Обсуждение: security: escaping user-supplied data
I am writing a C program that will collect data from various untrusted
sources, and put it in a postgresql database by PQexec'ing an INSERT
INTO statement.
The statements I generate are usually of the form:
INSERT INTO foo (bar, bas) VALUES ('abc', 'def');
but the 'abc' and 'def' come from an untrusted source, so if they supply
a string like "def'); delete from foo; '" they can make me do this:
INSERT INTO foo (bar, bas) VALUES ('abc', 'def'); delete from foo; '');
What do I need to do to prevent this? My current plan is to prepend a
backslash to every single-quote, backslash, and semicolon in the
untrusted string. Are there any other special characters I should watch
out for? Is it possible to do something evil despite your special
characters being prepended with a backslash?
At 02:31 +0200 on 12/10/1999, Jason Uhlenkott wrote:
> The statements I generate are usually of the form:
> INSERT INTO foo (bar, bas) VALUES ('abc', 'def');
> but the 'abc' and 'def' come from an untrusted source, so if they supply
> a string like "def'); delete from foo; '" they can make me do this:
> INSERT INTO foo (bar, bas) VALUES ('abc', 'def'); delete from foo; '');
>
> What do I need to do to prevent this? My current plan is to prepend a
> backslash to every single-quote, backslash, and semicolon in the
> untrusted string. Are there any other special characters I should watch
> out for? Is it possible to do something evil despite your special
> characters being prepended with a backslash?
I don't see why you would want to escape a semicolon. If you escape single
quotes and backslashes, the above situation won't happen - the string won't
be finished until the first unescaped quote - yours - is encountered.
Semicolons are not special in strings.
Herouth
--
Herouth Maoz, Internet developer.
Open University of Israel - Telem project
http://telem.openu.ac.il/~herutma
On Tue, Oct 12, 1999 at 11:53:44AM +0200, Herouth Maoz wrote:
> At 02:31 +0200 on 12/10/1999, Jason Uhlenkott wrote:
>
>
> > The statements I generate are usually of the form:
> > INSERT INTO foo (bar, bas) VALUES ('abc', 'def');
> > but the 'abc' and 'def' come from an untrusted source, so if they supply
> > a string like "def'); delete from foo; '" they can make me do this:
> > INSERT INTO foo (bar, bas) VALUES ('abc', 'def'); delete from foo; '');
> >
> > What do I need to do to prevent this? My current plan is to prepend a
> > backslash to every single-quote, backslash, and semicolon in the
> > untrusted string. Are there any other special characters I should watch
> > out for? Is it possible to do something evil despite your special
> > characters being prepended with a backslash?
>
> I don't see why you would want to escape a semicolon. If you escape single
> quotes and backslashes, the above situation won't happen - the string won't
> be finished until the first unescaped quote - yours - is encountered.
> Semicolons are not special in strings.
>
> Herouth
I once posted a similar question to the pgsql-novice mailing
list. There, David Rugge (1 Aug 1999) told me to escape ', ", and %,
even though I am not quite sure why you have to escape " and %. But
now that I think of it: you also need to escape \, of course, or
backslashes will either get lost or, even worse, may escape the
closing quote (think of $def="\"). Thus, using Perl and Pg, you would
do:
use Pg;
$conn = ...;
$abc="abc";
$def="def";
$conn->exec("INSERT INTO foo (bar, bas) VALUES ('" . &stdstr($abc) . "', '" . &stdstr($def) . "')";
sub stdstr { local $or = $_[0]; $or =~ s /\'/\\\'/g; $or =~ s /\"/\\\"/g; $or =~ s /%/\\%/g; $or =~ s
/\\/\\\\/g; return $or;
}
Hope that helps,
Albert.
--
--------------------------------------------------------------------------- Post an / Mail to / Skribu al: Albert
Reiner<areiner@tph.tuwien.ac.at>
---------------------------------------------------------------------------
PHP has some function that auto-escapes a query string so it is safe.
You may want to check that.
> On Tue, Oct 12, 1999 at 11:53:44AM +0200, Herouth Maoz wrote:
> > At 02:31 +0200 on 12/10/1999, Jason Uhlenkott wrote:
> >
> >
> > > The statements I generate are usually of the form:
> > > INSERT INTO foo (bar, bas) VALUES ('abc', 'def');
> > > but the 'abc' and 'def' come from an untrusted source, so if they supply
> > > a string like "def'); delete from foo; '" they can make me do this:
> > > INSERT INTO foo (bar, bas) VALUES ('abc', 'def'); delete from foo; '');
> > >
> > > What do I need to do to prevent this? My current plan is to prepend a
> > > backslash to every single-quote, backslash, and semicolon in the
> > > untrusted string. Are there any other special characters I should watch
> > > out for? Is it possible to do something evil despite your special
> > > characters being prepended with a backslash?
