A practical security guide for web developers (Work in progress)
The intended audience
Security issues happen for two reasons -

Developers who have just started and cannot really tell a difference between using MD5 or bcrypt.
Developers who know stuff but forget/ignore them.

Our detailed explanations should help the first type while we hope our checklist helps the second one create more secure systems. This is by no means a comprehensive guide, it just covers stuff based on the most common issues we have discovered in the past.
Contents

The Security Checklist
What can go wrong?
Securely transporting stuff: HTTPS explained
Authentication: I am who I say I am
4.1 Form based authentication
4.2 Basic authentication
4.3 One is not enough, 2 factor, 3 factor, ....
4.4 Why use insecure text messages? Introducing HOTP & TOTP
4.5 Handling password resets
Authorization: What am I allowed to do?
5.1 Token based Authorization
5.2 OAuth & OAuth2
5.3 JWT
Data Validation and Sanitation: Never trust user input
6.1 Validating and Sanitizing Inputs
6.2 Sanitizing Outputs
6.3 Cross Site Scripting
6.4 Injection Attacks
6.5 User uploads
6.6 Tamper-proof user inputs
Plaintext != Encoding != Encryption != Hashing
7.1 Common encoding schemes
7.2 Encryption
7.3 Hashing & One way functions
7.4 Hashing speeds cheatsheet
Passwords: dadada, 123456 and cute@123
8.1 Password policies
8.2 Storing passwords
8.3 Life without passwords
Public Key Cryptography
Sessions: Remember me, please
10.1 Where to save state?
10.2 Invalidating sessions
10.3 Cookie monster & you
Fixing security, one header at a time
11.1 Secure web headers
11.2 Data integrity check for 3rd party code
11.3 Certificate Pinning
Configuration mistakes
12.1 Provisioning in cloud: Ports, Shodan & AWS
12.2 Honey, you left the debug mode on
12.3 Logging (or not logging)
12.4 Monitoring
12.5 Principle of least privilege
12.6 Rate limiting & Captchas
12.7 Storing project secrets and passwords in a file
12.8 DNS: Of subdomains and forgotten pet-projects
12.9 Patching & Updates
Attacks: When the bad guys arrive
13.1 Clickjacking
13.2 Cross Site Request Forgery
13.3 Denial of Service
13.4 Server Side Request Forgery
Stats about vulnerabilities discovered in Internet Companies
On reinventing the wheel, and making it square
15.1 Security libraries and packages for Python
15.2 Security libraries and packages for Node/JS
15.3 Learning resources
Maintaining a good security hygiene
Security Vs Usability
Back to Square 1: The Security Checklist explained

Who are we?
We are full stack developers who just grew tired of watching how developers were lowering the barrier to call something a hack by writing unsecure code. In the past six months, we have prevented leaks of more than 15 million credit card details, personal details of over 45 million users and potentially saved companies from shutting down. Recently, we discovered an issue that could result in system takeover and data leak in a bitcoin institution. We have helped several startups secure their systems, most of them for free, sometimes without even getting a thank you in response :)
If you disagree with something or find a bug please open an issue or file a PR. Alternatively, you can talk to us on hello@fallible.co