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Yahoo! XSS Flaw Endangers its Users
Rarely Greys writes "A major Yahoo XSS flaw makes it possible to take over any Yahoo user's account, including their mail, instant messaging, photos, etc.
This is not a rare occurrence. So why aren't web sites doing more to protect their users? It's looking like most web developers don't even know or care about XSS."
As a web developer myself, I try dillagently to kill off any XSS attacks by writing good secure code, but there will always be a few corner cases in any non-trivial application that one does not count for. This is doubly so when dealing with web services that have to pump out huge amounts of data over an insecure medium.
What is most showing is how fast it will be till Yahoo fixes this vunerability as a sign of their metal.
imho...
Well, my take (multiple major webdev projects on the go NOW)...
1. MOVING TARGET
A lot of webdev security issues (DB input, etc.) are moving targets.
For example, take database input. Ten years ago, for many (beginning) developers, escaping quotes and backslashes manually was considered fine. Later developers had database libraries that provided these functions natively. All of a sudden, unicode came along. Suddenly you had to worry about extra characters. This was another step - for example, for developers using MySQL, it was pertinent to change all of your escape functions to a new, unicode-aware one.
With everything else on their plate, even if they're single-language developers, auditing old code to maintain current security best practice falls somewhere at the bottom of the todo list, between 'get some exercise' and 'catch up on sleep'.
2. EXPECTATIONS RISING
As individual leading sites like google's gmail or google earth appear, expectations from clients increase. Web developers have a hard time keeping up with meeting all of the new 'standard features' that are expected, and are often implementing certain aspects for the first time, relying on either poorly audited code (random downloaded scripts) or writing their own with insufficient time for testing and security auditing.
3. NEW OR RAPIDLY ELEVATED ISSUES IN WEB SECURITY
In the last ten years, issues have appeared such as:
1. Public tools and worms that can easily attack custom-made applications, rendering some older, unmaintained code more readily exploitable. (This is just another time pressure, and security is all about the combination of resources for the attacker and defender... not just technical know-how on either side.)
2. Cross site scripting... this is quite a complex issue and is not understood by all developers.
3. A large number of scripting languages, which are constantly being updated and take a lot of time to stay up to date with. For example, most web developers are not really competent with javascript/ecmascript...
4. Browser or other 'out of web developer control' bugs can make different tags or features dangerous 'at short notice'.
5. AJAX and web services, which emphasise providing structured, easily-accessible data to the public, make data scraping (ala screen scraping) that much more of a real and widespread threat. As of today, most developers still do not take this threat seriously.
6. Denial of service attacks.
7. New expectations of server-side image (or web services data) processing can expose extra code (often legacy tools, or tools in entirely new languages) to potentially hostile input.
4. GENERAL PROGRAMMING ISSUES
Add to the above the standard pressures that lead to security shortfalls:
- Web developers, like other programmers, are often lumbered with unrealistic delivery timeframes.
- A lot of webdev is single-developer stuff, not completed in teams. As only one person reads the code, errors are less likely to be spotted.
- Most webdev projects have no budget for code auditing as close-to-secure code is often merely a desirable part of the overall bundle, not a steadfast client requirement...
- Webdev people often aren't client facing. In today's highly comepetitive webdev market, client facing salespeople perhaps don't know enough about code security to sell it as a budget-worthy extra.
5. CLIENTS DONT CARE
They want a working site, not a working site with n^m behind-the-scenes feelgood features they have to take at face value and have no way to 'see', 'show the boss' or otherwise justify.
The NoScript addon has Yahoo as one of their exemptions to its anti-XSS protection by default.
If you want to secure your systems, make sure you do not allow userinput with certain tags (assuming this input is displayed later on in a html page).
/me *shudders* (not sure if this is still true in IE7, in quriks-mode however i am pretty sure this still works in ie7 and non standard compliant mode AKA quirks-mode is the default for most IE only or IE targetted sites).
;)
Tags like script, iframe, link, style, embed, object _MUST_ be stripped in an untrusted environment. why you may ask: script, iframe, link allow external references (for example injection of code of remote sites which you can not easily check).
script itself is the most evil tag because it allows an attacker to access any element in a page, modify it and inject further remote scripts not stored on your server.
ie interprets javascript and vbcode in style tags
embed and object tags are used to insert java and activeX code, I guess I do not have to say much about those two techniques, it's again about inserting remote code at runtime.
iframe is, by nature, a fairly secure tag. it can not harm the users page much but it can be used to trick the user in believing to be on another page/site or trick him in any other way. plus, many IE versions had security holes where scripts could travel up from iframe into its parent document to manipulate data from another domain (crossite
There might be some potentially evil tags missing in my list, this is just from the top of my head.
I usually go the other way, instead of restricting tags i define a white-list of tags which are useful for formatting reasons such as strong, em, front, etc. this seems to be a much more controllable way.
HTH,
-Simon