Securely zero out memory in NSString

Question

On iOS, I was wondering, say if I read user provided password value as such:

NSString* strPwd = UITextField.text;

//Check 'strPwd'
...

//How to clear out 'strPwd' from RAM?

I just don't like leaving sensitive data "dangling" in the RAM. Any idea how to zero it out?

Answer 1

Basically you really can't. There are bugs filed with Apple over this exact issue. Additionally there are problems with UITextField and NSString at a minimum.

To reiterate the comment in a now deleted answer by @Leo Natan:

Releasing the enclosing NSString object does not guarantee the string bytes are zeroes in memory. Also, if a device is jailbroken, all the sandboxing Apple promises will be of no use. However, in this case, there is little one can do, as it is possible to swap the entire runtime in the middle of the application running, this getting the password from the memory.

Please file another bug with apple requesting this, the more the better.

Apple Bug Reporter