Building Secure UserAgents

I have been working on making an HTTP client (also known as a user agent) that is safe for end-users to control. I investigated building it in Perl, Python, asynchronous Perl, and Go.

During my brief downtime during my paternity leave I’ve been toying with a new application. One of the things this application will do is make web requests on behalf of users. There are plenty of examples of applications that do this already: RSS Readers, anything that has OpenID login support, and things that do postbacks; when someone sends an SMS to my Twilio number, it hits an endpoint of my choosing.

Sometimes applications that do these kinds of requests can be vulnerable to attack. Last year Clint Ruoho found a handful of problems with Pocket, a service Mozilla had recently bundled with Firefox.

The vulnerabilities listed there are only the beginning. Here are some things that an attacker could do:

  • Connect to private services, listening only on localhost, assumed to be secure
  • Read from AWS EC2 UserData (which Ruoho did in the example above)
  • Connect to private services running on other servers, that are not normally addressible to the outside world

How do we protect against this?

I suspect that most people protect against this by analyzing the url in the request.

if ($req->url->host eq '') { ... }

For example, today, if you go to (or the localhost version) it knows that you are hitting a “non-internet” URL. I made a domain ( that resolves to and today, if you go to it claims that the site is actually up. And that’s just the tip of the iceberg. We can tell that is running in an AWS-like environment because seems to be “up” from the server’s perspective. There are a non-trivial number of private IP addresses like this (details in the appendix.)

So at the very least we cannot merely inspect the request, we need to verify the resolution of the domain.

use Socket 'getaddrinfo', 'NI_NUMERICHOST';
my (undef, @addrs) = getaddrinfo($req->uri->host, NI_NUMERICHOST);
my @ips = map {
   my (undef, $ip, $service) = getnameinfo($_->{addr}, NI_NUMERICHOST);
} @addrs;
if (grep { $_ eq '' } @ips) { ... }

Even that is insufficient though. As Ruoho found, many user agents will automatically handle redirects, so even though the implementor may have done all of the above (which I think is non-trivial; I left out a lot of error handling in the second part and none of it correctly handles all of the various IP masks,) a domain could be validated, and then redirect to an IP that should have been blocked.

There’s also what is sometimes called “tarpits.” Some user agents define timeouts as “stall” timeouts: they reset when any progress is made. Consider the Slowloris attack, but implemented at the server side instead of at the client. Similarly a DNS server can return long chains of CNAMEs to cause the same kind of problem. This should be fixed with a global timeout (instead of the more common stall timeouts referenced before.)

Another vulnerability is unexpected schemata for requests. Some clients are smart enough to access file://, ftp://, etc. Clients like this must be defanged such that they only access http:// and https://. I tend to only use less magical clients, but support for the above is only a patch away.


The redirect detail makes it clear that the post resolution verification must happen within the user agent. A solid user agent design should make this reasonably doable. The first user agent I’d heard of that tackled these problems (though likely not the first in existence) is LWPx::ParanoidAgent, made by Brad Fitzpatrick almost surely while at LiveJournal to protect against attacks originating from OpenID servers. LWP::UserAgent::Paranoid has since supplanted it with better, more modular code; but the general idea and usage is the same.


The problem with these two modules is that they are written in the classic blocking style. If you need to make 20 HTTP requests and each takes 0.5s you just spent 10s. Newer tools are asynchronous, and so could do 20 HTTP requests in parallel. When I do async in Perl I use IO::Async. In IO::Async here is how you could create a safe client:

#!/usr/bin/env perl

use 5.24.0;
use warnings;

use Net::Async::HTTP;
use IO::Async::Loop::Epoll;

use Net::Subnet;

# this list is incomplete, see the appendix
my $private = subnet_matcher qw(

my $loop = IO::Async::Loop::Epoll->new;
my $http = Net::Async::HTTP->new(
   timeout => 10,

$loop->add( $http );

my ( $response ) = $http->do_request(
   uri => URI->new( shift ),
   on_ready => sub {
      my $sock = $_[0]->read_handle->peerhost;
      if ($private->($sock)) {
        close $sock;
        return Future->fail('Illegal IP') 

print $response->code;

If I end up using Perl for this project I’ll likely publish a subclass of naHTTP, or submit a patch, allowing the on_ready handler to be set for the whole class instead of requiring it to be set per request.


