Alf.nu

>I'm also sure that SHA-1 remains pervasive in many codebases

I've done a lot of security reviews of backup solutions over the years and nearly universally they use SHA1 for de-duplication. Security professionals have been providing warnings about this practice, but when you are dealing with a cloud backup solution as an example, speed and low compute requirements are a must and it's hard to win the argument when someone tosses "theoretical" onto the table. Well, we are now past "theoretical" and that's what matters.

I just tried out the SHA1 collider: https://alf.nu/SHA1 and created two PDFs containing entirely different bits of text in JPGs. It took seconds and indeed the output of the files were SHA1 identical. This is no longer nation-state level stuff and computing colliding documents to upload to cloud backup providers who de-dupe across the file system and not just restricted within the boundaries of each account could result in the ability to wipe out and change other people's data. At a minimum that could be an annoyance. At worst, it could result in the intentional change of someone's content to cause significant financial or physical harm.

This was pretty enjoyable. It reminds me a bit of "return true to win" (https://alf.nu/ReturnTrue).

I think I cheated a bit on Level 7. I just copy-pasted the code that wrote to epochComms and changed it to write to interceptComms instead. I'm not quite sure what the intended solution is.

On Level 12, does Secret.extHash serve any purpose? It never gets read.

I was doing this on mobile (Chrome on Android) and I ran into a few odd problems. If you have two opening curly braces in a row and try to backspace to delete the first one, it instead inserts a newline and duplicates the rest of the line. That is, if you have "{{foo" with the cursor after the first curly brace and you press backspace you end up with two lines each saying "{foo". This doesn't happen for me on desktop.

Copy-paste also seems to be broken on mobile. You can't select anything and trying to paste is just ignored.

It would be nice if you go go back and view the hint after you've completed a level. Or even if you could try different solutions to a level after you've finished it. You may not always be happy with the first thing that works.

W3C: "Service workers create the opportunity for a bad actor to turn a bad day into a bad eternity."[1] With service workers, cross-site scripting vulnerabilities are forever.

Service workers install by drive-by, which is troubling. In Firefox, check "about:serviceworkers" to see what you have installed. Take a look. You probably have far more service workers active than you thought. There are supposed to be enough cross-site scripting restrictions to keep service workers contained to their origin domain, but some holes have been found. There was a successful service worker attack on Dropbox, since fixed. "If you run a site that serves user files with secret URLs from a shared domain, you need to look out for the Service-Worker: script HTTP header; if you see it, run for the hills."[2]

If an attacker can get one page loaded from a site being attacked, they then own all traffic between the user and the site. This has lots of attack potential.

[1] https://www.w3.org/TR/service-workers/#security-considerations
[2] https://alf.nu/ServiceWorker

My first jobs were heavily focused on parsing data from HTML and regex was (and still is) the most common solution for the majority of cases

To learn it, I played a lot of regex golf [1]

I also enabled regular expressions in my code editor's Find feature so every search I'd make used regex. Having it enabled in my editor made learning it more immersive and useful, especially when combined with things like find-and-replace. I highly recommend permanently enabling that in your editor as well

Also, challenge your coworkers to see who can make the shortest patterns for a variety of cases and see whose is the most versatile. It's always a fun time

[1] https://alf.nu/RegexGolf

https://xss-game.appspot.com/ (this has a lot of overlap with gruyere)

https://github.com/cure53/XSSChallengeWiki/wiki

https://sites.google.com/site/infosecrocks/ (this links to some of the other stuff I mentioned)

https://www.hacking-lab.com

There's also these 2 advanced XSS challenges:

https://alf.nu/alert1

https://prompt.ml/0

I'm not sure if they're still functional, I think some of their solutions require functionality that browsers have since changed.

You might also want to check out the Youtube channel Live Overflow:

https://www.youtube.com/channel/UClcE-kVhqyiHCcjYwcpfj9w

SHA-1 (and SHA-256 and many other hash functions) are subject to length extension, which is how the 2017 shattered attack works: find a pair of short prefixes that work as PDF headers and that have colliding hashes; then append a common suffix such that the different headers display different parts of the suffixes. Once the collision has been found you can easily make your own colliding PDFs - https://alf.nu/SHA1

This breaks anything that dedups on just the SHA-1 hashes of raw files