Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Date: Sun, 28 Dec 2008 11:26:12 +0100
From: Florian Weimer <>
Subject: Re:  Re: CVE Request - roundcubemail

* Steven M. Christey:

> On Wed, 17 Dec 2008, Florian Weimer wrote:
>> > I bet there's a chunk of these in various applications.  I believe Perl
>> > has similar functionality.
>> Not quite, the s///e operator uses a compile-time transformation for
>> the replacement expression, so it shouldn't be affected by this very
>> issue.
>> \Q \E pairs are an issue in the pattern, not the replacement.
>> Mistakes in this area increase the attack surface by exposing the
>> regular expression compiler to potentially hostile input, and it may
>> lead to denial-of-service vulnerabilities because some implementations
>> do not cope well with certain patterns.  Perhaps CWE-624 should be
>> split to reflect this?
> We'll take a closer look at it.


> I'm not exactly sure what you're saying here, though.  Do you mean that if
> attackers can insert a \Q or \E into the pattern, then they might be able
> to effectively modify the pattern in unexpected ways?

What I'm trying to say is: The PHP way of implementing
preg_replace("/$pattern/e", $expr, $subject) is something like this:

  my @captures = $subject =~ /$pattern/;
  if (@captures) {
    $expr =~ s/\$(\d+)/quotemeta($captures[$1])/ge; # expand captures
    $result = eval "$expr"; # run code
  } else {
    $result = $subject;

This means that capture contents can leak into $expr and be executed.

Perl translates 

  $subject =~ s/$pattern/$expr/e;


    eval "sub regexp001 {
      \$0 = \$_[0];
      \$1 = \$_[1];
      ... # number of assignments depends on \$expr

  if ($subject =~ /$pattern/) {
    substr $subject, $-[0], $+[0] - $-[0], regexp001($1, $2, ...);

Or something like that.  I can't find it in the source code, but it's
possible to reveal that the replacement expression is compiled early
by putting a BEGIN block into the replacement expression.

Powered by blists - more mailing lists

Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.