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XSS The Gloves are Off. Andy Prow Managing Director, Aura Software Security Ltd Andy@AuraSoftwareSecurity.co.nz . Kirk Jackson Senior Developer Xero kirk@xero.com . The Message. “XSS can fully compromise your site’s users’ machine – which might include you”
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XSSThe Gloves are Off Andy Prow Managing Director, Aura Software Security Ltd Andy@AuraSoftwareSecurity.co.nz Kirk Jackson Senior Developer Xero kirk@xero.com
The Message “XSS can fully compromise your site’s users’ machine – which might include you” “XSS is easy to protect against as long as take the right precautions”
Who are we? Andy Prow – Managing Director of Aura Software Security Ltd Security Consultants - Penetration Testers Performed web application pen-testing for both NZ, Ausy and UK companies. Govt, corporate and banking Wellington based. BSc Hons in Comp Sci and Soft Eng – 14 years software dev experience.
Who are we? Kirk Jackson – Developer & Security Officer, Xero http://www.xero.com Microsoft MVP – ASP.NET Organises the Wellington .NET user group – 25 user groups nationwide: http://www.dot.net.nz Blog: http://pageofwords.com
Are the Threats Real? XSS attacks include: Twitter, FaceBook, PayPal, Google, MySpace, WordPress, etc................... XSS attacks have / can: Inject rude images and abusive pop-ups Targeted CSRF “Own” their browser – example in a mo... Perform port scans from their machine Download full malware to compromise their machine
XSS 101 http://www.owasp.org/index.php/XSS
XSS Shell The Victim...
XSS Shell The Command and Control Console...
XSS Attack and Defence The gloves are off...
XSS Attack and Defence NOTE: slides 11 (this slide) to slide 49 are taken from the Live demo, using a .Net 3.5 web-application. Each set of slides shows Andy’s XSS attacks and then Kirk updates to the code – rebuild – rerun and therefore DEFEND the attack. The real preso slides start again at 50.
XSS Attack and Defence Attack: XSS 101 for dummies... <script>alert('xss');</script> Defence: sInput.Replace("<script>", "<script>“) Comments: Poor choice – only replacing the “<script>” tag is too specific, there are many ways around...
XSS Attack and Defence Attack: So if they’re checking for “<script>” we’ll change it... <ScRiPt>alert(‘XSS’);</ScRiPt> Defence: OK – ToLower() should fix that... Comments: Still very easily bypassed.
XSS Attack and Defence Attack: Try something other than “script” <IMG onmouseover="javascript:alert('XSS')" SRC="http://www.aurasoftwaresecurity.co.nz/images/Logo.jpg"> Defence: Replace a larger set of strings... Comments: So it picks up on of the recognisable text, but what about encoded values?
XSS Attack and Defence Attack: Perhaps HEX or UTF8 encoded <IMG onmouseover="javascript:alert('XSS')” SRC="http://www.aurasoftwaresecurity.co.nz/images/Logo.jpg"> Defence: HEX decode, then replace a larger set of strings... Comments: So it picks up on of the recognisable text, even encoded...But couldn’t it be simpler?
XSS Attack and Defence Attack: Any variation... Defence: http://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet Comments: Now it picks up all HTML special characters, whether encoded or not However, there are alternatives...
XSS Attack and Defence Attack: Any variation... Defence: Microsoft AntiXSS Library (for .Net) Comments: All done for you, and seems to work!
XSS Attack and Defence Attack: So the web-pages are secure – what about the web-services? Defence: Copy input cleansing to web-service, or move to data-layer. Picks up data entry points. Comments: It’s an option...
XSS Attack and Defence Attack: So all web-services and web-pages are secure. Perhaps we’ve got in via a back-end legacy system? Defence: Assume you cannot your own DB – cleanse the output to the browser. Comments: It’s an option...
XSS Attack and Defence Attack: All data-stores, input and output are clean. So if persistant XSS fails try reflected. Defence: Check EVERY input parameter, both on the querystring and from form data! Comments: Trust nothing, from anywhere!
Encoding • Encoding is "the process of transforming information from one format into another" [Wikipedia] • Taking some input text and making it appropriate to use in a given context • Untrusted input Safe to output • User enters: Kirk <script>... • We output: Kirk <script>....
