From a C program I want to call a shell script with a filename as a parameter. Users can control the filename. The C is something like (initialization/error checking omitted):
sprintf(buf, "/bin/sh script.sh \"%s\"", filename);
system(buf);
The target device is actually an embedded system so I don't need to worry about malicious users. Obviously this would be an attack vector in a web environment. Still, if there is a filename on the system which, for example, contains backquotes in its name, the command will fail because the shell will perform expansion on the name. Is there any to prevent command substitution?
Well, you could always reimplement system() using a call to fork() and then execv().
http://www.opengroup.org/onlinepubs/000095399/functions/system.html
Try triggering "unalias " in the system function.
Since you tagged this as C I will provide you with a C answer. You will need to escape the filename -- create a new string that will be treated properly by the shell, so that things like This is a file name produces This\ is\ a\ file\ name or bad;rm *;filename becomes bad\;rm\ \*\;filename. Then you can pass that to the shell.
Another way around this would be to run the shell directly with fork and one of the exec functions. Passing arguments directly to programs does not result in shell command line expansion or interpretation.
As sharth said, you should not use system but fork and execv yourself. But to answer the question of how you make strings safe to pass to the shell (in case you insist on using system), you need to escape the string. The simplest way to do this is to first replace every occurrence of ' (single quote) with '\'' (single quote, backslash, single quote, single quote) then add ' (single quote) at the beginning and end of the string. The other fairly easy (but usually less efficient) method is to place a backslash before every single character, but then you still need to do some special quotation mark tricks to handle embedded newlines, so I prefer the first method.
Related
I want to call an URL in a C program which contains some "&". The system only recognices the URL until the "&" (https://chart.googleapis.com/chart?cht=qr) and tells me it doesnt know the command cht and chl. How can I make it to use the whole URL?
char get_qr[100];
sprintf (get_qr, "start https://chart.googleapis.com/chart?cht=qr&chs=500x500&chl=Hello World!);
system (get_qr);
The issue isn’t the URL handling, it’s that you’re launching a shell command via system.
Shell commands need to be shell quoted. In particular, & is a special character of the shell. So put quotes around your shell command argument:
system("start \"https://chart.googleapis.com/chart?cht=qr&chs=500x500&chl=Hello World!\"");
(In your code there’s no need for sprintf anyway. If your actual code requires sprintf, don’t just allocate a static buffer; allocate a dynamic buffer of the correct size!)
I'm writing a GUI application for character recognition that uses Tesseract. I want to allow the user to specify a custom shell command to be executed with /bin/sh -c when the text is ready.
The problem is the recognized text can contain literally anything, for example && rm -rf some_dir.
My first thought was to make it like in many other programs, where
the user can type the command in a text entry, and then special strings (like in printf()) in the command are replaced by the appropriate data (in my case, it might be %t). Then the whole string is passed to execvp(). For example, here is a screenshot from qBittorrent:
The problem is that even if I properly escape the text before replacing %t, nothing prevents the user to add extra quotes around the specifier:
echo '%t' >> history.txt
So the full command to be executed is:
echo ''&& rm -rf some_dir'' >> history.txt
Obviously, that's a bad idea.
The second option is only let the user to choose an executable (with a file selection dialog), so I can manually put the text from Tesseract as argv[1] for execvp(). The idea is that the executable can be a script where users can put anything they want and access the text with "$1". That way, the command injection is not possible (I think). Here's an example script a user can create:
#!/bin/sh
echo "$1" >> history.txt
It there any pitfalls with this approach? Or maybe there's a better way to safely pass an arbitrary text as parameter to a program in shell script?
In-Band: Escaping Arbitrary Data In An Unquoted Context
Don't do this. See the "Out-Of-Band" section below.
To make an arbitrarily C string (containing no NULs) evaluate to itself when used in an unquoted context in a strictly POSIX-compliant shell, you can use the following steps:
Prepend a ' (moving from the required initial unquoted context to a single-quoted context).
Replace each literal ' within the data with the string '"'"'. These characters work as follows:
' closes the initial single-quoted context.
" enters a double-quoted context.
' is, in a double-quoted context, literal.
" closes the double-quoted context.
' re-enters single-quoted context.
Append a ' (returning to the required initial single-quoted context).
This works correctly in a POSIX-compliant shell because the only character that is not literal inside of a single-quoted context is '; even backslashes are parsed as literal in that context.
