Recent content in Adam Pritchard's blog and miscellany on adam-p
Getting the length of a string seems simple and is something we do in our code every day. Limiting the length of a string is also extremely common in both frontend and backend code. But both of those actions – especially length limiting – hide a lot of complexity, bug-risk, and even vulnerability danger. In this post, we’re going to examine string length limiting deeply enough to help us fully…
While reviewing a co-worker’s results-paging design I realized there was a bug in some paging code I wrote 1 a few years ago. It’s unlikely to manifest and kind of subtle, but I thought that describing it here might be useful to others writing such code (including my future self). It comes down to sorting by timestamp… We’ll start with a table like this: CREATE TABLE item ( id TEXT PRIMARY KEY…
These are some parenting tips/tricks/techniques that I’m capturing for a friend. I’m not pretending that the ideas are good or unique or will help anyone other that me, but… maybe? My frame of reference: I have one 8-year-old son, healthy and neurotypical. He’s basically well-behaved now, but we had a lot of behavioural troubles (hitting other kids, etc.) until recently. I’ll probably be using…
Here’s a quick-and-dirty explanation of why two-factor authentication is good, and why U2F/WebAuthn keys (like YubiKeys) are better than the alternatives. (So I have something to point friends and family at.) Note that when I use the word “attacker”, I don’t necessarily mean someone that’s specifically out to get you. In fact, it’s usually someone trying to crack thousands of accounts at once.…
We all know by now that the leftmost values in the X-Forwarded-For header can be spoofed and only the rightmost IPs – added by your own reverse proxies – can be trusted. The Forwarded header ( RFC 7239 , 2014) has that same problem, and a new one: If the header is parsed correctly, an attacker can sabotage the whole header . Let’s take a quick trip to understanding how that can happen and how…
When I first saw this machine in the doctor’s office (waiting, bored) I thought the duplication of the labels – especially on the power button – was silly. Then I realized that I was being Anglo-centric and that having both enables use by more people, in more places.
There have recently been three different (but related) contexts where I have asked or been asked that question: When a reverse proxy is adding the client IP to the X-Forwarded-For header. When the client IP is being used for rate limiting. When checking if a client IP is contained in a configured list of ranges/prefixes/CIDRs. As I understood more about zones my opinion on this changed. This is an…
Update 2022-03-23: Matt Layher created a Go issue about this. Update 2022-04-14: In response to that issue, two weeks ago a change was committed to Go that makes netip.ParsePrefix behave like net.ParseCIDR : they both return an error when a zone is present. It wasn’t released in 1.18.1, but I’m guessing it’ll be in 1.18.2. So that’s great! Does this surprise you? (Try it in the playground .)…
## Summary This post ended up being incredibly long comprehensive. I’m afraid that many people won’t read enough to get everything that’s important, so here are the key points: When deriving the “real client IP address” from the X-Forwarded-For header, use the rightmost IP in the list. The leftmost IP in the XFF header is commonly considered to be “closest to the client” and “most real”, but it’s…
IPv6 rate-limiting is scarily half-baked right now. If you run a server that does any kind of IP-based rate-limiting, consider not enabling IPv6 if possible. If you do use IPv6, check how your rate-limiter actually handles it. ## Four billion is a pretty small number Most IPv4 rate-limiters will block individual addresses as they exceed the limit. That’s mostly okay, because there are only 4…
Every now and then I need to make a choice between using git submodules or subtrees (or nothing), or I get asked about them by coworkers. This is infrequent enough that I forget some of the details each time and need to refresh my memory. So I wrote up these notes to share with my coworkers and to help my future self. Hopefully they’re of some use to others as well. Disclaimer: My experience still…
Recently, I was adding timeouts to a Go HTTP server and ended up exploring how the different settings and approaches act and interact. I’m going to publish my notes here, along with the code I used for testing. Hopefully this will help someone else (or myself) in the future. The timeout testing client can be found here: github.com/adam-p/httptimeout . There is a server in the examples directory…
How should travel time be estimated? What are the ethical implications of the approach taken? You enter your destination into your maps app. It finds a few likely routes. It determines the distance of each pretty easily. It checks traffic conditions along the routes. But we don’t yet have a travel time estimate. Time equals distance divided by speed, adjusted for traffic. What travel speed…
From the Neck Up does neck-up massage, including intra-oral. When the RMT has their fingers in your mouth it’s pretty hard to give directions. So there are hand signs for communicating with the RMT. And, since you’re laying on your back during the procedure, the guide to the signs are posted on the ceiling.
