cookie: fix tailmatching to prevent cross-domain leakage

Cookies set for 'example.com' could accidentaly also be sent by libcurl
to the 'bexample.com' (ie with a prefix to the first domain name).

This is a security vulnerabilty, CVE-2013-1944.

Bug: http://curl.haxx.se/docs/adv_20130412.html

lib/cookie.c

@@ -118,15 +118,29 @@ static void freecookie(struct Cookie *co)
   free(co);
 }
 
-static bool tailmatch(const char *little, const char *bigone)
+static bool tailmatch(const char *cooke_domain, const char *hostname)
 {
-  size_t littlelen = strlen(little);
+  size_t cookie_domain_len = strlen(cooke_domain);
-  size_t biglen = strlen(bigone);
+  size_t hostname_len = strlen(hostname);
 
-  if(littlelen > biglen)
+  if(hostname_len < cookie_domain_len)
     return FALSE;
 
-  return Curl_raw_equal(little, bigone+biglen-littlelen) ? TRUE : FALSE;
+  if(!Curl_raw_equal(cooke_domain, hostname+hostname_len-cookie_domain_len))
+    return FALSE;
+
+  /* A lead char of cookie_domain is not '.'.
+     RFC6265 4.1.2.3. The Domain Attribute says:
+       For example, if the value of the Domain attribute is
+       "example.com", the user agent will include the cookie in the Cookie
+       header when making HTTP requests to example.com, www.example.com, and
+       www.corp.example.com.
+   */
+  if(hostname_len == cookie_domain_len)
+    return TRUE;
+  if('.' == *(hostname + hostname_len - cookie_domain_len - 1))
+    return TRUE;
+  return FALSE;
 }
 
 /*