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How To Harden And Simplify Urlopen Function In Python

by Jonathan BowmanMarch 22nd, 2021

Too Long; Didn't Read

Python has a useful high-level HTTP client built into the standard library: urllib.request.urlopen. The default function opens a variety of protocols, even allowing local. filesystem access withURLs. Python's built-in OpenerDirector class offers just such an opportunity to streamline the function. It will still follow redirects automatically, and raise an exception for any HTTP status code that isn't in the 200s or 300s. The code above communicates the lesson "sanitize your user inputs"

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Python has a useful high-level HTTP client built into the standard library:

urllib.request.urlopen()

. While HTTP libraries such as requests and HTTPX are excellent,

urlopen

gets things done without external dependencies.

However, there is a notable security concern with

urlopen

: the url can open a variety of protocols, even allowing local filesystem access with

file:///

URLs. Of course, we should always sanitize user input, but what if we could configure

urlopen

to only load, say,

https

URLs?

The built-in OpenerDirector class offers just such an opportunity to streamline

urlopen

, make it more secure, and provide custom error handling.

Security problems with the default

urlopen

You might try the following on a Linux system:

from urllib.request import urlopen

url = "file:///etc/passwd"

with urlopen(url) as response:
  print(response.read().decode())

A little disturbing, yes? Bandit agrees. (Perhaps you want to consider scanning with Bandit or its related flake8 plugin.)

The code above certainly communicates the lesson "sanitize your user inputs". Of course, if you control that "url" string, or can ensure that it starts with the correct "https://" scheme, then things are looking better. If this url comes from user input, though, it would be good to check for protocol at least, and, better yet, ensure that the domain is as expected.

A suggested solution

The following code results in a

urlopen

command that only opens "https://" URLs by default:

import urllib.request


class SafeOpener(urllib.request.OpenerDirector):
    def __init__(self, handlers: typing.Iterable = None):
        super().__init__()
        handlers = handlers or (
            urllib.request.UnknownHandler,
            urllib.request.HTTPDefaultErrorHandler,
            urllib.request.HTTPRedirectHandler,
            urllib.request.HTTPSHandler,
            urllib.request.HTTPErrorProcessor,
        )

        for handler_class in handlers:
            self.add_handler(handler_class())


opener = SafeOpener()
urllib.request.install_opener(opener)

After running the above, using

urllib.request.urlopen()

should fail with a

URLError

if attempting to open http, ftp, file, data, or any other URL that doesn't have "https:" at the beginning. It will still follow redirects automatically, just as

urlopen

does, and raise an exception for any HTTP status code that isn't in the 200s or 300s.

By the way, if you prefer not to override the opener in the

urlopen

function, you could remove the

install_opener(opener)

line. Then call

opener.open()

anywhere you would have previously used

urlopen()

The above code assumes that all HTTP calls will be encrypted with TLS (aka "SSL", with "https:" at the beginning of the URL). That also means that testing will need to use "https:" URLs as well. Consider using the vcr library to mock-reproduce HTTP calls, or the trustme library to actually set up certs for testing. If you need to use unencrypted "http:" URLs, though, you can simply add

urllib.request.HTTPHandler

to the

handlers

iterable.

The handlers, defined

This is the chain of default handlers normally used by

urlopen

```
ProxyHandler
```
: searches system settings for proxies. If you are 100% sure that your tool or library will never be used with a proxy, then this is not necessary.
```
UnknownHandler
```
: raises a URLError if the protocol requested in the URL is not supported by a handler in this chain. Very helpful and recommended.
```
HTTPHandler
```
: handles unencrypted HTTP connections. Only add this if you are sure you need URL support other than HTTPS
```
HTTPDefaultErrorHandler
```
: a prefilter of sorts that turns all responses into exceptions, for handling downstream. This is necessary unless you plan on handling statuses, exceptions, and redirects yourself.
```
HTTPRedirectHandler
```
: handles redirects (status codes 301, 302, 303, or 307) and is necessary if automatic following of redirects is desired.
```
FTPHandler
```
: handles ftp: URLs. Not necessary for HTTP calls.
```
FileHandler
```
: handles file: URLs, and poses security risks. Rarely should this be necessary, if ever.
```
HTTPErrorProcessor
```
: The final response handler, raising any non-200 (OK) responses
```
DataHandler
```
: handles
```
data:
```
URLs. Hard to imagine why this would be necessary in normal use, and could pose potential security risks with user input.

As may be apparent, several of the above are rarely necessary, if ever, for HTTP API work. Instead, I recommend this list as a happy medium between security and usability:

```
ProxyHandler
```
```
UnknownHandler
```
```
HTTPHandler
```
```
HTTPDefaultErrorHandler
```
```
HTTPRedirectHandler
```
```
HTTPSHandler
```
```
HTTPErrorProcessor
```

Of the above,

ProxyHandler

could possibly be removed if you know you don't need it, and even

HTTPHandler

could be removed if you know that only HTTPS URLs will be called. Actually, this is a pretty good combination: the point of HTTPS is to ensure that nothing is intercepting the connection, and that there are no proxies. So a most-secure list is the same as what is in the example code above: