How safe is it to cat an arbitrary file?

Question

Sometimes when I cat a binary file by mistake, my terminal gets garbled up. Nothing a quick reset can't fix, but couldn't an attacker theoretically create a file that, when displayed on a terminal, would execute some arbitrary code? Through an exploit in the terminal emulator or otherwise.

Solution 1

Whether such output can be exploited depends on the terminal program, and what that terminal does depending on escape codes that are being sent. I am not aware of terminal programs having such exploitable features, and the only problem now would be if there is an unknown buffer overflow or something like that, that could be exploited.

With some older hardware terminals this could be a problem as you programmed e.g. function keys with these kind of escape sequences, by storing a command sequence for that key in the hardware. You would still need a physical key-press to activate that.

But there are always (as Hauke so righfully marked 'braindead') people willing to add such a feature if it solves a problem for them, not understanding the loophole they create. In my experience with open source software is that, because of the many eyes looking at the code, this is less likely to happen as with closed source. (I remember that in the mail program on Silicon Grahpics' Irix, in the mid ninetees, you could include commands to be executed on the receivers machine, real paths to executables, ....)

Solution 2

Most terminal emulators will send back some response, if they receive certain escape sequences (have a look at the xterm control sequences documentation). E.g., you can send \e[0c to a VT100-like emulator and it will send back the device attributes, something like \e[?1;2c (This is probably what Keith observed.) But these answers are not arbitrary strings. Still, having an executable named 2c somewhere on your system that does something fatal is a bad idea.

Update: The risks are in fact bigger than I thought, due to the possibility to set the title of an xterm window and to send back the title using appropriate escape sequences (http://www.securityfocus.com/bid/6940/). In contrast to the example above, the title can be an almost arbitrary string.

Solution 3

This changes the terminal title in GNOME Terminal 3.6.1, unless overridden by something like PS1:

printf "\033]2;Script Kiddie was here\007"

Now open a new GNOME Terminal window to test the cat version:

printf "\033]2;Script Kiddie was here\007" > test.bin
cat test.bin

Yep, this also sets the terminal title.

There used to be a security issue with an escape code resulting in the title being printed to the command line, so you could effectively create a file, which when cated would print (I'm not sure if you could put a newline in there) arbitrary commands. Ouch!

Solution 4

While using cat might not result in code execution, escape codes will be processed so you could easily be misled into thinking the script is harmless when in fact it is malicious.

Here is an example command you can run which will create a "malicious" shell script:

echo -e '#!/bin/sh\necho "...doing something bad here..."\nexit\n\033[A\033[Aecho "Hello dear reader, I am just a harmless script, safe to run me!"' > demo.sh
chmod a+x demo.sh

When you inspect the file, it seems harmless enough:

$ cat demo.sh
#!/bin/sh
echo "Hello dear reader, I am just a harmless script, safe to run me!"

But should you actually run it...

$ ./demo.sh 
...doing something bad here...

The script works by including raw escape codes to move the cursor up a couple of lines, so the rest of the script is written over the top of the malicious code, hiding it.

Nearly any other program will reveal the script for what it is. Only programs that don't process the file content (like cat, more and less -r) will produce the misleading output.

Note that tail and head also produce the same misleading output. Using "less +F" is therefore safer than "tail -f".

Solution 5

I have definitely experienced xterm inserting arbitrary characters into itself as if I had typed them. And on occasion this has apparently included newline character, so that I got ngwerm:0riu: command not found as a response. I see no reason why someone could not craft a file that would send specific, harmful commands. So yes, at least some terminals are susceptible to attacks with arbitrary impact.

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Comments

[Keith]

Sometimes when I do that my shell will say at the end "<garbage> unknown command". That makes me wonder if this is actually possible.

[sunnysideup]

There were exploits for the terminal emulator, e.g. linuxsecurity.com/content/view/104657 or securityfocus.com/bid/6936/discuss so it is not necessary particular safe

[Blrfl]

This is why it's better to use something that will balk at binary files (like more) or is terminal-aware (less) to examine the contents of files. Not only will it not put your terminal in a weird state, the whole file won't go flying by in one shot.

[bot47]

mosh documentation has some thoughts on that: mosh.mit.edu/#techinfo

[Gilles 'SO- stop being evil']

See also Can “cat-ing” a file be a potential security risk? and How to avoid escape sequence attacks in terminals?

[mosvy]

@ThorbjørnRavnAndersen No, it does not.

[Admin]

See also superuser.com/q/1520750/59284.

[Gunchars]

That's already cutting it very close.

[user]

"you could include commands to be executed on the receivers machine" You mean something like including in an email VBScript that calls out to the Windows Scripting Host? :)

[Anthon]

No exactly, you could start an executable that was already on the machine, like playing a sound. I don't recall the exact syntax (that was almost 20 years ago) nor whether you could switch that 'feature' off in a setup. We had some fun though with auto-playing videos stored in our network.

[Anthon]

@luserdroog No this was the standard GUI based mail program under Irix

[luser droog]

Oh, well. I'm a little obsessed with NeWS. :)

[Baard Kopperud]

@Anthon I'm not sure if it's still possible, but the possibility of using escape-codes to get a terminal to "repeat" text coming to it from the write command - thus executing commands/scripts as the user owning the terminal. It's supposedly the reason why many recommend turning off messages mesg -n for users most of the time, and for root always. AFAIK, this was actually done - though I don't know if it ever was exploited. So random text from a catted executable, could perhaps be executed.

[sendmoreinfo]

There's an even older feature -- 'answerback message', sent in response to ENQ (C-e) character. On a real VT100, it is set by the user in the terminal's SETUP menu; maybe there are terminal emulators that allow setting it remotely...

[Jan Wikholm]

running strings on an unknown file can also have problematic consequences. lcamtuf.blogspot.fi/2014/10/…

[Charlie]

This is quite problematic... You can see what's actually going on by running echo $(cat demo.sh), cat demo.sh | grep . --color=yes (Note: the --color=yes is what's showing the "malicious" code here) or the build-in cat -v demo.sh.

[Incnis Mrsi]

True or false, it’s an answer for a different question: how trustworthy is cat in displaying the file’s content.

[slm]

@IncnisMrsi - read the first sentence!!!!

[Incnis Mrsi]

OK, retracting my previous statement, The answer is short, using confusing terminology, unfounded, and evidently incomplete. Note that in security, “arbitrary” ≠ random as distributed in your favourite OS.

[G-Man Says 'Reinstate Monica']

(1) You do realize that Hauke Laging’s answer has been deleted, so most people can’t see it, right?  (2) Strictly speaking, Hauke said that the insecure feature was braindead, not the implementer.