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The Theme of Web Security

If you learn only one thing from this chapter, let it be this:

Never — under any circumstances — trust data from the browser.

You never know who’s on the other side of that HTTP connection. It might be one of your users, but it just as easily could be a nefarious cracker looking for an opening.

Any data of any nature that comes from the browser needs to be treated with a healthy dose of paranoia. This includes data that’s both “in band” (i.e., submitted from Web forms) and “out of band” (i.e., HTTP headers, cookies, and other request information). It’s trivial to spoof the request metadata that browsers usually add automatically.

Every one of the vulnerabilities discussed in this chapter stems directly from trusting data that comes over the wire and then failing to sanitize that data before using it. You should make it a general practice to continuously ask, “Where does this data come from?”

SQL Injection

SQL injection is a common exploit in which an attacker alters Web page parameters (such as GET/POST data or URLs) to insert arbitrary SQL snippets that a naive Web application executes in its database directly. It’s probably the most dangerous — and, unfortunately, one of the most common — vulnerabilities out there.

This vulnerability most commonly crops up when constructing SQL “by hand” from user input. For example, imagine writing a function to gather a list of contact information from a contact search page. To prevent spammers from reading every single email in our system, we’ll force the user to type in someone’s username before providing her email address:

def user_contacts(request):
    user = request.GET['username']
    sql = "SELECT * FROM user_contacts WHERE username = '%s';" % username
    # execute the SQL here...

Though at first this doesn’t look dangerous, it really is.

First, our attempt at protecting our entire email list will fail with a cleverly constructed query. Think about what happens if an attacker types "' OR 'a'='a" into the query box. In that case, the query that the string interpolation will construct will be:

SELECT * FROM user_contacts WHERE username = '' OR 'a' = 'a';

Because we allowed unsecured SQL into the string, the attacker’s added OR clause ensures that every single row is returned.

However, that’s the least scary attack. Imagine what will happen if the attacker submits "'; DELETE FROM user_contacts WHERE 'a' = 'a'". We’ll end up with this complete query:

SELECT * FROM user_contacts WHERE username = ''; DELETE FROM user_contacts WHERE 'a' = 'a';

Yikes! Where’d our contact list go?

foo.get_list(bar__exact="' OR 1=1")

SELECT * FROM foos WHERE bar = '\' OR 1=1'

from django.db import connection

def user_contacts(request):
    user = request.GET['username']
    sql = "SELECT * FROM user_contacts WHERE username = %s;"
    cursor = connection.cursor()
    cursor.execute(sql, [user])
    # ... do something with the results

Cross-Site Scripting (XSS)

Cross-site scripting (XSS), is found in Web applications that fail to escape user-submitted content properly before rendering it into HTML. This allows an attacker to insert arbitrary HTML into your Web page, usually in the form of <script> tags.

Attackers often use XSS attacks to steal cookie and session information, or to trick users into giving private information to the wrong person (aka phishing).

This type of attack can take a number of different forms and has almost infinite permutations, so we’ll just look at a typical example. Consider this extremely simple “Hello, World” view:

def say_hello(request):
    name = request.GET.get('name', 'world')
    return render_to_response("hello.html", {"name" : name})

This view simply reads a name from a GET parameter and passes that name to the hello.html template. We might write a template for this view as follows:

<h1>Hello, {{ name }}!</h1>

So if we accessed http://example.com/hello/name=Jacob, the rendered page would contain this:

<h1>Hello, Jacob!</h1>

But wait — what happens if we access http://example.com/hello/name=<i>Jacob</i>? Then we get this:

<h1>Hello, <i>Jacob</i>!</h1>

Of course, an attacker wouldn’t use something as benign as <i> tags; he could include a whole set of HTML that hijacked your page with arbitrary content. This type of attack has been used to trick users into entering data into what looks like their bank’s Web site, but in fact is an XSS-hijacked form that submits their back account information to an attacker.

The problem gets worse if you store this data in the database and later display it it on your site. For example, MySpace was once found to be vulnerable to an XSS attack of this nature. A user inserted JavaScript into his profile that automatically added him as your friend when you visited his profile page. Within a few days, he had millions of friends.

Now, this may sound relatively benign, but keep in mind that this attacker managed to get his code — not MySpace’s — running on your computer. This violates the assumed trust that all the code on MySpace is actually written by MySpace.

MySpace was extremely lucky that this malicious code didn’t automatically delete viewers’ accounts, change their passwords, flood the site with spam, or any of the other nightmare scenarios this vulnerability unleashes.

<h1>Hello, {{ name|escape }}!</h1>

Cross-Site Request Forgery

Cross-site request forgery (CSRF) happens when a malicious Web site tricks users into unknowingly loading a URL from a site at which they’re already authenticated — hence taking advantage of their authenticated status.

Django has built-in tools to protect from this kind of attack. Both the attack itself and those tools are covered in great detail in Chapter 14.

Session Forging/Hijacking

This isn’t a specific attack, but rather a general class of attacks on a user’s session data. It can take a number of different forms:

• A man-in-the-middle attack, where an attacker snoops on session data as it travels over the wire (or wireless) network.

• Session forging, where an attacker uses a session ID (perhaps obtained through a man-in-the-middle attack) to pretend to be another user.

  An example of these first two would be an attacker in a coffee shop using the shop’s wireless network to capture a session cookie. She could then use that cookie to impersonate the original user.

