Limit linkHeader length, throw error if exceeds (#25)

* Limit linkheader length, add checkHeader func, update documentation

* indentation

README.md

@@ -46,6 +46,14 @@ console.log(parsed);
 Parses the given link header containing [web links](http://tools.ietf.org/html/rfc5988) and returns an object keyed by
 the `rel` property that contains information about each link.
 
+### Environmental Variables
+
+To avoid redundantly parsing of extremely long (invalid) input, the package uses 2 env variabes:
+
+`PARSE_LINK_HEADER_MAXLEN` - Sets the number of characters the input should be limited to - longer inputs will not be handled. Defaults to `2000`.
+
+`PARSE_LINK_HEADER_THROW_ON_MAXLEN_EXCEEDED` - Defines behavior for when the `PARSE_LINK_HEADER_MAXLEN` parameter is exceeded. if defined, an error will be thrown; if it's `null`, the function fails silently by returning `null`. Defaults to `null`.
+
 ### Formatting a link header
 
 The purpose of this module is to parse the link header information. To format an object generated by this module back to the link header string, use the [format-link-header](https://github.com/jonathansamines/format-link-header) module.

index.js

@@ -4,6 +4,9 @@ var qs = require('querystring')
   , url = require('url')
   , xtend = require('xtend');
 
+const PARSE_LINK_HEADER_MAXLEN = parseInt(process.env.PARSE_LINK_HEADER_MAXLEN) || 2000;
+const PARSE_LINK_HEADER_THROW_ON_MAXLEN_EXCEEDED = process.env.PARSE_LINK_HEADER_THROW_ON_MAXLEN_EXCEEDED != null
+
 function hasRel(x) {
   return x && x.rel;
 }
@@ -46,8 +49,21 @@ function parseLink(link) {
   }
 }
 
+function checkHeader(linkHeader){
+  if (!linkHeader) return false;
+
+  if (linkHeader.length > PARSE_LINK_HEADER_MAXLEN) {
+    if (PARSE_LINK_HEADER_THROW_ON_MAXLEN_EXCEEDED) {
+      throw new Error('Input string too long, it should be under ' + PARSE_LINK_HEADER_MAXLEN + ' characters.');
+    } else {
+        return false;
+      }
+  }
+  return true;
+}
+
 module.exports = function (linkHeader) {
-  if (!linkHeader) return null;
+  if (!checkHeader(linkHeader)) return null;
 
   return linkHeader.split(/,\s*</)
    .map(parseLink)

test/parse-link-header.js

@@ -186,3 +186,21 @@ test('parsing a proper link header with matrix parameters', function (t) {
   )
   t.end()
 })
+
+test('parsing an extremely long link header', function (t) {
+  function payload (n) {
+    var ret = ""
+    for (var i = 0; i < n; i++) {
+      ret += " "
+    }
+    return ret
+    }
+  var linkHeader = '; rel="' + payload(10000) + '",'
+
+  t.equal(
+      parse(linkHeader)
+    , null
+    , 'correctly returns null when dealing with an extremely long link header'
+  )
+  t.end()
+})