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An integer overflow occurs when a computed value is larger than the storage space it s assigned to This often happens in expressions used to compute the allocation size for an array of objects because the expression is of the form object_size object_count Listing 3-2 shows an example of how to overflow an integer
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Listing 3-2
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int * x = malloc(sizeof (*x ) * n); for (i = 0; i < n; i++) x[i] = 0;
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If n is larger than 1 billion (when sizeof(int) is 4), the computed value of
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sizeof (*x) * n
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will be larger than 4 billion and will result in a smaller value than intended This means the allocation size will be unexpectedly small When the buffer is later accessed, some reads and writes will be performed past the end of the allocated length, even though they are within the expected limits of the array
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It is possible to detect these integer overflows either as they occur or before they are allowed to occur by examining the result of the multiplication or by examining the arguments Listing 3-3 shows an example of how to detect an integer overflow
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Listing 3-3
Detecting an Integer Overflow
void *array_alloc(size_t count, size_t size) { if (0 == count || MAX_UINT / count > size) return (0); return malloc(count * size); }
It s worth noting at this point that NSInteger will behave exactly the same way: It s not even actually an object, but simply an Objective-C way to say int
Format String Attacks
Format string vulnerabilities are caused when the programmer fails to specify how user-supplied input should be formatted, thus allowing an attacker to specify their own format string Apple s NSString class does not have support for the %n format string, which allows for writing to the stack of the running program However, there is still the threat of allowing an attacker to read from process memory or crash the program
NOTE
Valid format strings for the iPhone OS can be found at http://developerapplecom/iphone/library/ documentation/CoreFoundation/Conceptual/CFStrings/formatSpecifiershtml
Listing 3-4 shows an example of passing user-supplied input to NSLog without using a proper format string
Listing 3-4
No Format Specifier Used
int main(int argc, char *argv[]) { NSString * test = @"%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x"; NSLog(test); NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; int retVal = UIApplicationMain(argc, argv, nil, nil);
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[pool release]; return retVal; }
Running this results in the following:
[Session started at 2009-03-14 22:09:06 -0700] 2009-03-14 22:09:08874 DemoApp[2094:20b] 000070408fe0154b10000bffff00cbfffef842a4e1bfffef8cbfffef94bffff00c0
Whoops! Our user-supplied string resulted in memory contents being printed out in hexadecimal Because we re just logging this to the console, it isn t too big a deal However, in an application where this output would be exposed to a third party, we d be in trouble If we change our NSLog to format the user-supplied input as an Objective-C object (using the % format specifier), we can avoid this situation, as shown in Listing 3-5
Listing 3-5
Proper Use of Format Strings
int main(int argc, char *argv[]) { NSString * test = @"%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x"; NSLog(@"%@", test); NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; int retVal = UIApplicationMain(argc, argv, nil, nil); [pool release]; return retVal; }
NSLog makes for a good demo but isn t going to be used that often in a real iPhone app (given that there s no console to log to) Common NSString methods to watch out for are stringByAppendingFormat, initWithFormat, stringWithFormat, and so on One thing to remember is that even when you re using a method that emits NSString objects, you still must specify a format string As an example, say we have a utility class that just takes an NSString and appends some user-supplied data:
+ (NSString*) formatStuff:(NSString*)myString { myString = [myString stringByAppendingString:userSuppliedString]; return myString }
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When calling this method, we use code like the following:
NSString myStuff = @"Here is my stuff"; myStuff = [myStuff stringByAppendingFormat:[UtilityClass formatStuff:unformattedStufftext]];
Even though we re both passing in an NSString and receiving one in return, stringByAppendingFormat will still parse any format string characters contained within that NSString The correct way to call this code would be as follows:
NSString myStuff = @"Here is my stuff"; myStuff = [myStuff stringByAppendingFormat:@"%@", [UtilityClass formatStuff:unformattedStufftext]];
When regular C primitives are used, format strings become an even more critical issue because the use of the %n format can allow for code execution If you can, stick with NSString Either way, remember that you, the programmer, must explicitly define a format string
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