• A cookie-forging attack, where an attacker overrides the supposedly read-only data stored in a cookie. Chapter 12 explains in detail how cookies work, and one of the salient points is that it’s trivial for browsers and malicious users to change cookies without your knowledge.

  There’s a long history of Web sites that have stored a cookie like IsLoggedIn=1 or even LoggedInAsUser=jacob. It’s dead simple to exploit these types of cookies.

  On a more subtle level, though, it’s never a good idea to trust anything stored in cookies; you never know who’s been poking at them.

• Session fixation, where an attacker tricks a user into setting or reseting the user’s session ID.

  For example, PHP allows session identifiers to be passed in the URL (e.g., http://example.com/?PHPSESSID=fa90197ca25f6ab40bb1374c510d7a32). An attacker who tricks a user into clicking a link with a hard-coded session ID will cause the user to pick up that session.

  Session fixation has been used in phishing attacks to trick users into entering personal information into an account the attacker owns. He can later log into that account and retrieve the data.

• Session poisoning, where an attacker injects potentially dangerous data into a user’s session — usually through a Web form that the user submits to set session data.

  A canonical example is a site that stores a simple user preference (like a page’s background color) in a cookie. An attacker could trick a user into clicking a link to submit a “color” that actually contains an XSS attack; if that color isn’t escaped, the user could again inject malicious code into the user’s environment.

Email Header Injection

SQL injection’s less well-known sibling, email header injection, hijacks Web forms that send email. An attacker can use this technique to send spam via your mail server. Any form that constructs email headers from Web form data is vulnerable to this kind of attack.

Let’s look at the canonical contact form found on many sites. Usually this sends a message to a hard-coded email address and, hence, doesn’t appear vulnerable to spam abuse at first glance.

However, most of these forms also allow the user to type in his own subject for the email (along with a “from” address, body, and sometimes a few other fields). This subject field is used to construct the “subject” header of the email message.

If that header is unescaped when building the email message, an attacker could submit something like "hello\ncc:spamvictim@example.com" (where "\n” is a newline character). That would make the constructed email headers turn into:

To: hardcoded@example.com
Subject: hello
cc: spamvictim@example.com

Like SQL injection, if we trust the subject line given by the user, we’ll allow him to construct a malicious set of headers, and he can use our contact form to send spam.

Directory Traversal

Directory traversal is another injection-style attack, wherein a malicious user tricks filesystem code into reading and/or writing files that the Web server shouldn’t have access to.

An example might be a view that reads files from the disk without carefully sanitizing the file name:

def dump_file(request):
    filename = request.GET["filename"]
    filename = os.path.join(BASE_PATH, filename)
    content = open(filename).read()

    # ...

Though it looks like that view restricts file access to files beneath BASE_PATH (by using os.path.join), if the attacker passes in a filename containing .. (that’s two periods, a shorthand for “the parent directory”), she can access files “above” BASE_PATH. It’s only a matter of time before she can discover the correct number of dots to successfully access, say, ../../../../../etc/passwd.

Anything that reads files without proper escaping is vulnerable to this problem. Views that write files are just as vulnerable, but the consequences are doubly dire.

Another permutation of this problem lies in code that dynamically loads modules based on the URL or other request information. A well-publicized example came from the world of Ruby on Rails. Prior to mid-2006, Rails used URLs like http://example.com/person/poke/1 directly to load modules and call methods. The result was that a carefully constructed URL could automatically load arbitrary code, including a database reset script!

import os
import posixpath

# ...

path = posixpath.normpath(urllib.unquote(path))
newpath = ''
for part in path.split('/'):
    if not part:
        # strip empty path components
        continue

    drive, part = os.path.splitdrive(part)
    head, part = os.path.split(part)
    if part in (os.curdir, os.pardir):
        # strip '.' and '..' in path
        continue

    newpath = os.path.join(newpath, part).replace('\\', '/')

Exposed Error Messages

During development, being able to see tracebacks and errors live in your browser is extremely useful. Django has “pretty” and informative debug messages specifically to make debugging easier.

However, if these errors get displayed once the site goes live, they can reveal aspects of your code or configuration that could aid an attacker.

Furthermore, errors and tracebacks aren’t at all useful to end users. Django’s philosophy is that site visitors should never see application-related error messages. If your code raises an unhandled exception, a site visitor should not see the full traceback — or any hint of code snippets or Python (programmer-oriented) error messages. Instead, the visitor should see a friendly “This page is unavailable” message.

Naturally, of course, developers need to see tracebacks to debug problems in their code. So the framework should hide all error messages from the public, but it should display them to the trusted site developers.

A Final Word on Security

We hope all this talk of security problems isn’t too intimidating. It’s true that the Web can be a wild and wooly world, but with a little bit of foresight, you can have a secure Web site.

Keep in mind that Web security is a constantly changing field; if you’re reading the dead-tree version of this book, be sure to check more up to date security resources for any new vulnerabilities that have been discovered. In fact, it’s always a good idea to spend some time each week or month researching and keeping current on the state of Web application security. It’s a small investment to make, but the protection you’ll get for your site and your users is priceless.

What’s Next

In the `next chapter`_, we’ll finally cover the subtleties of deploying Django: how to launch a production site and how to set it up for scalability.

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