Allow setuid on shell scripts

  • The setuid permission bit tells Linux to run a program with the effective user id of the owner instead of the executor:

    > cat setuid-test.c
    #include <stdio.h>
    #include <unistd.h>
    int main(int argc, char** argv) {
        printf("%d", geteuid());
        return 0;
    > gcc -o setuid-test setuid-test.c
    > ./setuid-test
    > sudo chown nobody ./setuid-test; sudo chmod +s ./setuid-test
    > ./setuid-test

    However, this only applies to executables; shell scripts ignore the setuid bit:

    > cat setuid-test2
    id -u
    > ./setuid-test2
    > sudo chown nobody ./setuid-test2; sudo chmod +s ./setuid-test2
    > ./setuid-test2

    Wikipedia says:

    Due to the increased likelihood of security flaws, many operating systems ignore the setuid attribute when applied to executable shell scripts.

    Assuming I'm willing to accept those risks, is there any way to tell Linux to treat the setuid bit the same on shell scripts as it does on executables?

    If not, is there a common workaround for this problem? My current solution is to add a sudoers entry to allow ALL to run a given script as the user I want it run as, with NOPASSWD to avoid the password prompt. The main downsides to that is the need for a sudoers entry every time I want to do this, and the need for the caller to sudo some-script instead of just some-script

  • Linux ignores the setuid¹ bit on all interpreted executables (i.e. executables starting with a #! line). The comp.unix.questions FAQ explains the security problems with setuid shell scripts. These problems are of two kinds: shebang-related and shell-related; I go into more details below.

    If you don't care about security and want to allow setuid scripts, under Linux, you'll need to patch the kernel. As of 3.x kernels, I think you need to add a call to install_exec_creds in the load_script function, before the call to open_exec, but I haven't tested.

    Setuid shebang

    There is a race condition inherent to the way shebang (#!) is typically implemented:

    1. The kernel opens the executable, and finds that it starts with #!.
    2. The kernel closes the executable and opens the interpreter instead.
    3. The kernel inserts the path to the script to the argument list (as argv[1]), and executes the interpreter.

    If setuid scripts are allowed with this implementation, an attacker can invoke an arbitrary script by creating a symbolic link to an existing setuid script, executing it, and arranging to change the link after the kernel has performed step 1 and before the interpreter gets around to opening its first argument. For this reason, most unices ignore the setuid bit when they detect a shebang.

    One way to secure this implementation would be for the kernel to lock the script file until the interpreter has opened it (note that this must prevent not only unlinking or overwriting the file, but also renaming any directory in the path). But unix systems tend to shy away from mandatory locks, and symbolic links would make a correct lock feature especially difficult and invasive. I don't think anyone does it this way.

    A few unix systems (mainly OpenBSD, NetBSD and Mac OS X, all of which require a kernel setting to be enabled) implement secure setuid shebang using an additional feature: the path /dev/fd/N refers to the file already opened on file descriptor N (so opening /dev/fd/N is roughly equivalent to dup(N)). Many unix systems (including Linux) have /dev/fd but not setuid scripts.

    1. The kernel opens the executable, and finds that it starts with #!. Let's say the file descriptor for the executable is 3.
    2. The kernel opens the interpreter.
    3. The kernel inserts /dev/fd/3 the argument list (as argv[1]), and executes the interpreter.

    Sven Mascheck's shebang page has a lot of information on shebang across unices, including setuid support.

    Setuid interpreters

    Let's assume you've managed to make your program run as root, either because your OS supports setuid shebang or because you've used a native binary wrapper (such as sudo). Have you opened a security hole? Maybe. The issue here is not about interpreted vs compiled programs. The issue is whether your runtime system behaves safely if executed with privileges.

    • Any dynamically linked native binary executable is in a way interpreted by the dynamic loader (e.g. /lib/, which loads the dynamic libraries required by the program. On many unices, you can configure the search path for dynamic libraries through the environment (LD_LIBRARY_PATH is a common name for the environment variable), and even load additional libraries into all executed binaries (LD_PRELOAD). The invoker of the program can execute arbitrary code in that program's context by placing a specially-crafted in $LD_LIBRARY_PATH (amongst other tactics). All sane systems ignore the LD_* variables in setuid executables.