However, this only works correctly when sigils are used only in an unquoted context (thus putting onus on your users to get things right), and when a shell is strictly POSIX-compliant. Also, in a worst-case scenario, you can have the string generated by this transform be up to 5x longer than the original; one thus needs to be cautious around how the memory used for the transform is allocated.
(One might ask why '"'"' is advised instead of '\''; this is because backslashes change their meaning used inside legacy backtick command substitution syntax, so the longer form is more robust).
Out-Of-Band: Environment Variables, Or Command-Line Arguments
Data should only be passed out-of-band from code, such that it's never run through the parser at all. When invoking a shell, there are two straightforward ways to do this (other than using files): Environment variables, and command-line arguments.
In both of the below mechanisms, only the user_provided_shell_script need be trusted (though this also requires that it be trusted not to introduce new or additional vulnerabilities; invoking eval or any moral equivalent thereto voids all guarantees, but that's the user's problem, not yours).
Using Environment Variables
Excluding error handling (if setenv() returns a nonzero result, this should be treated as an error, and perror() or similar should be used to report to the user), this will look like:
setenv("torrent_name", torrent_name_str, 1);
setenv("torrent_category", torrent_category_str, 1);
setenv("save_path", path_str, 1);
# shell script should use "$torrent_name", etc
system(user_provided_shell_script);
A few notes:
While values can be arbitrary C strings, it's important that the variable names be restricted -- either hardcoded constants as above, or prefixed with a constant (lowercase 7-bit ASCII) string and tested to contain only characters which are permissible shell variable names. (A lower-case prefix is advised because POSIX-compliant shells use only all-caps names for variables that modify their own behavior; see the POSIX spec on environment variables, particularly the note that "The name space of environment variable names containing lowercase letters is reserved for applications. Applications can define any environment variables with names from this name space without modifying the behavior of the standard utilities").
Environment space is a limited resource; on modern Linux, the maximum combined storage for both environment variables and command-line arguments is typically on the scale of 128kb; thus, setting large environment variables will cause execve()-family calls with large command lines to fail. Validating that length is within reasonable domain-specific limits is wise.
Using Command-Line Arguments:
This version requires an explicit API, such that the user configuring the trigger command knows which value will be passed in $1, which will be passed in $2, etc.
/* You'll need to do the usual fork() before this, and the usual waitpid() after
* if you want to let it complete before proceeding.
* Lots of Q&A entries on the site already showing the context.
*/
execl("/bin/sh", "-c", user_provided_shell_script,
"sh", /* this is $0 in the script */
torrent_name_str, /* this is $1 in the script */
torrent_category_str, /* this is $2 in the script */
path_str, /* this is $3 in the script */
NUL);
Any time you're runnng commands with even the possibility of user input making its way into them you must escape for the shell context.
There's no built-in function in C to do this, so you're on your own, but the basic idea is to render user parameters as either properly escaped strings or as separate arguments to some kind of execution function (e.g. exec family).
I am new to Unix systems and trying to learn some thing with help of terminal. I have following question in my mind. If we can write filepath without single quotes in terminal (for ex : mv path1 path2) then why we sometime use single quotes to specify paths. What is the difference between these two?
This is not a question of the operating system, but of the shell you use. You can actually chose what shell you want to use on a unixoid system if multiple are installed (which usually is the case).
In general the shell has to interpret the input you make. It has to decide how to handle the tokens of the input. What to consider as the "command" you want to execute, what as arguments. For the arguments it has to decide if the string is meant as a single argument or multiple arguments.
Without quotes (single or double quotes), whitespace characters are considered separators between words, words are typically considered separate arguments. So you can specify multiple arguments for a single command. If that is not desired then you can use quote characters to group multiple words separated by whitespace characters into a single argument, for example a folder name containing a space character. This works because now the shell knows that you want everything following the quote character to be considered as a single argument up to the next matching quote character (actually except escaped ones...).
It's used to escape spaces in file names, otherwise, a backslash is needed. For instance:
$ rm spaces\ in\ file\ name
$ rm 'spaces in file name'
If your file path does not have spaces, it's probably safe to omit the quotes.
I have two strings, both of which can be set by the user, e.g.
char *command = "vim $VAR";
char *myVar = "/tmp/something";
I want to execute *command using *myVar for $VAR.
