JavaScript for hackers

http://dev.opera.com/articles/view/opera-javascript-for-hackers-1/

by lelf • 13 years ago

89 11 13 years ago

dev.opera.com

jt2190• 13 years ago

This article is dated 2009. Are these still security holes in Opera?

yeahbutbut• 13 years ago

These are more issues with how Javascript is handled on the web. I'd imagine that js sanitizers have gotten more sophisticated so the callback (first) example may be a bit weak; if you're accepting code from an unknown source, even if you're filtering it first, you're gonna have a bad time.

dubcanada• 13 years ago

Opera using Chromium (v8) rather then Carakan. And these aren't really security holes so much as Javascript quirks that you should watch out for.

parakh1989• 13 years ago

I doubt. I would like to go through the release notes.

jdaley• 13 years ago

I don't get it. His first example is that since the string replace method takes a callback as its second argument, you can use it to run code. So if you happen to find an app that injects unescaped user input into Javascript, you could enter the string alert(1)".replace(/.+/,eval)// and have it run the alert.

But if the app is injecting unescaped user input into code, wouldn't a simple ";alert(1);// achieve the same thing? Why do you need to be tricky?

The examples seem to be based on the premise that web apps commonly forgo the simple escaping of quotes, and instead implement a complex parsing and analysis of user input as code to identify malicious statements, which can be defeated as long as you obfuscate enough.

raganwald• 13 years ago

He gives an example of such a system.

The larger issue is that the premise behind the design of web browsers is itself flawed with respect to displaying user content. As long as it is acceptable to execute JavaScript that is embedded directly in a web page (such as within script tags or as the target of an HREF), and as long as you are also trying to display user-supplied content, security is going to be extremely fragile.

We are perpetually one careless oversight away from pwnage, and we are vulnerable to mistakes that might creep into our libraries as well as into our own code.

johnsoft• 13 years ago

Here's something I didn't know - you can use unicode escapes outside of strings. Code like `\u0061\u006c\u0065\u0072\u0074(1)` translates to `alert(1)` and runs just fine.

ep103• 13 years ago

Reddit had a fun challenge where they did exactly this for their frontend position.

Its actually possible to encode your js entirely to whitespace, which is, imho, far more amazing.

infinity• 13 years ago

This article is written by Gareth Heyes. He has a blog about JavaScript weirdness and hacking:

http://www.thespanner.co.uk/

thedufer• 13 years ago

JavaScript doesn't know the result of a function until it is executed, and obviously it has to call the function to return the variable type.
+alert(1)--

That's a particularly poor example. There's no reason to expect `alert` to run there (in current Chrome, it doesn't) - a function can't return an l-value, so you don't have to wait for the return value to determine that it'll be invalid.

Also, I'm not sure what the unary + is supposed to achieve - it has the same precedence as post-decrement, with right to left associativity, so knowing that the post-increment will fail shows us that the + will never run.

saurik• 13 years ago

This doesn't work in Apple's JSC either.

JavaScript for hackers

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