    • In shells such as sh, csh and derivatives, environment variables automatically become shell parameters. Through parameters such as PATH, IFS, and many more, the invoker of the script has many opportunities to execute arbitrary code in the shell scripts's context. Some shells set these variables to sane defaults if they detect that the script has been invoked with privileges, but I don't know that there is any particular implementation that I would trust.

    • Most runtime environments (whether native, bytecode or interpreted) have similar features. Few take special precautions in setuid executables, though the ones that run native code often don't do anything fancier than dynamic linking (which does take precautions).

    • Perl is a notable exception. It explicitly supports setuid scripts in a secure way. In fact, your script can run setuid even if your OS ignored the setuid bit on scripts. This is because perl ships with a setuid root helper that performs the necessary checks and reinvokes the interpreter on the desired scripts with the desired privileges. This is explained in the perlsec manual. It used to be that setuid perl scripts needed #!/usr/bin/suidperl -wT instead of #!/usr/bin/perl -wT, but on most modern systems, #!/usr/bin/perl -wT is sufficient.

    Note that using a native binary wrapper does nothing in itself to prevent these problems. In fact, it can make the situation worse, because it might prevent your runtime environment from detecting that it is invoked with privileges and bypassing its runtime configurability.

    A native binary wrapper can make a shell script safe if the wrapper sanitizes the environment. The script must take care not to make too many assumptions (e.g. about the current directory) but this goes. You can use sudo for this provided that it's set up to sanitize the environment. Blacklisting variables is error-prone, so always whitelist. With sudo, make sure that the env_reset option is turned on, that setenv is off, and that env_file and env_keep only contain innocuous variables.


    • Setuid shebang is insecure but usually ignored.
    • If you run a program with privileges (either through sudo or setuid), write native code or perl, or start the program with a wrapper that sanitizes the environment (such as sudo with the env_reset option).

    ¹ This discussion applies equally if you substitute “setgid” for “setuid”; they are both ignored by the Linux kernel on scripts

    Wow, you just scared the crap out of me, and I think I need to spend the next 4 days re-writing a lot of shell scripts in native code instead! ;-)

    @Josh: Secure setuid shell scripts are possible, but only if the both the shell implementer and the script writer are very careful. Rather than native code, I recommend Perl, where the implementers have taken care that setuid scripts should be secure with little effort on the script writer's part.

    apparently the `suidperl` stuff has been deprecated and marked for removal for years (but persists non-the-less)

    are there any other languages that support setuid safely?

    Actually `suidperl` has been removed as of perl 5.11 (5.12 stable): perl5110delta: > "suidperl" has been removed. It used to provide a mechanism to emulate setuid permission bits on systems that don't support it properly. perl5120delta: > "suidperl" is no longer part of Perl. It used to provide a mechanism to emulate setuid permission bits on systems that don't support it properly.

    Also note this line from perl 5.6.1 docs (nearly a decade ago)... perl561delta: > Note that suidperl is neither built nor installed by default in any recent version of perl. **Use of suidperl is highly discouraged**. If you think you need it, try alternatives such as sudo first. See .

    Doesn't sudo sanitize the environment (or can at least be set to do so)?

    @Demetri Yes, sudo can sanitize the environment if configured appropriately (`env_reset` option).

    I don't understand: this seems to be an explanation of the reasons for the problems, but is there an actual answer to the OP's question here? Is there a way to tell my OS to run setuid shell scripts?

    @Tom No, unless you patch the kernel, or use *BSD. I've added an explicit note about patching the Linux kernel.

    What old system are you using that hasn't closed the LD_PRELOAD and LD_LIBRARY_PATH holes yet? Normally the loader just ignores them if ruid != euid.

    @Joshua None. I wrote “All sane systems ignore the `LD_*` variables in setuid executables”.

    Commenting on the situation, not your explanation: This approach is so damn shortsighted. It makes not only attacks harder, but also *good* practices, such as running things under a separate, restricted account.

    OpenBSD (6.5 and 6.6 snapshot as of 2019) comes with "secure" setuid scripts enabled by default; no kernel setting has to be enabled. See my demo.

