Simon Willison’s Weblog

Pysa: An open source static analysis tool to detect and prevent security issues in Python code (via) Interesting new static analysis tool for auditing Python for security vulnerabilities—things like SQL injection and os.execute() calls. Built by Facebook and tested extensively on Instagram, a multi-million line Django application. # 7th August 2020, 8:50 pm

James Bennett on why Django should not support JWT in core (via) The topic of adding JWT support to Django core comes up occasionally—here’s James Bennett’s detailed argument for not doing that. The short version is that the JWT specification isn’t just difficult to implement securely: it’s fundamentally flawed, which results in things like five implementations in three different languages all manifesting the same vulnerability. Third party modules exist that add JWT support to Django, but baking it into core would act as a form of endorsement and Django’s philosophy has always been to encourage people towards best practices. # 1st August 2020, 12:28 am

CSRF: Flash + 307 redirect = Game Over. Here’s the exploit that Django and Rails both just released fixes for. It’s actually a flaw in the Flash player. Flash isn’t meant to be able to make cross-domain HTTP requests with custom HTTP headers unless the crossdomain.xml file on the other domain allows them to, but it turns out a 307 redirect (like a 302, but allows POST data to be forwarded) confuses the Flash player in to not checking the crossdomain.xml on the host it is being redirect to. # 10th February 2011, 10:07 pm

Bleach, HTML sanitizer and auto-linker. HTML sanitisation is notoriously difficult to do correctly, but Bleach (a Python library) looks like an excellent effort. It uses the html5lib parsing library to deal with potentially malformed HTML, uses a whitelist rather than a blacklist and has a neat feature for auto-linking URLs that is aware of the DOM (so it won’t try to auto-link a URL that is already wrapped in a link element). It was written by the Mozilla team for addons.mozilla.org and support.mozilla.org so it should be production ready. # 25th October 2010, 1:32 pm

Timing attack in Google Keyczar library. An issue I also need to fix in the proposed Django signing code. If you’re comparing two strings in crypto (e.g. seeing if the provided signature matches the expected signature) you need to use a timing independent string comparison function or you risk leaking information. This kind of thing is exactly why I want an audited signing module in Django rather than leaving developers to figure it out on their own. # 4th January 2010, 3:23 pm

Design and code review requested for Django string signing / signed cookies. Do you know your way around web app security and cryptography (in particular signing things using hmac and sha1)? We’d appreciate your help reviewing the usage of these concepts in Django’s proposed string signing and signed cookie implementations. # 4th January 2010, 1:24 pm

Django security updates released. A potential denial of service vulnerability has been discovered in the regular expressions used by Django form library’s EmailField and URLField—a malicious input could trigger a pathological performance. Patches (and patched releases) for Django 1.1 and Django 1.0 have been published. # 10th October 2009, 12:24 am

Adding signing (and signed cookies) to Django core. I’ve been increasing my participation in Django recently—here’s my proposal for adding signing and signed cookies to Django, which I’d personally like to see ship as part of Django 1.2. # 24th September 2009, 7:31 pm

Django: Security updates released. A fix for a directory traversal attack in the Django development server (the one with the big “never run this in production” warnings in the documentation). Also reminds that the release of 1.1 means that 0.96, released over two years ago, has reached end of life and will not receive any further bug fixes after the just-released 0.96.4. # 29th July 2009, 1:45 pm

Reducing XSS by way of Automatic Context-Aware Escaping in Template Systems (via) The Google Online Security Blog reminds us that simply HTML-escaping everything isn’t enough—the type of escaping needed depends on the current markup context, for example variables inside JavaScript blocks should be escaped differently. Google’s open source Ctemplate library uses an HTML parser to keep track of the current context and apply the correct escaping function automatically. # 14th April 2009, 9:26 am

Django: Security fix released. The Django admin used to save partially-submitted forms if your session expired, and continue the submission when you logged in. It turns out that’s actually an unblockable CSRF exploit and is hence broken as designed, so it’s now been removed. Thanks Ed Eliot and other GCap colleagues for helping me flesh out the potential attack. # 3rd September 2008, 12:14 am

Django snippets: Sign a string using SHA1, then shrink it using url-safe base65. I needed a way to create tamper-proof URLs and cookies by signing them, but didn’t want the overhead of a full 40 character SHA1 hash. After some experimentation, it turns out you can knock a 40 char hash down to 27 characters by encoding it using a custom base65 encoding which only uses URL-safe characters. # 27th August 2008, 10:18 pm

Changeset 8162. “Implemented a secure password reset form that uses a token and prompts user for new password”—also sneaks base36 encoding and decoding in to Django. # 31st July 2008, 10:54 pm

mod_rpaf for Apache. A more secure alternative to Django’s equivalent middleware: sets the REMOTE_ADDR of incoming requests from whitelisted load balancers to the X-Forwarded-For header, without any risk that if the load balancers are missing attackers could abuse it to spoof their IP addresses. # 24th June 2008, 5:02 pm

Django: security fix released. XSS hole in the Admin application’s login page—updates and patches are available for trunk, 0.96, 0.95 and 0.91. # 14th May 2008, 7:49 am

Why the h can’t Rails escape HTML automatically? It would be a pretty huge change, but auto-escaping in Rails 2.0 could close up a lot of accidental XSS holes. # 1st December 2007, 8:34 pm

Django Changeset 6671. Malcolm Tredinnick: “Implemented auto-escaping of variable output in templates”. Fantastic—Django now has protection against accidental XSS holes, turned on by default. # 14th November 2007, 5:05 pm

Django security fix released. Django’s internationalisation system has a denial of service hole in it; you’re vulnerable if you are using the i18n middleware. Fixes have been made available for trunk, 0.96, 0.95 and 0.91. # 26th October 2007, 9:47 pm

Two months with Ruby on Rails. Good rant—covers both the good and the bad. The first complaint is the lack of XSS protection by default in the template language. Django has the same problem, but the solution was 90% there when I saw Malcolm at OSCON. # 9th October 2007, 12:23 pm