TL;DR: When trying to prevent timing attacks (e.g., against login username enumeration) by making a request take constant time, make sure you exclude the network read and write time. If you don’t, an attacker can slow down their request to bypass it. I’ll be covering some background and contextual information here. If you don’t need it, skip to “Exclude network time from constant-time limiting”.…
There’s a ton of “what to expect when you’re expecting” stuff out there that I have no intention of repeating. I found there was a lot of stuff I wasn’t prepared for – mentally or logistically – that I want to call out here. A lot of what I’m going to say is negative. I feel like the negative experience for (some) fathers isn’t discussed very much and that that’s a major disservice to us – and a…
For four years I carried a Yubikey NEO (USB Type-A) in my pocket, on my keychain. And then it died (would no longer be recognized by any computer). Yubikey’s durability claim was: Crush-resistant and waterproof, YubiKey NEO is practically indestructible during normal use, weighs only 3g, and attaches to your keychain alongside your house and car keys So, they didn’t explicitly say “carrying it on…
I have enjoyed reading other people’s design and debugging train-of-thought posts, so after I spent two days wrestling with a code problem, I thought I’d write it up. It’s not technically exciting, but I think that describing it might be useful to someone – or my future self – someday. Or, at the very least, a little amusing. (Bonus: While writing this I discovered an error I made while doing the…
In October 2017, Troy Hunt of Have I Been Pwned held a contest inviting people to do something cool with the HIBP API. I decided a) that I would kind of like the special edition ThinkPad he was giving away, and b) that I could probably whip something up pretty quickly. I decided to create a browser extension that would simply pull HIBP breach information and show a browser notification – with the…
I often come across digital and physical interface designs that are either great (coherent) or terrible (incoherent). For some mysterious reason I’ve been wanting to capture such interfaces and share them. To start, a curious headstone. The goal of the design of a headstone is to be read by the living. For a few decades some of the people who read it will have known the deceased, but many won’t,…
[This started as notes to myself to help clarify the problem and solution. It’s probably more suited to a Github issue than a blog post, and it may get copied into one.] # The story so far The Firefox and Thunderbird versions of Markdown Here both used nearly the same code – an old-style XUL extension. Tb is only capable of using a XUL extension, while Fx supports at least three extension types:…
While poking around in my Android phone’s developer options, I realized that if you steal a phone that’s currently unlocked because it’s in a “trusted place”, then you can force it to remain unlocked forever . (And then I got schooled about that not being a problem.) ## Security Feature: Smart Lock with Trusted Places Android’s Smart Lock allows users to configure conditions under which to keep…
## Background and Motivation I received a pull request for Markdown Here that was great: it found a bug, fixed it, and included tests for the fix. However, the PR submitter didn’t write the tests using the existing framework, so I figured I’d massage his test code into the proper form. And then I noticed that he included a copyright line in the test file. It says “MIT License”, which is the…
This is just a stub test post to allow me to try out Markdown Here in Disqus comments. Right now MDH won’t work with Disqus in Chrome because of cross-origin restrictions. See: https://github.com/adam-p/markdown-here/issues/124 Update : The Disqus edit box is contenteditable , and MDH will render in it, but all formatting seems to get stripped out when you actually post the comment. Seems like the…
Trying to get Markdown Here listed in the Safari Extensions Gallery is by far the worst browser extension “store” experience I’ve had so far. Shockingly bad. ## No hosting First of all, but least of all: There’s no hosting. Unlike the Chrome and Mozilla stores, the Safari store doesn’t host the extension for you – it’s really more of a listing of links to wherever you host your extension files.…
Page actions – the buttons in a browser’s address bar – are a surprising UI failure . When adding a button for a browser extension, a choice must be made whether to make it a “page action” or a “browser action” (button on the toolbar). But browsers have failed to communicate the interactiveness of page actions, and almost no one – techy or layman – realizes that they’re clickable . To complement…