SecuriTeam - Avenger's News System Command Execution Vulnerability

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CanSecWest 2012

2nd Annual Cyber Security

Hack In Paris

Avenger's News System (ANS) is a PERL-based solution to creating an easy-to-update and easy-to-maintain web site. Instead of constantly uploading new news pages and wrestling with HTML, you can post stuff via a web-based form. A security vulnerability in the product allows command execution by remote attackers who have access to the ANS page.

Credit:
The information has been provided by b0iler _.

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The variable $QUERY is defined in the configuration file as:
<define QUERY>"$ENV{'QUERY_STRING'}"

When the script runs, it checks for a POST and then checks for a plugin. The problem is in the plugin subroutine:

  if (substr($QUERY, 0, 2) eq "p=")
  {
    $plugin = substr((split /&/, $QUERY)[0], 2);
    if (index("$QUERY", "&") < 0) { $QUERY = ""; }
                             else { $QUERY = substr($QUERY, index("$QUERY", "&")+1); }

    open (PLUGIN, "$FILE_LOCATION/$plugin");
    @plugin = <PLUGIN>;
    close (PLUGIN);

    eval("@plugin");
    exit;
  }

No input filtering is done on user input so command execution is possible.

Exploit:
ans.pl?p=../../../../bin/command argument|&blah

Fix:
Filter characters such as '..' and use < << > >> when calling open().

Replace the above code with this:

if (substr($QUERY, 0, 2) eq "p="){
$QUERY =~ s/([\&;\`'\\\|"*?~<>^\[\]\{\}\$\n\r])/\\$1/g; #filter meta characters
$QUERY =~ s/\.\.//g; #filter double dot (..)
$plugin = substr((split /&/, $QUERY)[0], 2);
if (index("$QUERY", "&") < 0) { $QUERY = ""; }
else { $QUERY = substr($QUERY, index("$QUERY", "&")+1); }

open (PLUGIN, "<$FILE_LOCATION/$plugin"); #added a < to the open() - readonly
@plugin = <PLUGIN>;
close (PLUGIN);

eval("@plugin");
exit;
}

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