    And suidperl went the way of the dodo a long time ago

  • One way of solving this problem is to call the shell script from a program that can use the setuid bit.
    its something like sudo. For example, here is how you would accomplish this in a C program:

    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/types.h>
    #include <unistd.h>
    int main()
        setuid( 0 );   // you can set it at run time also
        system( "/home/pubuntu/" );
        return 0;

    Save it as setuid-test2.c.
    Now do the setuid on this program binary:

    su - nobody   
    [enter password]  
    chown nobody:nobody a.out  
    chmod 4755 a.out  

    Now, you should be able to run it, and you'll see your script being executed with nobody permissions.
    But here also either you need to hardcode the script path or pass it as command line arg to above exe.

    I would advise against the suggestion to allow passing of the script as a command line argument, as that essentially gives anyone who can execute the program the ability to run any script as that defined user.

    I would add some sanity checking if you implement that C solution: ensure that the script being run is owned by the right user+group, is not world-writable, and not of the directories in its path are world- or group-writable (essentially the same checks that suPHP performs) - this will reduce the chance of your tool becoming a security problem through other users being able to somehow edit or replace the script.

    I voted down for suggestion of passing as parameter. **IT WOULD MEAN ANY SCRIPT CAN BE RUN WITH ROOT PRIVILAGES. YES - ONE CONTAINING `rm -rf /` AS WELL**.

    @Maciej: your point is valid. My assumption was from original Q. "Assuming I'm willing to accept those risks, is there any way to tell Linux to treat the setuid bit the same on shell scripts as it does on executables?"

    @Hemant: The problem is that passing script as parameter means that I can execute any script (i.e. effectively giving any user root rights). Giving setuid to one program does not involve such risk.

    **Note that THIS IS INSECURE even if the full path to the script is hardcoded.** The shell will inherit variables from the environment, and many of them allow the invoker to inject arbitrary code. `PATH` and `LD_LIBRARY_PATH` are obvious vectors. Some shells execute `$ENV` or `$BASHENV` or `~/.zshenv` even before they start executing the script proper, so you can't protect from these at all from within the script. The only safe way to invoke a shell script with privileges is to **clean up the environment**. Sudo knows how to do it safely. So **do not write your own wrapper, use sudo**.

    I feel bad that he's suddenly getting downvoted for this -- I did specifically say I wanted to hear insecure versions too, and I was imagining an executable that took a shell script argument when I said it. Obviously it's massively insecure, but I wanted to know what possibilities exist

    @Gilles: FYI, Linux unsets `LD_LIBRARY_PATH` among other things when it encounters the setuid bit.

    Instead of using `system`, you may find it simpler (and more efficient) to use one of the `exec` family - most likely `execve`. That way, you don't create a new process or start a shell, and you can forward arguments (assuming that your privileged script can safely handle arguments).

    Excellent suggestion! We don't need our forks taken from us if we want to use them in dangerous ways... ;)

    I'm surprized how come nobody asked what if we write a C program and set it's setuid bit where this C program will calculate the script's checksum (eg. sha1) and filesize, and will run the script with sudo if those results match with the ones hardcoded in the same C program.

  • I prefix a few scripts that are in this boat thus:

    [ "root" != "$USER" ] && exec sudo $0 "[email protected]"

    Note that this does not use setuid but simply executes the current file with sudo.

    This does not use setuid but just gives you a `sudo` prompt. (For me, the whole point of setuid is allowing things to run as root without needing sudo.)

  • If you want to avoid calling sudo some_script you can just do:

      #!/ust/bin/env sh
      sudo /usr/local/scripts/your_script

    SETUID programs need to be designed with extreme care as they run with root privileges and users have large control over them. They need to sanity-check everything. You cannot do it with scripts because:

    • Shells are large pieces of software which interact heavily with user. It is nearly impossible to sanity check everything — especially since most of the code is not intended to run in such mode.
    • Scripts are a mostly quick'n'dirty solution and usually are not prepared with such care that they would allow setuid. They have many potentially dangerous features.
    • They depend heavily on other programs. It is not sufficient that the shell was checked. sed, awk, etc. would need to be checked as well

    Please note that sudo provides some sanity-checking but it isn't sufficient — check every line in your own code.

    As a last note: consider using capabilities. They allow you to give a process running as a user special privileges that would normally require root privileges. However for example, while ping needs to manipulate the network, it does not need to have access to files. I'm not sure however if they are inherited.

    I presume `/ust/bin/env` should be `/usr/bin/env`.

  • super [ -r reqpath] command [ args ]

    Super allows specified users to execute scripts (or other commands) as if they were root; or it can set the uid, gid, and/or supplementary groups on a per-command basis before executing the command. It is intended to be a secure alternative to making scripts setuid root. Super also allows ordinary users to supply commands for execution by others; these execute with the uid, gid, and groups of the user offering the command.

    Super consults a ``'' file to see if the user is allowed to execute the requested command. If permission is granted, super will exec pgm [ args ], where pgm is the program that is associated with this command. (Root is allowed execution by default, but can still be denied if a rule excludes root. Ordinary users are disallowed execution by default.)

    If command is a symbolic link (or hard link, too) to the super program, then typing % command args is equivalent to typing % super command args (The command must not be super, or super will not recognize that it's being invoked via a link.)

    Hi and welcome to the site! We expect answers to be more detailed here. Could you [edit] your answer and explain what this program is and how it could help solve the OP's problem?

    Thank you Nizam, for hours trying to find something like super for accounts to be allowed to be executed by another user as the user of the file.

  • You can create an alias for sudo + the name of the script. Of course, that is even more work to set up, since you then have to setup an alias, too, but it saves you from having to type sudo.

    But if you don't mind horrible security risks, use a setuid shell as the interpreter for the shell script. Don't know whether that'll work for you, but I guess it might.

    Let me state that I advise against actually doing this, though. I'm just mentioning it for educational purposes ;-)

    It will work. Horribly as you stated. SETUID bit allows execution with the owner right. Setuid shell (unless it was designed to work with setuid) will run as root for any user. I.e. anyone can run `rm -rf /` (and other commands from series **DON'T DO IT AT HOME**).

    @MaciejPiechotka by *DON'T DO IT AT HOME* you mean *Feel free to do that at work?* :)

  • If for some reason sudo is not available, you can write a thin wrapper script in C:

    #include <unistd.h>
    int main() {
        execle("/bin/bash","bash","/full/path/to/script",(char*) NULL,(char*) NULL);

    And once you compile it set it as setuid with chmod 4511 wrapper_script.

    This is similar to another posted answer, but runs the script with a clean environment and explicitly uses /bin/bash instead of the shell called by system(), and so closes some potential security holes.

    Note that this discards the environment entirely. If you want to use some environmental variables without opening up vulnerabilities, you really just need to use sudo.

    Obviously, you want to make sure the script itself is only writable by root.

  • I first found this question, unconvinced by all the answers, here is a much better one, which also allows you to completely obfuscate your bash script, if you are so inclined!

    It should be self-explanatory.

    #include <string>
    #include <unistd.h>
    template <typename T, typename U>
    T &replace (
              T &str, 
        const U &from, 
        const U &to)
        size_t pos;
        size_t offset = 0;
        const size_t increment = to.size();
        while ((pos = str.find(from, offset)) != T::npos)
            str.replace(pos, from.size(), to);
            offset = pos + increment;
        return str;
    int main(int argc, char* argv[])
        // Set UUID to root
        std::string script = 
    echo $1
        // Escape single quotes.
        replace(script, std::string("'"), std::string("'\"'\"'"));
        std::string command;
        command = command + "bash -c '" + script + "'"; 
        // Append the command line arguments.
        for (int a = 0; a < argc; ++a)
            command = command + " " + argv[a];
        return system(command.c_str());

    Then you run

    g++ embedded.cpp -o embedded
    sudo chown root embedded
    sudo chmod u+s embedded

    Why the downvotes???

    Probably because it's an overly complicated solution that does string manipulation and has embedded shell code. Either you make a simple launcher, or you use sudo. This is the worst of all options.

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Content dated before 6/26/2020 9:53 AM