From 0c17c3470e0888f592a93f8caa67cbbadfd76efb Mon Sep 17 00:00:00 2001
From: David Cook <dcook@prosentient.com.au>
Date: Thu, 23 May 2019 16:53:57 +1000
Subject: [PATCH] Bug 23290: XSLT system preferences allow administrators to
 exploit XML and XSLT vulnerabilities

The problem is that administrators can provide XSLTs that
can read from the server and network and write to the server. The

This patch prevents the Koha::XSLT_Handler from running
XSLT stylesheets that call actions such as read_file, write_file,
read_net, and write_net as documented at
https://metacpan.org/pod/XML::LibXSLT#XML::LibXSLT::Security

(Previous tests suggested issues with XML external entities
causing read file like vulnerabilities but these were not
reproducible)
---
 Koha/XSLT_Handler.pm | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/Koha/XSLT_Handler.pm b/Koha/XSLT_Handler.pm
index 0682c3ca20..55ce24c53f 100644
--- a/Koha/XSLT_Handler.pm
+++ b/Koha/XSLT_Handler.pm
@@ -320,8 +320,41 @@ sub _load {
         return;
     }
 
+    my $security = XML::LibXSLT::Security->new();
+    $security->register_callback( read_file  => sub {
+        warn "read_file called in XML::LibXSLT";
+        #i.e. when using the exsl:document() element or document() function (to read a XML file)
+        my ($tctxt,$value) = @_;
+        return 0;
+    });
+    $security->register_callback( write_file => sub {
+        warn "write_file called in XML::LibXSLT";
+        #i.e. when using the exsl:document element (or document() function?) (to write an output file of many possible types)
+        #e.g.
+        #<exsl:document href="file:///tmp/breached.txt">
+        #   <xsl:text>breached!</xsl:text>
+        #</exsl:document>
+        my ($tctxt,$value) = @_;
+        return 0;
+    });
+    $security->register_callback( read_net   => sub {
+        warn "read_net called in XML::LibXSLT";
+        #i.e. when using the document() function (to read XML from the network)
+        #e.g. <xsl:copy-of select="document('http://localhost')" />
+        my ($tctxt,$value) = @_;
+        return 0;
+    });
+    $security->register_callback( write_net  => sub {
+        warn "write_net called in XML::LibXSLT";
+        #NOTE: it's unknown how one would invoke this, but covering our bases anyway
+        my ($tctxt,$value) = @_;
+        return 0;
+    });
+
     #parse sheet
     my $xslt = XML::LibXSLT->new;
+    $xslt->security_callbacks( $security );
+
     $rv = $code? $digest.$codelen: $filename;
     $self->{xslt_hash}->{$rv} = eval { $xslt->parse_stylesheet($style_doc) };
     if ($@) {
-- 
2.16.4