advocate 0.3.0

A wrapper around the requests library for safely making HTTP requests on behalf of a third party

Advocate

Advocate is a set of tools based around the requests library for safely making HTTP requests on behalf of a third party. Specifically, it aims to prevent common techniques that enable SSRF attacks.

Advocate was inspired by fin1te’s SafeCurl project.

Installation

pip install advocate

Advocate is officially supported on CPython 2.7+, CPython 3.4+ and PyPy 2. PyPy 3 may work as well, but you’ll need a copy of the ipaddress module from elsewhere.

This seems like it’s been done before

There’ve been a few similar projects, but in my opinion Advocate’s approach is the best because:

It sees URLs the same as the underlying HTTP library

Parsing URLs is hard, and no two URL parsers seem to behave exactly the same. The tiniest differences in parsing between your validator and the underlying HTTP library can lead to vulnerabilities. For example, differences between PHP’s parse_url and cURL’s URL parser allowed a blacklist bypass in SafeCurl.

Advocate doesn’t do URL parsing at all, and lets requests handle it. Advocate only looks at the address requests actually tries to open a socket to.

It deals with DNS rebinding

Two consecutive calls to socket.getaddrinfo aren’t guaranteed to return the same info, depending on the system configuration. If the “safe” looking record TTLs between the verification lookup and the lookup for actually opening the socket, we may end up connecting to a very different server than the one we OK’d!

Advocate gets around this by only using one getaddrinfo call for both verification and connecting the socket. In pseudocode:

def connect_socket(host, port):
    for res in socket.getaddrinfo(host, port):
        # where `res` will be a tuple containing the IP for the host
        if not is_blacklisted(res):
            # ... connect the socket using `res`

See Wikipedia’s article on DNS rebinding attacks for more info.

It handles redirects sanely

Most of the other SSRF-prevention libs cover this, but I’ve seen a lot of sample code online that doesn’t. Advocate will catch it since it inspects every connection attempt the underlying HTTP lib makes.

It understands IPv6

Admittedly, Advocates IPv6 support is still a work-in-progress, since I’m not that familiar with the spec, and there are so many ways to tunnel IPv4 over IPv6, as well as other things we’d rather avoid. IPv6 records are ignored by default for now, but you can enable them with allow_ipv6=True.

If you can think of any improvements to the IPv6 handling, please submit an issue or PR!

Examples

Advocate is more-or-less a drop-in replacement for requests. In most cases you can just replace requests with advocate where necessary and be good to go:

import advocate
print advocate.get("http://google.com/")

Advocate also provides a subclassed requests.Session with sane defaults for blacklisting already set up:

import advocate
sess = advocate.Session()
print sess.get("http://google.com/")

If you have more nuanced rules but still want a drop-in replacement for requests, there’s RequestsAPIWrapper :

from advocate import Blacklist, RequestsAPIWrapper
from advocate.packages import ipaddress

dougs_advocate = RequestsAPIWrapper(Blacklist(ip_blacklist={
    # Contains data incomprehensible to mere mortals
    ipaddress.ip_network("42.42.42.42/32")
}))
print dougs_advocate.get("http://42.42.42.42/")
# ^ blocked!

Caveats

• This is alpha-quality software, the API might change without warning!

• mount() ing other adapters is disallowed to prevent Advocate’s blacklisting adapters from being clobbered.

• Advocate does not (yet) support the use of HTTP proxies.

• Proper IPv6 support is still a WIP as noted above.

Acknowledgements

• https://github.com/fin1te/safecurl for inspiration

• https://github.com/kennethreitz/requests for the lovely requests module

• https://bitbucket.org/kwi/py2-ipaddress for the backport of ipaddress