Dealing with User Input Securely Kim Carter – OWASP Day 2013-09-12 

Demonstrate vulnerabilities Increase knowledge, awareness and desire to test Discuss practical techniques and approaches that increase our defences Agenda 

Why the hacker always has the advantage Learn to enjoy breaking your own software. It'll make you a better developer. Our builders must think like breakers Developers Day Job Write Code Hackers Day Job Break Code 

What does Poor Sanitisation look like? 

OWASP ZAP also has a REST API. Useful for regression test suites If we have time at the end, we'll go over some AJAX XSS 

Quality What is Quality? Do we as builders care? Why we should care 

No content

Quality But increasing quality is expensive right? 

Quality Not necessarily 

My Philosophy on Quality Everyone on the team needs to be thinking about it. Not just the testers. Reducing faults much earlier in the cycle. 

User Input Sanitisation Strategies All code should be driven by executable specifications. Especially sanitisation logic Based around my following two blog posts http://blog.binarymist.net/2012/11/04/sanitising-user-input-from-browser-part-1/ http://blog.binarymist.net/2012/11/16/sanitising-user-input-from-browser-part-2/ Main components were a WCF service which dished up XSL'd XML as HTML to an existing web app 

User Input Sanitisation Strategies Threat modelling Defence in depth Minimising attack surface Field length validation, incl structured data Parametrised Queries / Prepared Statements Least privilege White lists How to escape untrusted data for the different execution contexts File uploads not covered Why bother with client side Leveraging existing libraries 

Threat modelling Ideally performed at design time Identify the real risks. How? Decomposition Determine entry points, assets, trust levels of users Analyse dependencies Determine & rank threats Determine security controls to prevent threats 

Defence in depth Multiple layers may seem redundant Think of each layer as the only layer Attempt to stop the attack as soon as possible User Interface (Mark-up, JavaScript, CSS) Client – Server Comms Server side (internet facing) Back end code Data store 

Minimising attack surface Field length validation (client side) 

Minimising attack surface Field length validation (server side) 

Minimising attack surface Constrain fields to well structured data. Dates, post codes, e-mail addresses, check boxes, radio buttons Minimise free-form text input Hard to create small white lists with free-form 

Parametrised Queries / Prepared Statements Least privilege 

White lists Decide which characters are essential for each input Can now use the HTML5 pattern attribute on input tag. Doesn't cover textareas 

Client Side 1.type the characters in 2.[ctrl]+[v] characters in clipboard 3.right click -> Paste 

Server Side 

Escaping Escape all characters depending on potential execution contexts they may end up in. Even if they are not in your white lists Get away with the following escaping example only if you deal with untrusted data in HTML elements and you're sure your attributes are all quoted Escaping details for additional contexts here: https://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet 

Client Side 

Server Side 

Why bother with client side User Experience Server side sanitisation can be a lot slower When an honest user submits their data, they're not going to get server side exceptions due to validation 

Leveraging existing libraries Useful ● OWASP Encoding Project (Reform library) Supports Perl, Python, PHP, JavaScript, ASP, Java, .NET ● OWASP Enterprise Security API Not so Useful ● Microsoft Anti-Cross Site Scripting Library A lot more detail on my blog blog.binarymist.net 

Using: http://google-gruyere.appspot.com/ Stored XSS via AJAX 

When the user clicks refresh button, response looks like In the mark-up the snippet looks like: 

Resources Threat Modelling ● https://www.owasp.org/index.php/Application_Threat_Modeling ● https://www.owasp.org/index.php/Threat_Risk_Modeling Cheat Sheets and Check Lists I found helpful ● https://www.owasp.org/index.php/Input_Validation_Cheat_Sheet ● https://www.owasp.org/index.php/OWASP_Validation_Regex_Repository ● https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat _Sheet ● https://www.owasp.org/index.php/DOM_based_XSS_Prevention_Cheat_Sheet ● https://www.owasp.org/index.php/OWASP_AJAX_Security_Guidelines 

No content