Simon Willison’s Weblog

Tip for changing cookie subdomains: change the cookie name too. This is a really useful tip I hadn’t encountered before. If you make a change to the way cookies are configured—changing the cookie domain or path for example—it’s a good idea to change the name of the cookie as well. If you don’t change the cookie name you’ll see weird behaviour for users who have previously had the cookie set using the older configuration. This definitely explains bugs I’ve seen in the past. Filing this tip away for future cookie-related development work. # 9th June 2020, 6:41 pm

Cookies-over-HTTP Bad (via) Mike West from the Chrome security team proposes a way for browsers to start discouraging the use of tracking cookies sent over HTTP—which represent a significant threat to user privacy from network attackers. It’s a clever piece of thinking: browsers would slowly ramp up the forced expiry deadline for non-HTTPS cookies, further encouraging sites to switch to HTTPS cookies while giving them ample time to adapt. # 7th April 2018, 2:39 pm

Firesheep (via) Oh wow. A Firefox extension that makes sniffing for insecured (non-HTTPS) cookie requests on your current WiFi network and logging in as that person a case of clicking a couple of buttons. Always possible of course, but it’s never been made easy before. Private VPNs are about to become a lot more popular. # 25th October 2010, 9:11 am

Facebook’s Instant Personalization: An Analysis of Fundamental Privacy Flaws (via) Oh FFS. “Instant Personalization” means you visit one of Facebook’s “partner websites” and Facebook instantly tells them your full identity and gives them access to full Facebook connect functionality—without you performing any action other than visiting the site. This will not end well. # 2nd October 2010, 11:53 pm

Internet Explorer Cookie Internals (FAQ). Grr... IE 6, 7 and 8 don’t support the max-age cookie argument, forcing you to use an explicit expiry date instead. This appears to affect the cache busting cookie pattern, where you set a cookie to expire in 30 seconds for any user who posts content and use the presence of that cookie to skip caches and/or send their queries to a master instead of slave database. If you have to use expires, users with incorrect system clocks may get inconsistent results. Anyone know of a workaround? # 26th February 2010, 12:25 pm

Negative Cashback from Bing Cashback (via) Some online stores show you a higher price if you click through from Bing—and set a cookie that continues to show you the higher price for the next three months. It’s unclear if this is Bing’s fault—comments on Hacker News report that Google Shopping sometimes suffers from the same problem (POST UPDATED: I originally blamed Bing for this). # 23rd November 2009, 9:24 pm

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

You Deleted Your Cookies? Think Again (via) Flash cookies last longer than browser cookies and are harder to delete. Some services are sneakily “respawning” their cookies—if you clear the regular tracking cookie it will be reinstated from the Flash data next time you visit a page. # 17th August 2009, 3:23 pm

Towards a Standard for Django Session Messages. I completely agree that Django’s user.message_set (which I helped design) is unfit for purpose, but I don’t think sessions are the right solution for messages sent to users. A signed cookie containing either the full message or a key referencing the message body on the server is a much more generally useful solution as it avoids the need for a round trip to a persistent store entirely. # 19th June 2009, 9:57 pm

Integrating Facebook Connect with Django in 15 minutes. Django authentication middleware that calls the Facebook REST API using a cookie set by Facebook Connect and checks if that person is your Facebook friend. Despite most of the magic happening on the server you still need Facebook’s JavaScript to set that cookie in the first place. # 17th December 2008, 1:18 pm

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

Major Update to Prism (via) Mozilla’s site-specific browser tool can now use separate profiles (and hence separate cookie jars) for each instance, making it an excellent tool for protecting yourself against CSRF vulnerabilities in the web applications you rely on. # 10th March 2008, 2:03 pm

Fluid. Another site-specific browser toolkit for OS X (Leopard only), from Todd Ditchendorf. Again, it’s not clear if this does the Right Thing and creates separate cookie jars for every application. # 28th December 2007, 11:42 pm

Site-specific browsers and GreaseKit. New site-specific browser tool which lets you include a bunch of Greasemonkey scripts. For me, the killer feature of site-specific browsers is still cookie isolation (to minimise the impact of XSS and CSRF holes) but none of the current batch of tools advertise this as a feature, and most seem to want to share the system-wide cookie jar. # 25th October 2007, 7:56 am

IE vulnerability allows cookie stealing. Full exploit against the same-domain cookie origin policy, so malicious sites can steal cookies from elsewhere. Avoid using IE until this is patched. # 6th June 2007, 9:53 am

IE and 2-letter domain-names (via) IE won’t let you set a cookie on XX.YY, where YY is anything other than .pl or .gr. Other browsers have better exception lists. # 15th February 2007, 12:33 am

Ubuntu sugar cookies (via) Different coloured dough is used to bake the Ubuntu logo in to the cookies themselves, kind of like making sushi rolls. # 11th January 2007, 2:49 pm