# How Reasonable Decisions Created a Security Incident 
_合理的な判断がセキュリティインシデントを招いた事例_

- **When:** Thu, Nov 26, 2026 18:30 (JST)
- **Where:** OPTiM Tokyo, Minato — Shiodome Building 18F, 1-2-20 Kaigan, Minato City, Tokyo
- **Map:** https://www.google.com/maps/search/?api=1&query=35.6567277,139.7593743
- **Price:** price on the official site
- **Category:** Tech & Meetups
- **Official site:** https://trbmeetup.doorkeeper.jp/events/200155
- **On Tokyo Events:** https://eventstokyo.com/en/events/how-reasonable-decisions-created-a-security-incident

New Venue!

Please take note that the venue for this event is OPTiM Tokyo!

Agenda

18:30 〜 19:00 Doors open

Grab a drink, grab some stickers, and catch up with your fellow Rubyists.

19:00 ~ 19:05 Opening 

We'll kick things off by welcoming everyone and giving a short introduction of the event.

19:05 ~ 19:10 OPTiM Lightning Talk 

We'll take a moment to hear a quick word from our sponsor. 

19:10 〜 19:40 How Reasonable Decisions Created a Security Incident - 
Paul McMahon 

A security incident exposed the resumes of 460 job seekers. The surprising part? There was no single bug or obviously wrong decision to blame. Instead, a series of reasonable technical choices interacted in unexpected ways, creating a vulnerability that went unnoticed until it was too late. In this talk, I’ll share how the incident was discovered, investigated, and resolved, along with practical lessons on security, complexity, and hidden assumptions in Ruby on Rails applications.

Profile

Paul McMahon is the founder of TokyoDev and has been building Ruby on Rails applications for over 20 years. He also founded the Tokyo Rubyist Meetup in 2010, and previously co-founded the event platform Doorkeeper. Today he runs TokyoDev, helping international software developers advance their careers in Japan, while staying hands-on with the development and operation of the site’s Rails application.

19:40 〜 20:00 Rubyist show and tell

Do you have a cool snippet of Ruby code that you'd like to share, but it doesn't warrant an entire presentation? A challenge with your Ruby application you've been struggling with, and would like some advice? A helpful library or tool you've come across? This is your chance to share it.

For those who'd like to participate, you'll have up to five minutes. There's no need to create a slide deck, but you'll be able to use the projector if you have a code sample or something else to share. While we'll ask you if you'd like to participate when signing up, this is just to gauge…

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Source: Tokyo Events — https://eventstokyo.com/en/events/how-reasonable-decisions-created-a-security-incident