ls into ~ftp/bin. ls is owned by root with permissions 111 (noread, nowrite, execute). Any other commands you put in ~ftp/bin should have the same permissions as well. 5) Make the directory ~ftp/etc. This directory is owned by root with permissions 111. 6) Create from scratch the files /etc/passwd and /etc/group in ~ftp/etc. These files should be mode 444. The passwd file should only contain root, daemon, uucp, and ftp. The group file must contain ftp's group. Use your /etc/passwd and /etc/group files as a template for creating passwd and group files going to ~ftp/etc. You may even change the user names in this file, they are used only for 'ls' command. So for example if all files in your ~ftp/pub/linux hierarchy will be maintained by a real user 'balon' with uid=156 you may put linux:*:156:120:Kazik Balon:: in the ~ftp/etc/passwd file (regardless of his real username). Leave only these users who will own files under ftp hierarchy (e.g. root, daemon, ftp...) and definitely remove *ALL* passwords by replacing them with '*' so the entry looks like: root:*:0:0:Ftp maintainer:: ftp:*:400:400: Anonymous ftp:: For more security, you can just remove ~ftp/etc/passwd and ~ftp/etc/group (the effect is that ls -l will not show the directories' group names). Wuarchive ftp daemon (and some others) have some extensions based on the contents of the group/passwd files, so read the appropriate documentation. 7) Make the directory ~ftp/pub. This directory is owned by you and has the same group as ftp with permissions 555. On most systems (like SunOS) you may want to make this directory 2555, ie. set-group-id, in order to create new files with the same group ownership. Files are left here for public distribution. All folders inside ~ftp/pub should have the same permissions as 555. *** Neither the home directory (~ftp) nor any directory below it should be owned by ftp! No files should be owned by ftp either. Modern ftp daemons support all kinds of useful commands, such as chmod, that allow outsiders to undo your careful permission settings. They also have configuration options like the following (WuFTP) to disable them: # all the following default to "yes" for everybody delete no guest,anonymous # delete permission? overwrite no guest,anonymous # overwrite permission? rename no guest,anonymous # rename permission? chmod no anonymous # chmod permission? umask no anonymous # umask permission? 8) If you wish to have a place for anonymous users to leave files, create the directory ~ftp/pub/incoming. This directory is owned by root with permissions 733. Do a 'chmod +t ~ftp/pub/incoming'. The ftp daemon will normally not allow an anonymous user to overwrite an existing file, but a normal user of the system would be able to delete anything. By setting the mode to '1733' you prevent this from happening. In wuftpd you may configure the daemon to create new files with permissions '600' owned by root or any other user. Many times, incoming directories are abused by exchanging pirated and pornographic material. Abusers often create hidden directories there for this purpose. Making the incoming directory unreadable by anonymous ftp helps to some extent. With ordinary ftp severs there is no way to prevent directories being created in incoming. The WUarchive ftp server can limit uploads to certain directories and can restrict characters used in file names like this: # specify the upload directory information upload /var/spool/ftp * no upload /var/spool/ftp /incoming yes ftp staff 0600 nodirs # path filters # path-filter... path-filter anonymous /etc/msgs/pathmsg ^[-A-Za-z0-9_\.]*$ ^\. ^- path-filter guest /etc/msgs/pathmsg ^[-A-Za-z0-9_\.]