Before I came up with the async Perl option above I had come to the conclusion that it was be a ton of work to get it working in IO::Async and that I should just use Go. I might still use Go, as it’s more well supported for code of this nature. In Go I was able to basically use the same technique as above:

package main

import (

func main() {
  _, net1, _ := net.ParseCIDR("")
  _, net2, _ := net.ParseCIDR("")
  _, net3, _ := net.ParseCIDR("")
  _, net4, _ := net.ParseCIDR("")
  _, net5, _ := net.ParseCIDR("")
  nets := [](*net.IPNet){net1, net2, net3, net4, net5}

  internalClient := &http.Client{
    Timeout: 10 * time.Second,
    Transport: &http.Transport{
      Dial: func(network, addr string) (net.Conn, error) {
        conn, err := net.Dial(network, addr)

        if err != nil {
          return nil, err

        ipStr, _, err := net.SplitHostPort(conn.RemoteAddr().String())
        // no idea how this could happen
        if err != nil {
          return nil, err

        ip := net.ParseIP(ipStr)
        for _, net := range nets {
          if net.Contains(ip) {
            err := conn.Close()
            if err != nil {
              // wtf
            return nil, errors.New("Illegal IP")

        return conn, nil

  res, err := internalClient.Get(os.Args[1])

  if err != nil {


The above is very similar to the IO::Async version. Basically we set a global timeout on the client, and then in the code that connects to a socket, vet the socket before continuing.


Perl is not really the “big dog” of dynamic languages anymore, so I figured I’d document how to do this with a more popular language. I mentioned that I’ve been toying with Python lately already, so it seemed like the most natural choice. If you know how to do this with other languages hit me up.

I looked at urllib2, urllib3, and requests, and it seemed like this kind of feature is impossible in these popular Python libraries without significant rewriting, duplication, or patches. I would love to be wrong here, and will update this post if someone can show me how to do what needs to be done. Otherwise, if you are using Python and need to do requests on behalf of the user, best of luck: you may end up writing your own HTTP client.

Also beware that at least urllib2 is helpful enough to provide support for file://. Make sure that if you are using urllib2, even indirectly, you remove support for untrusted handlers.

As with all security concerns, this is about measuring the cost of failure. There is no bug free code; the cost of eternal vigilance and perfection are too high. The only other option I know of would be to spin up a completely separate virtual machine isolated as much as possible from the rest of your system, in it’s own DMZ maybe. This is feasible, but it is certainly a high cost alternative to something that’s not technically difficult.

I was surprised at how easy this was in both Go and IO::Async after striking upon the post-connection verification idea. Initially I had assumed that this was a nearly impossible to solve problem, because I assumed it needed to hook into DNS resolution directly.

The other big win in this modern day and age is that timeouts are easier to implement, and tend to be more trustworthy.

I hope this helps!

Appendix: Private Ranges

Please do not assume that this list is complete. I would love for it to be up-to-date and trustworthy, but it requires knowing all of the relevant RFC’s. Here are the ones I know about and where they are from, almost all of these were informed by RFC6890, Sections 2.2.2 and 2.2.3. Note also that some of these may not be a security vulnerability, like, but generally I doubt that the extra check is going to be expensive enough to matter.

Address Block Relevant RFC RFC1122 RFC1918 RFC6598 RFC1122 RFC3927 RFC1918 RFC6890 RFC6333 RFC5737 RFC3068 RFC1918 RFC2544 RFC5737 RFC5737 RFC1112 RFC0919

The IPv6 ranges have a lot of weird stuff in them. One block, for example, was terminated already a couple years ago. Again, I suspect that for most of them it’s safe to block them and then remove the block later if you find that you need to (like if you absurdly end up on an IPv6 only network.)

Address Block Relevant RFC
::1/128 RFC4291
::/128 RFC4291
64:ff9b::/96 RFC6052
::ffff:0:0/96 RFC4291
100::/64 RFC6666
2001::/23 RFC2928
2001::/32 RFC4380
2001:2::/48 RFC5180
2001:db8::/32 RFC3849
2001:10::/28 RFC4843
2002::/16 RFC3056
fc00::/7 RFC4193
fe80::/10 RFC4291

There are likely more. I think the definitive listings are here and here respectively, but some of the blocks in those listings don’t look private to me.

Posted Mon, Jul 25, 2016