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Locking Ruby in the Safe


Walter Webcoder has a great idea for a portal site: The Web Arithmetic Page. Surrounded by all sorts of cool mathematical links and banner ads that will make him rich is a simple central frame, containing a text field and a button. Users type an arithmetic expression into the field, press the button, and the answer is displayed. All the world's calculators become obsolete overnight, and Walter cashes in and retires to devote his life to his collection of car license plate numbers.

Implementing the calculator is easy, thinks Walter. He accesses the contents of the form field using Ruby's CGI library, and uses the eval method to evaluate the string as an expression. 

require 'cgi'


cgi = CGI::new("html4")


# Fetch the value of the form field "expression"
expr = cgi["expression"].to_s


begin
  result = eval(expr)
rescue Exception => detail
  # handle bad expressions
end


# display result back to user...

Roughly seven seconds after Walter puts the application online, a twelve-year-old from Waxahachie with glandular problems and no real life types ``system("rm *")'' into the form and, like his application, Walter's dreams come tumbling down.

Walter learned an important lesson: All external data is dangerous. Don't let it close to interfaces that can modify your system. In this case, the content of the form field was the external data, and the call to eval was the security breach.

Fortunately, Ruby provides support for reducing this risk. All information from the outside world can be marked as tainted. When running in a safe mode, potentially dangerous methods will raise a SecurityError if passed a tainted object.

Safe Levels

The variable $SAFE determines Ruby's level of paranoia. Table 20.1 on page 257 gives details of the checks performed at each safe level.

$SAFE Constraints
0 No checking of the use of externally supplied (tainted) data is performed. This is Ruby's default mode.
>= 1 Ruby disallows the use of tainted data by potentially dangerous operations.
>= 2 Ruby prohibits the loading of program files from globally writable locations.
>= 3 All newly created objects are considered tainted.
>= 4 Ruby effectively partitions the running program in two. Nontainted objects may not be modified. Typically, this will be used to create a sandbox: the program sets up an environment using a lower $SAFE level, then resets $SAFE to 4 to prevent subsequent changes to that environment.

The default value of $SAFE is zero under most circumstances. However, if a Ruby script is run setuid or setgid,[A Unix script may be flagged to be run under a different user or group id than the person running it. This allows the script to have privileges that the user does not have; the script can access resources that the user would otherwise be prohibited from using. These scripts are called setuid or setgid.] its safe level is automatically set to 1. The safe level may also be set using the -T command-line option, and by assigning to $SAFE within the program. It is not possible to lower the value of $SAFE by assignment.

The current value of $SAFE is inherited when new threads are created. However, within each thread, the value of $SAFE may be changed without affecting the value in other threads. This facility may be used to implement secure ``sandboxes,'' areas where external code may run safely without risking the rest of your application or system. Do this by wrapping code that you load from a file in its own, anonymous module. This will protect your program's namespace from any unintended alteration. 

f=open(fileName,"w")
f.print ...   # write untrusted program into file.
f.close
Thread.start {
  $SAFE = 4
  load(fileName, true)
}

With a $SAFE level of 4, you can load only wrapped files. See Kernel::load on page 418 for details.

Tainted Objects

Any Ruby object derived from some external source (for example, a string read from a file, or an environment variable) is automatically marked as being tainted. If your program uses a tainted object to derive a new object, then that new object will also be tainted, as shown in the code below. Any object with external data somewhere in its past will be tainted. This tainting process is performed regardless of the current safe level. You can inspect the tainted status of an object using Object#tainted? .

# internal data
# =============
x1 = "a string"
x1.tainted?
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