*$ ^\. ^- Suggestion: Create an extra file-system for your ftp-area (or at least for your incoming-area) to prevent a denial-of-service attack by filling your disk with garbage (inside your incoming directory). If you have wuftpd you may want to add some ftp extensions like compression/decompression 'on the fly' or creation of tar files for the directory hierarchies. Get the appropriate sources (gzip, gnutar, compress), compile them and link statically, put in the ~ftp/bin directory and edit the appropriate file containing the definitions of the allowed conversions. /usr/bin/tar is already statically-linked. You may wish to use gnu tar anyway. Gary Mills wrote a small program to support the following: I got compress from ftp.uu.net, in the root directory, I believe, and compiled it. To do tar and compress, I wrote a tiny program called `pipe', and statically-linked it. My /etc/ftpconversions file looks like this: #strip prefix:strip postfix:addon prefix:addon postfix:external command: #types:options:description :.Z: : :/bin/compress -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS :-z: : :/bin/compress -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS : : :.Z:/bin/compress -c %s:T_REG:O_COMPRESS:COMPRESS : : :.tar:/bin/tar cf - %s:T_REG|T_DIR:O_TAR:TAR : : :.tar.Z:/bin/pipe /bin/tar cf - %s | /bin/compress -c:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+COMPRESS : : :.tar:/bin/gtar -c -f - %s:T_REG|T_DIR:O_TAR:TAR : : :.tar.Z:/bin/gtar -c -Z -f - %s:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+COMPRESS : : :.tar.gz:/bin/gtar -c -z -f - %s:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+GZIP Here it is: -----------------8<-------------cut--------------- /* pipe.c: exec two commands in a pipe */ #define NULL (char *)0 #define MAXA 16 main(argc, argv) int argc; char *argv[]; { char *av1[MAXA], *av2[MAXA]; int i, n, p[2], cpid; i = 0; n = 0; while ( ++i < argc && n < MAXA ) { if ( *argv[i] == '|' && *(argv[i]+1) == '\0' ) break; av1[n++] = argv[i]; } if ( n == 0 ) uexit(); av1[n] = NULL; n = 0; while ( ++i < argc && n < MAXA ) av2[n++] = argv[i]; if ( n == 0 ) uexit(); av2[n] = NULL; if ( pipe(p) != 0 ) exit(1); if ( ( cpid = fork() ) == (-1) ) exit(1); else if ( cpid == 0 ) { (void)close(p[0]); (void)close(1); (void)dup(p[1]); (void)close(p[1]); (void)execv(av1[0], av1); _exit(127); } else { (void)close(p[1]); (void)close(0); (void)dup(p[0]); (void)close(p[0]); (void)execv(av2[0], av2); _exit(127); } /*NOTREACHED*/ } uexit() { (void)write(2, "Usage: pipe <command> | <command>\n", 34); exit(1); } 9) Other things to do: as root: touch ~ftp/.rhosts ~ftp/.forward chmod 400 ~ftp/.rhosts ~ftp/.forward ie. make these files zero-length and owned by root. Due to the last /bin/mail bugs in SunOS: touch /usr/spool/mail/ftp; chmod 400 /usr/spool/mail/ftp Consider an email-alias for the ftp-admin(s) to provide an email-address for problems-reports. If you are mounting some disks from other machines (or even your own) to the ~ftp hierarchy, mount it read-only. The correct entry for the /etc/fstab (on the host with ftpd) is something like: other:/u1/linux /home/ftp/pub/linux nfs ro,noquota,nosuid,intr,bg 1 0 This mounts under /home/ftp/pub/linux the disk from host 'other' with no quota, no 'suid' programs (just in case), interruptible (in case 'other' goes down) and 'bg' - so if 'other' is down when you reboot it will not stop you trying to mount /home/ftp/pub/linux all over again. Part 2 - Setting up a chrooted Secure Anonymous ftp server. This part was contributed by Marcus J Ranum <mjr@tis.com> Steps ----- 1) Build a statically linked version of ftpd and put it in ~ftp/bin. Make sure it's owned by root. 2) Build a statically linked version of /bin/ls if you'll need one. Put it in ~ftp/bin. If you are on a Sun, and need to build one, there's a ported version of the BSD net2 ls command for SunOs on ftp.tis.com: pub/firewalls/toolkit/patches/ls.tar.Z Make sure it's owned by root. 3) Chown ~ftp to root and make it mode 755 THIS IS VERY IMPORTANT 4) Set up copies of ~ftp/etc/passwd and ~ftp/etc/group just as you would normally, EXCEPT make 'ftp's home directory '/' -- make sure they are owned by root. 5) Write a wrapper to kick ftpd off and install it in /etc/inetd.conf The wrapper should look something like: (assuming ~ftp = /var/ftp) main() { if(chdir("/var/ftp")) { perror("chdir /var/ftp"); exit(1); } if(chroot("/var/ftp")) { perror("chroot /var/ftp"); exit(1); } /* optional: seteuid(FTPUID); */ execl("/bin/ftpd","ftpd","-l",(char *)0); perror("exec /bin/ftpd"); exit(1); } Options: You can use 'netacl' from the toolkit or tcp_wrappers to achieve the same effect. We use 'netacl' to switch so that a few machines that connect to the FTP service *don't* get chrooted first. This makes transferring files a bit less painful. You may also wish to take your ftpd sources and find all the places where it calls seteuid() and remove them, then have the wrapper do a setuid(ftp) right before the exec. This means that if someone knows a hole that makes them "root" they still won't be. Relax and imagine how frustrated they will be. If you're hacking ftpd sources, I suggest you turn off a bunch of the options in ftpcmd.y by unsetting the "implemented" flag in ftpcmd.y. This is only practical if your FTP area is read-only. 6) As usual, make a pass through the FTP area and make sure that the files are in correct modes and that there's nothing else in there that can be executed. 7) Note, now, that your FTP area's /etc/passwd is totally separated from your real /etc/passwd. This has advantages and disadvantages. 8) Some stuff may break, like syslog, since there is no /dev/log. Either build a version of ftpd with a UDP-based syslog() routine or run a second syslogd based on the BSD Net2 code, that maintains a unix-domain socket named ~ftp/dev/log with the -p flag. REMEMBER: If there is a hole in your ftpd that lets someone get "root" access they can do you some damage even chrooted. It's just lots harder. If you're willing to hack some code, making the ftpd run without permissions is a really good thing. The correct operation of your hacked ftpd can be verified by connecting to it and (while it's still at the user prompt) do a ps-axu and verify that it's not running as root. Part 3 - OS Specific needed information and suggestions. Older SVR2 and SVR3 system, RTU 6.0 (Masscomp, now Concurrent Real Time UNIX), AT&T 3B1 and 3B2 machines - May need dev/tcp: [dev/tcp] These ftpd implementations may require a ~ftp/dev/tcp in order for anonymous ftp to work. You have to create a character special device with the appropriate major and minor device numbers. The appropriate major and minor numbers of ~ftp/dev/tcp are what the major and minor numbers of /dev/tcp are. The ~ftp/dev is a directory and ~ftp/dev/tcp is a character special device. Make them owned and grouped by root. Permissions for ~ftp/dev is root read/write/exec and other & group read and exec. The permissions for ~ftp/dev/tcp is root read/write, other & group read. HPUX [Logging] If you're using HP's native ftpd, the line in /etc/inetd.conf should execute ftpd -l, which does extra logging. SunOS [Libraries] To set up SunOS to use its shared dynamic libraries, follow these steps: 1) Create the directory ~ftp/usr. This directory is owned by root with permissions 555. 2) Create the directory ~ftp/usr/lib. This directory is owned by root with permissions 555. 3) Copy the runtime loader ld.so into ~ftp/usr/lib for use by ls. ld.so is owned by root with permissions 555. 4) Copy the latest version of the shared C library, libc.so.* into ~ftp/usr/lib for use by ls. libc.so.* is owned by root with permissions 555. ***4.1.2(or above) users: you also need to copy /usr/lib/libdl.so.* to ~ftp/lib. 5) Create the directory ~ftp/dev. This directory is owned by root with permissions 111. 