Developing a web-security mindset

Concat 2015 Frederic Hemberger

»Security and lack thereof is almost always a design choice and a business decision.«

Security is an ongoing process throughout your projectʼs lifetime. Itʼs not an item to tick off in a feature sprint.

Defensive programming 101

Secure input and output handling

What is user input?

  • The URL (including path or query parameters)
  • HTTP method (GET, POST, PUT, DELETE, …) and headers
  • Form values, file uploads
  • Cookies, browser storage
  • Data exchanged with APIs, Flash, applets, etc.

Validate user input

Data type validation
String, Number, Boolean, Date, undefined, …

Range and constraint validation
Negative/exponential numbers, String lengths,
"Your age is not 150 years", …

Business logic validation
"You must be at least 14 years old to register"

Sanitize user input

  • Remove ASCII control characters, null bytes, Unicode LTR/RTL control characters, Unicode whitespace, …
  • (Usually) remove all HTML

Encode output where necessary

Choose the correct encoding depending on the context


Donʼt assume frameworks and libraries use secure defaults.

On Alexaʼs Top 100,000 websites,
75% use a jQuery version older than 2½ years.
On 35% itʼs even older than 5 years.

Node Security Project
+ Retire.js

Make security part of your build process

  • Check for outdated/vulnerable dependencies
  • Lint your code, aim for high code coverage in your unit tests
  • Check inputs (forms, APIs, etc.) during integration tests:
    Invalid input, null bytes, Unicode characters, HTML snippets, XSS vectors, etc.

Secure communication

Use HTTPS with Transport Layer Security (TLS)

The succeeding protocol to SSL

  • … for site storing user data
  • … for downloads/software updates
  • … for the Internet of Things

All your siteʼs content must be served via HTTPS.
Including ads, analytics, widgets, etc.
Make it stick using the HTTP Strict Transport Security header.

Be aware of possible side channels.
And seal them using Content Security Policy.

Gather metrics

Use centralized logging to detect unusual behavior patterns. Logins and password resets, file access attempts, activity from different IP ranges, …
Set up automated notifications and alerts.

Security is always an ongoing effort.
Revise your tools and processes when necessary.

Thank you and stay safe!

Web-Security Resources

Frederic Hemberger