> >
> > I don't see why you would want to escape a semicolon. If you escape single
> > quotes and backslashes, the above situation won't happen - the string won't
> > be finished until the first unescaped quote - yours - is encountered.
> > Semicolons are not special in strings.
> >
> > Herouth
>
> I once posted a similar question to the pgsql-novice mailing
> list. There, David Rugge (1 Aug 1999) told me to escape ', ", and %,
> even though I am not quite sure why you have to escape " and %. But
> now that I think of it: you also need to escape \, of course, or
> backslashes will either get lost or, even worse, may escape the
> closing quote (think of $def="\"). Thus, using Perl and Pg, you would
> do:
>
> use Pg;
> $conn = ...;
>
> $abc="abc";
> $def="def";
> $conn->exec("INSERT INTO foo (bar, bas) VALUES ('" .
> &stdstr($abc) . "', '" . &stdstr($def) . "')";
>
> sub stdstr {
> local $or = $_[0];
> $or =~ s /\'/\\\'/g;
> $or =~ s /\"/\\\"/g;
> $or =~ s /%/\\%/g;
> $or =~ s /\\/\\\\/g;
> return $or;
> }
>
> Hope that helps,
>
> Albert.
>
> --
>
> ---------------------------------------------------------------------------
> Post an / Mail to / Skribu al: Albert Reiner <areiner@tph.tuwien.ac.at>
> ---------------------------------------------------------------------------
>
> ************
>
>
-- Bruce Momjian | http://www.op.net/~candle maillist@candle.pha.pa.us | (610)
853-3000+ If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill,
Pennsylvania19026
Here's a function in perl which should work for cleaning
a string which is sent in a query:
# The checks for already backslashed apostrophies and backslashes
# prevent tricks with these, and also allow us to run clean_text()
# several times.
sub clean_text {
my $text = shift;
return "" if (! defined $text || ! length($text));
# First change backslashed backslashes back to single backslashes.
$text =~ s/\\\\/\\/g;
# Find backslash single quote combinations and convert them to single quotes.
# while... is theoretically not necessary. It's there just as an assurance.
while ($text =~ /\\\'/) {
$text =~ s/\\\'/'/g;
}
# Now there should be no single quotes preceded by backslashes left.
# Then find all backslashes and convert them to doubles.
$text =~ s/\\/\\\\/g;
# Now all characters preceded by a backslash should be in their
# original state, while all backslashed single quotes are
# just single quotes.
# Find all single quotes and backslash them.
# (Note: There are no single quotes preceded by a backslash, so
# it is not possible to have a backslash which would hide a
# backslash preceding a single quote.)
$text =~ s/\'/\\'/g;
return $text;
}
The function could be more efficient if you made $text a reference.
If you know how to use references in Perl, then the conversion
is easy. Just drop the "return" from the end and change all
references to $text with $$text. You should also add a check
such as "! defined $text || ! ref $text ||" to the second line's if
statement.
If you find a way to exploit this, I would like to know.
Troy
>
> At 02:31 +0200 on 12/10/1999, Jason Uhlenkott wrote:
>
>
> > The statements I generate are usually of the form:
> > INSERT INTO foo (bar, bas) VALUES ('abc', 'def');
> > but the 'abc' and 'def' come from an untrusted source, so if they supply
> > a string like "def'); delete from foo; '" they can make me do this:
> > INSERT INTO foo (bar, bas) VALUES ('abc', 'def'); delete from foo; '');
> >
> > What do I need to do to prevent this? My current plan is to prepend a
> > backslash to every single-quote, backslash, and semicolon in the
> > untrusted string. Are there any other special characters I should watch
> > out for? Is it possible to do something evil despite your special
> > characters being prepended with a backslash?
>
> I don't see why you would want to escape a semicolon. If you escape single
> quotes and backslashes, the above situation won't happen - the string won't
> be finished until the first unescaped quote - yours - is encountered.
> Semicolons are not special in strings.
>
> Herouth
>
> --
> Herouth Maoz, Internet developer.
> Open University of Israel - Telem project
> http://telem.openu.ac.il/~herutma
>
>
>
> ************
>
>