6) ~ftp/dev/zero is needed by the runtime loader. Move into the directory ~ftp/dev and create it with the command: mknod zero c 3 12 chown ~ftp/dev/zero to root. Make sure it's readable. ***For novices: WARNING!! Don't try to copy /dev/zero to ~ftp/dev/zero!! This is an endless file of zeroes and it will completely fill your filesystem! 7) If you want to have the local time showing when people connect, create the directory ~ftp/usr/share/lib/zoneinfo and copy /usr/share/lib/zoneinfo/localtime 8) If you are bothered by the need for copying your libraries so that you can use Sun's 'ls', which is dynamically linked, you can try to get a statically linked copy of 'ls' instead. The CD-ROM that contains Sun's OS has a statically-linked version of ls. In this case, you can dispense with steps #6-8. Statically linked versions may be available from the following sources: If you want a statically linked "ls" get the GNU fileutils off a archive site near you and statically link it. [Logging] Sun's standard ftpd logs *all* password information. To correct it, install patch: 101640-03 SunOS 4.1.3: in.ftpd logs password info when -d option is used. In /etc/inetd.conf find the line that starts with "ftp". At the end of that line, it should read "in.ftpd". Change that to "in.ftpd -dl". In /etc/syslog.conf, add a line that looks like: daemon.* /var/adm/daemonlog The information can be separated, such as: daemon.info /var/adm/daemon.info daemon.debug /var/adm/daemon.debug daemon.err /var/adm/daemon.err Note that the whitespace between the two columns must include at least one TAB character, not just spaces, or it won't work. Of course your log file could be anything you want. Then, create the logfile (touch /var/adm/daemonlog should do). Finally, restart inetd and syslogd, either individually, or by rebooting the system. You should be good to go. If you do not install the patch, make sure the log file is owned by root and mode 600, as the ftp daemon will log *everything*, including users' passwords. *** You want to make all logs root only readable for security reasons *** If a user mistypes his password for his username, it could be compromised *** if anyone can read the log files. Part 4 - Where to get other FTP daemons Wuarchive FTP 2.4- A secure FTP daemon that allows improved access-control, logging, pre-login banners, and is very configurable: Can be ftp'd from ftp.uu.net in "/networking/ftp/wuarchive-ftpd" directory. Be certain to verify the checksum information to confirm that you have retrieved a valid copy. [Warning: Older versions of Wu-FTP are extremely insecure and in some cases have been trojaned.] BSD SVR4 File Checksum Checksum MD5 Digital Signature ----------------- -------- --------- -------------------------------- wu-ftpd-2.4.tar.Z 38213 181 20337 362 cdcb237b71082fa23706429134d8c32e patch_2.3-2.4.Z 09291 8 51092 16 5558a04d9da7cdb1113b158aff89be8f For DECWRL ftpd, sites can obtain version 5.93 via anonymous FTP from gatekeeper.dec.com in the "/pub/misc/vixie" directory. BSD SVR4 File Checksum Checksum MD5 Digital Signature ----------------- -------- --------- -------------------------------- ftpd.tar.gz 38443 60 1710 119 ae624eb607b4ee90e318b857e6573500 For BSDI systems, patch 005 should be applied to version 1.1 of the BSD/386 software. You can obtain the patch file via anonymous FTP from ftp.bsdi.com in the "/bsdi/patches-1.1" directory. BSD SVR4 File Checksum Checksum MD5 Digital Signature ----------------- -------- --------- -------------------------------- BU110-005 35337 272 54935 543 1f454d4d9d3e1397d1eff0432bd383cf Public Domain Sources: ftp.uu.net ~ftp/systems/unix/bsd-sources/libexec/ftpd gatekeeper.dec.com ~ftp/pub/DEC/gwtools/ftpd.tar.Z Part 5 - How to Know if your Anonymous FTP Server is secure This section is intended for the administrator to go down a small check list of things to make sure his server is not easily compromised. a) Check to make sure your ftp server does not have SITE EXEC command by telneting to port 