--- a/netwerk/base/ProxyAutoConfig.cpp
+++ b/netwerk/base/ProxyAutoConfig.cpp
@@ -33,34 +33,45 @@ static const char *sPacUtils =
   "    return (host.length >= domain.length &&\n"
   "            host.substring(host.length - domain.length) == domain);\n"
   "}\n"
   ""
   "function dnsDomainLevels(host) {\n"
   "    return host.split('.').length - 1;\n"
   "}\n"
   ""
+  "function isValidIpAddress(ipchars) {\n"
+  "    var matches = /^(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})$/.exec(ipchars);\n"
+  "    if (matches == null) {\n"
+  "        return false;\n"
+  "    } else if (matches[1] > 255 || matches[2] > 255 || \n"
+  "               matches[3] > 255 || matches[4] > 255) {\n"
+  "        return false;\n"
+  "    }\n"
+  "    return true;\n"
+  "}\n"
+  ""
   "function convert_addr(ipchars) {\n"
   "    var bytes = ipchars.split('.');\n"
   "    var result = ((bytes[0] & 0xff) << 24) |\n"
   "                 ((bytes[1] & 0xff) << 16) |\n"
   "                 ((bytes[2] & 0xff) <<  8) |\n"
   "                  (bytes[3] & 0xff);\n"
   "    return result;\n"
   "}\n"
   ""
   "function isInNet(ipaddr, pattern, maskstr) {\n"
-  "    var test = /^(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})$/.exec(ipaddr);\n"
-  "    if (test == null) {\n"
+  "    if (!isValidIpAddress(pattern) || !isValidIpAddress(maskstr)) {\n"
+  "        return false;\n"
+  "    }\n"
+  "    if (!isValidIpAddress(ipaddr)) {\n"
   "        ipaddr = dnsResolve(ipaddr);\n"
-  "        if (ipaddr == null)\n"
+  "        if (ipaddr == null) {\n"
   "            return false;\n"
-  "    } else if (test[1] > 255 || test[2] > 255 || \n"
-  "               test[3] > 255 || test[4] > 255) {\n"
-  "        return false;    // not an IP address\n"
+  "        }\n"
   "    }\n"
   "    var host = convert_addr(ipaddr);\n"
   "    var pat  = convert_addr(pattern);\n"
   "    var mask = convert_addr(maskstr);\n"
   "    return ((host & mask) == (pat & mask));\n"
   "    \n"
   "}\n"
   ""