diff --git a/blacklisting/blacklist.xml b/blacklisting/blacklist.xml
index 7c8c61733f..0ae2b21d2c 100644
--- a/blacklisting/blacklist.xml
+++ b/blacklisting/blacklist.xml
@@ -16,13 +16,25 @@
 
 <item id='zlib-1.2.1-security'>
   <condition>
+    <containsSource
+        hash="sha256:0yp7z8ask4b8m2ia253apnnxdk0z0zrs70yr079m2rjd4297chgv"
+        origin="zlib-1.2.1.tar.gz" />
+<!--
     <or>
-      <containsSource
-          hash="sha256:0yp7z8ask4b8m2ia253apnnxdk0z0zrs70yr079m2rjd4297chgv"
-          origin="zlib-1.2.1.tar.gz" />
+      <and>
+        <containsSource
+            hash="sha256:0yp7z8ask4b8m2ia253apnnxdk0z0zrs70yr079m2rjd4297chgv"
+            origin="zlib-1.2.1.tar.gz" />
+        <not>
+          <containsSource
+              hash="..."
+              origin="zlib-1.2.1-dos.patch" />
+        </not>
+      </and>
       <containsOutput
           name="/nix/store/gxbdsvlwz6ixin94jhdw7rwdbb5mxxq3-zlib-1.2.1" />
     </or>
+    -->
   </condition>
   <reason>
     Zlib 1.2.1 is vulnerable to a denial-of-service condition.  See
diff --git a/blacklisting/check-env.pl b/blacklisting/check-env.pl
new file mode 100755
index 0000000000..f73ad558b8
--- /dev/null
+++ b/blacklisting/check-env.pl
@@ -0,0 +1,107 @@
+#! /usr/bin/perl -w
+
+use strict;
+use XML::Simple;
+
+my $blacklistFN = shift @ARGV;
+die unless defined $blacklistFN;
+my $userEnv = shift @ARGV;
+die unless defined $userEnv;
+
+
+# Read the blacklist.
+my $blacklist = XMLin($blacklistFN,
+    forcearray => [qw()],
+    keyattr => ['id'],
+    suppressempty => '');
+
+
+# Get all the elements of the user environment.
+my $userEnvElems = `nix-store --query --references '$userEnv'`;
+die "cannot query user environment elements" if $? != 0;
+my @userEnvElems = split ' ', $userEnvElems;
+
+
+my %storePathHashes;
+
+
+# Function for evaluating conditions.
+sub evalCondition {
+    my $storePaths = shift;
+    my $condition = shift;
+
+    if (defined $condition->{'containsSource'}) {
+        my $c = $condition->{'containsSource'};
+        my $hash = $c->{'hash'};
+
+        foreach my $path (keys %{$storePathHashes{$hash}}) {
+            # !!! use a hash for $storePaths
+            foreach my $path2 (@{$storePaths}) {
+                return 1 if $path eq $path2;
+            }
+        }
+        return 0;
+    }
+    
+    return 0;
+}
+
+
+# Iterate over all elements, check them.
+foreach my $userEnvElem (@userEnvElems) {
+
+    # Get the deriver of this path.
+    my $deriver = `nix-store --query --deriver '$userEnvElem'`;
+    die "cannot query deriver" if $? != 0;
+    chomp $deriver;
+
+    if ($deriver eq "unknown-deriver") {
+#        print "  deriver unknown, cannot check sources\n";
+        next;
+    }
+
+    print "CHECKING $userEnvElem\n";
+
+
+    # Get the requisites of the deriver.
+    my $requisites = `nix-store --query --requisites --include-outputs '$deriver'`;
+    die "cannot query requisites" if $? != 0;
+    my @requisites = split ' ', $requisites;
+
+
+    # Get the hashes of the requisites.
+    my $hashes = `nix-store --query --hash @requisites`;
+    die "cannot query hashes" if $? != 0;
+    my @hashes = split ' ', $hashes;
+    for (my $i = 0; $i < scalar @requisites; $i++) {
+        die unless $i < scalar @hashes;
+        my $hash = $hashes[$i];
+        $storePathHashes{$hash} = {} unless defined $storePathHashes{$hash};
+        my $r = $storePathHashes{$hash}; # !!! fix
+        $$r{$requisites[$i]} = 1;
+    }
+
+
+    # Evaluate each blacklist item.
+    foreach my $itemId (sort (keys %{$blacklist->{'item'}})) {
+#        print "  CHECKING FOR $itemId\n";
+
+        my $item = $blacklist->{'item'}->{$itemId};
+        die unless defined $item;
+
+        my $condition = $item->{'condition'};
+        die unless defined $condition;
+
+        # Evaluate the condition.
+        if (evalCondition(\@requisites, $condition)) {
+
+            # Oops, condition triggered.
+            my $reason = $item->{'reason'};
+            $reason =~ s/\s+/ /g;
+            $reason =~ s/^\s+//g;
+
+            print "    VULNERABLE TO `$itemId': $reason\n";
+        }
+    }
+}
+