I tried concatenating them as an environment variable (e.g. (pseudo-code) system("VAR=" + *myVar + "; " + *command), but the user controls myVar so this would be very insecure and buggy.
I considered tokenizing on spaces to directly replace $var and passing the results to exec(), but it's too awkward to worry about tokenizing shell command arguments correctly.
I think the solution is to emulate system() with exec by doing something like exec("sh", "-c", command, "--argument", "VAR", myVar), but I can't see anything in the sh/dash/bash man pages to permit setting environment variables in this way.
Edit: I just saw execvpe() which has an argument for setting environment variables from key=value strings. Would this be safe to use with untrusted input for the value?
How do I do this safely?
You can perform some string replacement on the value of myVar — put it inside single quotes, and replace all single quotes (the character ') by the four-character string '\''. Fiddly but safe if you don't make an implementation mistake. If possible, use a library that does it for you.
If your program is single-threaded, I recommend a different solution that doesn't involve fiddly quoting. You talk of setting environment variables… Well, just do it: make VAR an environment variable.
setenv("VAR", myVar, 1);
system(command);
unsetenv("VAR")
I've omitted error checking, and I assume that VAR isn't needed elsewhere in your program (if it is, this solution becomes more tedious because you need to remember the old value).
If you want fine control over the environment in which the command runs, you can reimplement system on top of fork, execve (or execvpe) and waitpid, or on top of posix_spawn (or posix_spawnp) and waitpid. It's more effort but you gain flexibility.
Note that whatever solution you adopt other than doing string replacement to "vim $VAR" inside the C program, the command will need to be vim "$VAR" and not vim $VAR. This is because in shell syntax, $VAR means “the value of the variable VAR” only if it's inside double quotes — otherwise, $VAR means “take the value of VAR, split it into words, and expand each word as a file name wildcard pattern”.
You need to quote the string contained in myVar; this may mean escaping naughty characters (eg with backslash).
You could use g_shell_quote from Glib
So as Ben pointed out, command is probably loaded at runtime.
I think the best approach is to tokenize command, rather than to tokenize myVar. You can then find which word in command is $VAR and replace that with the value of myVar. Then you can use posix_spawnp as per below.
If you really want command to be an arbitrary shell command, then your only option is to escape myVar before assigning it to an environment variable. Otherwise the shell will expand spaces and other special characters in it regardless of how you set it.
Third option is to make sure command is vim "$VAR" instead of vim $VAR. In that case you can assign it to environment using setenv, then call system, and then unset it after.
Old answer in case command is static:
It looks like what you actually want to do is
extern char *environ[];
posix_spawnp(NULL, "vim", NULL, NULL, (char*[]){"vim", myVar, NULL}, environ);
wait(NULL);
i.e. exec vim directly without any shell, with myVar as the first argument.
How about:
fork (if emulating system or spawn, skip this if doing exec)
setenv("VAR", myVar) in the fork child
exec "sh -c " + command
I've got a program which accepts a set of rules in the form of a single rules file.
When one of the conditions are considered met by my program, I seek to treat the block of commands associated with the condition as an independent bash script which needs to be executed. I would rather not deal with storing these commands in files as that leaves an undesirable attack vector. Is there a way to feed a line delimited list of bash commands to bash as a single group? I want if conditions and other things from the bash script to function correctly, not just executing each line raw on its own.
Example rules file:
if CONDITION
some nice
bash commands
pkill some process
./launching something!
endif
I want to be able to run the four lines of bash code as a group of bash commands, not independently of each other, when CONDITION is true, as determined by my C program.
Obviously this is from Linux, using C as the programming language.
You could also perhaps popen a bash process.
However, your approach suggests also to embed some scripting interpreter inside your application. Did you consider embedding e.g. lua inside it?
The simplest approach is probably to use sh -c "string containing commands to be executed". What's tricky is the embedded newlines. If the commands themselves won't contain single quotes, then you can wrap that multi-line string in single quotes. If it can contain single quotes, you'd want to escape the string to ensure that they are unchanged.
So:
read the commands into a buffer
do escape processing on the buffer; replace each ' with '\'' (remembering that the backslash must be in the output, so the string in C looks like "'\\''")
format the command: snprintf(command, sizeof(command), "sh -c '%s'", escaped_buffer);
ensure there was enough room
run system(command);