javascript - How to copy text to the client's clipboard using jQuery?

Copying to the clipboard is a tricky task to do in Javascript in terms of browser compatibility. The best way to do it is using a small flash. It will work on every browser. You can check it in this article.

Here's how to do it for Internet Explorer:

function copy (str) {     //for IE ONLY!     window.clipboardData.setData('Text',str); } 

Understanding Padding Oracle Attacks

Lets assume your application accepts an encrypted string as a parameter - whether the parameter is a cookie, a url parameter or something else is immaterial. When the application tries to decode it, there are 3 possible outcomes -

1. Outcome 1 : The encrypted string decrypted properly, and the application was able to make sense of it. Meaning, if the encrypted string was an 10 digit account number, after decryption the application found something like "1234567890" and not "abcd1213ef"

2. Outcome 2 : The padding was correct, but after decryption the string obtained was gibberish that the app couldn't understand. For example, the string decrypted to "abcd1213ef", but the app was expecting only numbers. Most apps will show a message like "Invalid account number".

3. Outcome 3 : The padding was incorrect, and the application threw some kind of error message. Most apps will show a generic message like "Some error occurred".

In order for a Padding Oracle attack to be successful, the attacker must be able to make several thousands of requests, and must be able to classify the response into one of the above 3 buckets without error.

If these two conditions are met, the attacker can eventually decrypt the message, and then re-encrypt it with whatever he wishes. Its just a question of time.

What can be done to prevent it?

1. Simplest thing - anything sensitive should never be sent to the client, encrypted or no encrypted. Keep it on the server.

2. Make sure that outcome 2 and outcome 3 in the above list appear exactly the same to the attacker. There should be no way to figure out one from the other. This is not all that easy, though - an attacker can discriminate using some kind of timing attack.

3. As a last line of defence, have a Web Application Firewall. The padding oracle attack needs to make several requests that look almost similar (changing one bit at a time), so it should be possible for a WAF to catch and block such requests.

P.S. A good explanation of Padding Oracle Attacks can be found in this blog post. Disclaimer: Its NOT my blog.

From what I read until now...

The attack allows someone to decrypt sniffed cookies, which could contain valuable data such as bank balances

They need the encrypted cookie of a user that have been already logged in, on any account. They also need to find data in cookies - I hope that developers do not store critical data in cookies :). And there is a way that I have below to not let asp.net store data in the login cookie.

How can someone get the cookie of a user that is online if he doesn't get his hands on the browser data? Or sniff the IP packet ?

One way to prevent that is to not allow cookies to transport without ssl encryption.

<httpCookies httpOnlyCookies="true" requireSSL="true" /> 

Also one more measure is to prevent storing Roles in cookies.

<roleManager enabled="true" cacheRolesInCookie="false"> 

Now about the cookies that are not secure for the regular pages, this needs some more thinking what you left your user do and what not, how you trust him, what extra check you can do (for example if you see a change on the ip, maybe stop trust him until relogin from security page).

Reference:
Can some hacker steal the cookie from a user and login with that name on a web site?

How to check from where attacks come and not give back informations. I wrote here a simple way to prevent the padding is invalid and logging at the same time to track down attackers: CryptographicException: Padding is invalid and cannot be removed and Validation of viewstate MAC failed

The way to track the attacker is to check the padding is invalid. With a simple procedure you can track them down and block them - they need some thousands of call on your page to find the key !

Update 1.

I have download the tool that suppose that's find the KEY and decrypt the data, and as I say its trap on the above code that's check the viewstate. From my tests this tool have many more to fix, for example can not scan compressed view state as it is and its crash on my tests.

If some one try to use this tool or this method the above code can track them down and you can block them out of your page with simple code like this one "Prevent Denial Of Service (DOS)", or like this code for preventing Denial of service.

Update 2

Its seems from what I read until now that the only think that is really need it to not give information back about the error, and just place a custom error page and if you like you can just create and a random delay to this page.

a very interesting video on this issue.

So all the above its more measure for more protections but not 100% necessaries for this particular issue. For example to use ssl cookie is solve the snif issue, the not cache the Roles in cookies it good to not send and get back big cookies, and to avoid some one that have all ready crack the code, to just place the admin role on the cookie of him.

The viewstate track its just one more measure to find attack.

Here is the MS response. It all boils down to "use a custom error page" and you won't be giving away any clues.

EDIT
Here is some more detailed info from scottgu.

Adding ScottGu's responses taken from discussion at http://weblogs.asp.net/scottgu/archive/2010/09/18/important-asp-net-security-vulnerability.aspx

Is custom IHttpModule instead of customErrors affected?

Q: I don't have a element declared in my web.config, I have instead an IHttpModule inside the section. This module logs the error and redirects to either a search page (for 404's) or to an error page (for 500's). Am I vulnerable?

A: I would recommend temporarily updating the module to always redirect to the search page. One of the ways this attack works is that looks for differentiation between 404s and 500 errors. Always returning the same HTTP code and sending them to the same place is one way to help block it.

Note that when the patch comes out to fix this, you won't need to do this (and can revert back to the old behavior). But for right now I'd recommend not differentiating between 404s and 500s to clients.

Can I continue using different errors for 404 and 500 errors?

Q: I take it we can still have a custom 404 page defined in addition to the default redirect on error, without violating the principles described above?

A: No - until we release a patch for the real fix, we recommend the above workaround which homogenizes all errors. One of the ways this attack works is that looks for differentiation between 404s and 500 errors. Always returning the same HTTP code and sending them to the same place is one way to help block it.

Note that when the patch comes out to fix this, you won't need to do this (and can revert back to the old behavior). But for right now you should not differentiate between 404s and 500s to clients.

How does this allow exposure of web.config?

Q: How does this allow exposure of web.config? This seems to enable decrypting of ViewState only, is there another related vulnerability that also allows the information disclosure? Is there a whitepaper that details the attack for a better explanation of what's going on?

A: The attack that was shown in the public relies on a feature in ASP.NET that allows files (typically javascript and css) to be downloaded, and which is secured with a key that is sent as part of the request. Unfortunately if you are able to forge a key you can use this feature to download the web.config file of an application (but not files outside of the application). We will obviously release a patch for this - until then the above workaround closes the attack vector.

EDIT: additional FAQ available in the second blogpost at http://weblogs.asp.net/scottgu/archive/2010/09/20/frequently-asked-questions-about-the-asp-net-security-vulnerability.aspx