Safe mode

From Rosetta Code
Safe mode is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Does the language implementation allow for a "safer mode" of execution? Usually termed Safe mode, a more realistic view is probably Safer mode or restricted mode. It is one thing to place restrictions on execution, and another thing entirely to allow execution of scripts from untrusted sources and assume nothing untoward will happen.

Along with a simple yes/no answer, describe what features are restricted when running in safe mode.


# syntax: GAWK --sandbox -f SAFE_MODE.AWK
# Per The GNU Awk User’s Guide, edition 4.2.1
# --sandbox or -S
# Disable the system() function, input redirections with getline, output
# redirections with print and printf, and dynamic extensions. This is
# particularly useful when you want to run awk scripts from questionable
# sources and need to make sure the scripts can't access your system (other
# than the specified input data file).
# Error message when running in sandbox mode:
# gawk: SAFE_MODE.AWK:16: fatal: redirection not allowed in sandbox mode
print("hello world") >"A.TMP"


Any code written in Go is considered to be 'safe' unless it uses one or more of the following features:

  • The 'unsafe' package.
  • The 'reflect' package.
  • cgo.

Although 'normal' Go code uses pointers, arithmetic on them is not permitted and so they cannot be made to point to arbitrary locations in memory. However, the 'unsafe' package contains features which do allow one to perform pointer arithmetic with all the risks this entails.

The 'reflect' package allows one to inspect and manipulate objects of arbitrary types and exposes internal data structures such as string and slice headers. This can result in fragile code where mistakes which would normally be identified at compile time will instead manifest themselves as runtime panics.

'cgo' is Go's bridge to using C code. As such it is just as unsafe as writing C code directly.


The jsish interpreter allows a -s, --safe command line switch to restrict access to the file system.

For example, given safer.jsi:

File.write('/tmp/safer-mode.txt', 'data line');
prompt$ jsish safer.jsi
prompt$ jsish -s safer.jsi
/home/btiffin/lang/jsish/safer.jsi:2: error: write access denied by safe interp: /tmp/safer-mode.txt    (at or near "data line")


The Jsish implementation borrows many ideas from Tcl, and also includes an Interp module. These sub interpreters can also be set to run in a safer mode.

prompt$ jsish
# var si = new Interp({isSafe:true});
# si.source('safer.jsi');
error: read access denied: /home/btiffin/lang/jsish/safer.jsi

Some control is allowed over the restrictions provided by safer mode.

var interp1 = new Interp({isSafe:true, safeWriteDirs:['/tmp'], , safeReadDirs:['/tmp']});


The only built-in 'safer' way to run Perl is to invoke it in 'taint mode' with the command line option -T. While in this mode input from the user, and all variables derived from it, cannot be used in certain contexts until 'sanitized' by being passed through a regular expression.

There is a CPAN module Safe that purports to allow Perl to be compiled and executed in restricted compartments, isolated namespaces with limits on allowed operators. However, it has not been updated in over 6 years, and so not tested with recent releases of Perl. Further, it does not appear to work at all on BSD-derived versions of UNIX.

There's really no switch to flip to make Perl code more secure. It is up to the programmer to follow security best-practices, such as employing the strict and warnings pragmas, using 3-argument form of open for filehandles, being careful about the contents of $ENV{PATH}, and so forth. The CPAN module Perl::Critic can be helpful in this regard. Read further on this topic in the language documentation on Perl security

Perl 6[edit]

Mostly a cut-n-paste from the Untrusted environment task.

Perl 6 doesn't really provide a high security mode for untrusted environments. By default, Perl 6 is sort of a walled garden. It is difficult to access memory directly, especially in locations not controlled by the Perl 6 interpreter, so unauthorized memory access is unlikely to be a threat with default Perl 6 commands and capabilities.

