Simon Willison’s Weblog

A deep dive into an NSO zero-click iMessage exploit: Remote Code Execution (via) Fascinating and terrifying description of an extremely sophisticated attack against iMessage. iMessage was passing incoming image bytes through to a bunch of different libraries to figure out which image format should be decoded, including a PDF renderer that supported the old JBIG2 compression format. JBIG2 includes a mechanism for programatically swapping the values of individual black and white pixels... which turns out to be Turing complete, and means that a sufficiently cunning “image” can include a full computer architecture defined in terms of logical bit operations. Combine this with an integer overflow and you can perform arbitrary memory operations that break out of the iOS sandbox. # 16th December 2021, 8:33 pm

How to win at CORS (via) Jake Archibald’s definitive guide to CORS, including a handy CORS playground interactive tool. Also includes a useful history explaining why we need CORS in the first place. # 12th October 2021, 2:07 pm

New tool: an nginx playground. Julia Evans built a sandbox tool for interactively trying out an nginx configuration and executing test requests through it. I love this kind of tool, and Julia’s explanation of how they built it using a tiny fly.io instance and a network namespace to reduce the amount of damage any malicious usage could cause is really interesting. # 24th September 2021, 6:44 pm

API Tokens: A Tedious Survey. Thomas Ptacek reviews different approaches to implementing secure API tokens, from simple random strings stored in a database through various categories of signed token to exotic formats like Macaroons and Biscuits, both new to me.

Macaroons carry a signed list of restrictions with them, but combine it with a mechanism where a client can add their own additional restrictions, sign the combination and pass the token on to someone else.

Biscuits are similar, but “embed Datalog programs to evaluate whether a token allows an operation”. # 25th August 2021, 12:12 am

MDN: Subdomain takeovers (via) MDN have a page about subdomain takeover attacks that focuses more on CNAME records: if you have a CNAME pointing to a common delegated hosting provider but haven’t yet provisioned your virtual host there, someone else might beat you to it and use it for an XSS attack.

“Preventing subdomain takeovers is a matter of order of operations in lifecycle management for virtual hosts and DNS.”

I now understand why Google Cloud make your “prove” your ownership of a domain before they’ll let you configure it to host e.g. a Cloud Run instance. # 22nd August 2021, 5:31 am

I stumbled across a nasty XSS hole involving DNS A records. Found out today that an old subdomain that I had assigned an IP address to via a DNS A record was serving unexpected content—turned out I’d shut down the associated VPS and the IP had been recycled to someone else, so their content was now appearing under my domain. It strikes me that if you got really unlucky this could turn into an XSS hole—and that new server could even use Let’s Encrypt to obtain an HTTPS certificate for your subdomain.

I’ve added “audit your A records” to my personal security checklist. # 22nd August 2021, 5:27 am

OkCupid had a CSRF vulnerability (via) Good write-up of a (now fixed) CSRF vulnerability on OkCupid. Their site worked by POSTing JSON objects to an API. JSON POSTs are usually protected against CSRF because they can only be sent using fetch() or XMLHttpRequest, which are protected by the same-origin policy. Yan Zhu notes that you can use the enctype="text/plain" attribute on a form (introduced in HTML5) and a crafty hidden input element with name='{"foo":"' value='bar"}' to construct JSON in an off-site form, which enabled CSRF attacks. # 2nd August 2021, 10:12 pm

Probably Are Gonna Need It: Application Security Edition (via) Jacob Kaplan-Moss shares his PAGNIs for application security: “basic security mitigations that are easy to do at the beginning, but get progressively harder the longer you put them off”. Plenty to think about in here—I particularly like Jacob’s recommendation to build a production-to-staging database mirroring solution that works from an allow-list of columns, to avoid the risk of accidentally exposing new private data as the product continues to evolve. # 8th July 2021, 6:31 pm

Reflected cross-site scripting issue in Datasette (via) Here’s the GitHub security advisory I published for the XSS hole in Datasette. The fix is available in versions 0.57 and 0.56.1, both released today. # 5th June 2021, 11:14 pm

M1RACLES: M1ssing Register Access Controls Leak EL0 State. You need to read (or at least scan) all the way to the bottom: this security disclosure is a masterpiece. It not only describes a real flaw in the M1 silicon but also deconstructs the whole culture of over-hyped name-branded vulnerability reports. The TLDR is that you don’t really need to worry about this one, and if you’re writing this kind if thing up for a news article you should read all the way to the end first! # 26th May 2021, 3:25 pm

Why you shouldn’t use ENV variables for secret data (via) I do this all the time, but this article provides a good set of reasons that secrets in environment variables are a bad pattern—even when you know there’s no multi-user access to the host you are deploying to. The biggest problem is that they often get captured by error handling scripts, which may not have the right code in place to redact them. This article suggests using Docker secrets instead, but I’d love to see a comprehensive write-up of other recommended patterns for this that go beyond applications running in Docker. # 14th April 2021, 6:22 pm

Behind GitHub’s new authentication token formats (via) This is a really smart design. GitHub’s new tokens use a type prefix of “ghp_” or “gho_” or a few others depending on the type of token, to help support mechanisms that scan for accidental token publication. A further twist is that the last six characters of the tokens are a checksum, which means token scanners can reliably distinguish a real token from a coincidental string without needing to check back with the GitHub database. “One other neat thing about _ is it will reliably select the whole token when you double click on it”—what a useful detail! # 5th April 2021, 9:28 pm

How we found and fixed a rare race condition in our session handling. GitHub had a terrifying bug this month where a user reported suddenly being signed in as another user. This is a particularly great example of a security incident report, explaining how GitHub identified the underlying bug, what caused it and the steps they are taking to ensure bugs like that never happen in the future. The root cause was a convoluted sequence of events which could cause a Ruby Hash to be accidentally shared between two requests, caused as a result of a new background thread that was introduced as a performance optimization. # 18th March 2021, 11:06 pm

The SOC2 Starting Seven (via) "So, you plan to sell your startup’s product to big companies one day. Congratu-dolences! [...] Here’s how we’ll try to help: with Seven Things you can do now that will simplify SOC2 for you down the road while making your life, or at least your security posture, materially better in the immediacy. # 5th March 2021, 7:50 pm

How to secure an Ubuntu server using Tailscale and UFW. This is the Tailscale tutorial I’ve always wanted: it explains in detail how you can run an Ubuntu server (from any cloud provider) such that only devices on your personal Tailscale network can access it. # 26th February 2021, 8:31 pm

Dependency Confusion: How I Hacked Into Apple, Microsoft and Dozens of Other Companies (via) Alex Birsan describes a new category of security vulnerability he discovered in the npm, pip and gem packaging ecosystems: if a company uses a private repository with internal package names, uploading a package with the same name to the public repository can often result in an attacker being able to execute their own code inside the networks of their target. Alex scored over $130,000 in bug bounties from this one, from a number of name-brand companies. Of particular note for Python developers: the --extra-index-url argument to pip will consult both public and private registries and install the package with the highest version number! # 10th February 2021, 8:42 pm