21 and typing SITE EXEC. If your ftp daemon has SITE EXEC make sure it is the most current version (ie, Wu-FTP 2.4). In older versions this allows anyone to gain shell via port 21. b) Check to make sure no one can log in and make files or directories in the main directory. If anyone can log in as anonymous FTP and make files such as .rhosts and .forward, instant access is granted to any intruder. c) Check to make sure the main directory is NOT owned by ftp. If it is owned by FTP, an intruder could SITE CHMOD 777 the main directory and then plant files to give him instant access. SITE CHMOD command should be removed because anonymous users do not need any extra priviledges. d) Check to make sure NO files or directories are owned by ftp. If they are, it is possible an intruder could replace them with his own trojan versions. e) There were several bugs in old daemons, so it is very important to make sure you are running the most current ftp daemons. Part 6 - Archie Searches FTP sites for programs. Login into these sites as archie or use client software for faster access. To get your own anonymous site added to Archie's search list, e-mail archie-updates@bunyip.com. archie.ac.il 132.65.20.254 (Israel server) archie.ans.net 147.225.1.10 (ANS server, NY (USA)) archie.au 139.130.4.6 (Australian Server) archie.doc.ic.ac.uk 146.169.11.3 (United Kingdom Server) archie.edvz.uni-linz.ac.at 140.78.3.8 (Austrian Server) archie.funet.fi 128.214.6.102 (Finnish Server) archie.internic.net 198.49.45.10 (AT&T server, NY (USA)) archie.kr 128.134.1.1 (Korean Server) archie.kuis.kyoto-u.ac.jp 130.54.20.1 (Japanese Server) archie.luth.se 130.240.18.4 (Swedish Server) archie.ncu.edu.tw 140.115.19.24 (Taiwanese server) archie.nz 130.195.9.4 (New Zealand server) archie.rediris.es 130.206.1.2 (Spanish Server) archie.rutgers.edu 128.6.18.15 (Rutgers University (USA)) archie.sogang.ac.kr 163.239.1.11 (Korean Server) archie.sura.net 128.167.254.195 (SURAnet server MD (USA)) archie.sura.net(1526) 128.167.254.195 (SURAnet alt. MD (USA)) archie.switch.ch 130.59.1.40 (Swiss Server) archie.th-darmstadt.de 130.83.22.60 (German Server) archie.unipi.it 131.114.21.10 (Italian Server) archie.univie.ac.at 131.130.1.23 (Austrian Server) archie.unl.edu 129.93.1.14 (U. of Nebraska, Lincoln (USA)) archie.uqam.ca 132.208.250.10 (Canadian Server) archie.wide.ad.jp 133.4.3.6 (Japanese Server) Part 7 - Acknowledgements Thanks to the following people for suggestions that help shape this FAQ: Tomasz Surmacz (tsurmacz@asic.ict.pwr.wroc.pl) Wolfgang Ley (Ley@rz.tu-clausthal.de) Russel Street (russells@ccu1.auckland.ac.nz) Gary Mills (mills@CC.UManitoba.CA) Nicholas Ironmonger (ndi@sam.math.ethz.ch) Morten Welinder (terra@diku.dk) Nick Christenson (npc@minotaur.jpl.nasa.gov) Mark Hanning-Lee (markhl@romoe.caltech.edu) Marcus J Ranum <mjr@tis.com> Copyright This paper is Copyright (c) 1994 by Christopher Klaus of Internet Security Systems, Inc. Permission is hereby granted to give away free copies. You may distribute, transfer, or spread this paper. You may not pretend that you wrote it. This copyright notice must be maintained in any copy made. Disclaimer The information within this paper may change without notice. Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties with regard to this information. In no event shall the author be liable for any damages whatsoever arising out of or in connection with the use or spread of this information. Any use of this information is at the user's own risk. Address of Author Please send suggestions, updates, and comments to: Christopher Klaus <cklaus@shadow.net> of Internet Security Systems, Inc. <iss@shadow.net> -- Christopher William Klaus <cklaus@shadow.net> <iss@shadow.net> Internet Security Systems, Inc. Computer Security Consulting 2209 Summit Place Drive, Penetration Analysis of Networks Atlanta,GA 30350-2430. (404)998-5871.