It is possible (and quite easy) to run Perl 6 with a restricted setting which will disable many IO commands that can be used to access or modify things outside of the Perl 6 interpreter. However, a determined bad actor could theoretically work around the restrictions, especially if the nativecall interface is available. The nativecall interface allows directly calling in to and executing code from C libraries so anything possible in C is now possible in Perl 6. This is great for all of the power it provides, but along with that comes the responsibility and inherent security risk. The same issue arises with unrestricted loading of modules. If modules can be loaded, especially from arbitrary locations, then any and all restriction imposed by the setting can be worked around.

The restricted setting is modifiable, but by default places restrictions on or completely disables the following things:

User Subroutines (disabled)
  • sub chmod() modify filesystem permissions
  • sub copy() copy a file
  • sub link() create a link to a file
  • sub mkdir() make a filesystem directory
  • sub open() open a filesystem location / file
  • sub pipe() open a pipe
  • sub QX() execute arbitrary code
  • sub rename() rename a file
  • sub rmdir() remove a directory
  • sub run() run arbitrary code
  • sub shell() execute code in a shell
  • sub socket() open a socket
  • sub spurt() write a file
  • sub symlink() create a symbolic link to a location
  • sub unlink() delete a file
Internal Subroutines (disabled)
  • sub CHANGE-DIRECTORY change directory
  • sub CHMOD-PATH change permissions
  • sub COPY-FILE copy a file
  • sub MAKE-DIR make a directory
  • sub REMOVE-DIR remove a directory
  • sub RENAME-PATH rename a directory
  • sub SYMLINK-PATH create a symbolic link
  • sub UNLINK-PATH delete a file
Classes (disabled)
  • class IO::CatHandle streaming file handle
  • class IO::Handle file handle
  • class IO::Path filesystem path
  • class IO::Pipe OS pipe
  • class IO::Socket OS socket
  • class IO::Socket::INET Network socket
  • class NativeCall Nativecall interface to foreign code (C mostly)
  • class Proc OS Process
  • class Proc::Async Asynchronous OS Process
Method Mixins / Roles (locked down so can't be overridden)
  • method FALLBACK() handle unknown method calls
  • method new() create a new instance
  • method gist() display method

Really, if you want to lock down a Perl 6 instance so it is "safe" for unauthenticated, untrusted, general access, you are better off running it in some kind of locked down virtual machine or sandbox managed by the operating system rather than trying to build an ad hoc "safe" environment.


For running REXX on IBM mainframes,   REXX supports the option   Scan   for the   trace   statement.

This allows the program to be processed (and be checked for syntax errors),   but commands to the "host system" won't be executed.

However, not all REXXes support this option.

Regina REXX supports a --restricted command line option, and embedded interpreters can also be set to run restricted. Many commands are disabled in this mode, including most access to hosted services. The intrinsic FUNCTION REXX() extension in GnuCOBOL defaults to restricted mode, and programmers must explicitly use FUNCTION REXX-UNRESTRICTED(script, args...) for access to the full REXX programming environment from that COBOL implementation.

       identification division.
program-id. rexxtrial.
environment division.
configuration section.
function all intrinsic.
data division.
working-storage section.
procedure division.
*> First attempt fails and return statement does not execute
display rexx("ADDRESS SYSTEM; 'ls rexxtrial.cob'; return 'fail'")
display "Exception: " exception-status
*> Second is allowed and succeeds
display "Try with rexx-unrestricted"
display rexx-unrestricted(
"ADDRESS SYSTEM; 'ls -l rexxtrial.cob'; return 'success'")
display "No exception raised: " exception-status
end program rexxtrial.
$ cobc -xj rexxtrial.cob
     1 +++ 'ls rexxtrial.cob'
Error 95 running "gnucobol", line 1: [Restricted feature used in "safe" mode]
Error 95.5: [Running external commands invalid in "safe" mode]

Exception: EC-IMP-SCRIPT
Try with rexx-unrestricted
-rw-rw-r--. 1 btiffin btiffin 727 Feb 19 04:26 rexxtrial.cob
No exception raised: