Scanf vulnerability examples
It still exists today partly because of programmers carelessness while writing a code. The least we can do is to avoid writing bad code that gives a chance to even script kiddies to attack your program and exploit it. In this buffer overflow tutorial, we will discuss the basics of the following :.
Similar to the first example, arr refers to the left boundary while arr refers to the right boundary. By now it should be clear what a buffer means. Moving on lets understand when a buffer overflows. A buffer is said to be overflown when the data meant to be written into memory buffer gets written past the left or the right boundary of the buffer.
This way the data gets written to a portion of memory which does not belong to the program variable that references the buffer. In the above example, we declared an array of size 10 bytes. Please note that index 0 to index 9 can used to refer these 10 bytes of buffer. This is the point where buffer overrun happens because data gets written beyond the right boundary of the buffer. It is also important for you to understand how GCC compilation process works to create a C executable.
Some of us may think that though a buffer overflow is a bad programming practice but so is an unused variable on stack, then why there is so much hullabaloo around it? What is the harm buffer overrun can cause to the application?
Then this may lead to crash in most of the cases. The reason being, a pointer is not allowed to access heap memory that does not belong to it. Now, depending on the compiler you are using, chances are high that this will get unnoticed during compilation and would not crash during execution. The simple reason being that stack memory belongs to program so any buffer overflow in this memory could get unnoticed.
So in these kind of scenarios, buffer over flow quietly corrupts the neighbouring memory and if the corrupted memory is being used by the program then it can cause unexpected results. You also need to understand how you can prevent stack smashing attacks with GCC. Until now we discussed about what buffer overflows can do to your programs.
We learned how a program could crash or give unexpected results due to buffer overflows. But, that it is not the worst part. Consider this example :. The program above simulates scenario where a program expects a password from user and if the password is correct then it grants root privileges to the user.
But do you know that there is a possibility of buffer overflow in this program. The gets function does not check the array bounds and can even write string of length greater than the size of the buffer to which the string is written. Now, can you even imagine what can an attacker do with this kind of a loophole? In the above example, even after entering a wrong password, the program worked as if you gave the correct password.
There is a logic behind the output above. To avoid buffer overflow attacks, the general advice that is given to programmers is to follow good programming practices. For example:. Thanks for the article. The example works because the memory location of the pass variable is just next to the location of the buffer, so it gets overwritten! My question: Is it always like this? Is is true? Defensive coding is important when it comes to accepting input, no matter if it is from a carbon unit or another machine.
Now we beginner know buffer overflow because of this article.Reads formatted data from the standard input stream. Returns the number of fields successfully converted and assigned. The return value doesn't include fields that were read but not assigned. A return value of 0 indicates no fields were assigned. The return value is EOF for an error, or if the end-of-file character or the end-of-string character is found in the first attempt to read a character.
Each argument must be a pointer to a variable type that corresponds to the type specifier in format. If copying occurs between strings that overlap, the behavior is undefined.
Specify the sizes for all cCsSor string control set  parameters. The buffer size in characters is passed as an additional parameter. It immediately follows the pointer to the buffer or variable.
For example, if you're reading a string, the buffer size for that string is passed as follows:. The buffer size includes the terminal null. You can use a width specification field to ensure the token that's read in fits into the buffer. When a token is too large to fit, nothing is written to the buffer unless there's a width specification. The buffer size parameter describes the maximum number of characters, not bytes. In this example, the width of the buffer type doesn't match the width of the format specifier.
The S format specifier means use the character width that's "opposite" the default width supported by the function. The character width is single byte, but the function supports double-byte characters. This example reads in a string of up to nine single-byte-wide characters and puts them in a double-byte-wide character buffer. The characters are treated as single-byte values; the first two characters are stored in wsthe second two are stored in wsand so on.
When multiple characters for non-null-terminated strings are read, integers are used for both the width specification and the buffer size. For more information, see scanf Width Specification. For more information, see Format Specification Fields: scanf and wscanf Functions.
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The vulnerabilities here are usually very technical in nature, but the issue is that the scanf function takes as parameters the address into which the values read should be written. If you don't pass enough arguments to the function specifying where they should go, or you give the wrong type of arguments, then a malicious attacker can craft a format string for scanf such that the values read in are written into locations in memory that control sensitive information.
For example, the most common way in which functions are implemented work by storing somewhere on the stack the address of which instruction to execute next after the function returns. If the attacker can somehow write data to this address, they can change where the function will return to. Moreover, if the attacker can put binary data into memory from scanf perhaps by feeding into stdin a bunch of datathey could in theory change the return address to jump into that binary data, hijacking the program and causing it to execute code of the attacker's choice.
This could be malicious, such as code to format the hard drive, or even worse could steal personal information, add a backdoor into the computer, etc. For more information on how to execute generic attacks that work by writing data where it shouldn't go, the article "Smashing the Stack for Fun and Profit" is a good reference.
It doesn't specifically address this type of issue, but the sorts of attacks described here can similarly be executed.
Learn more. What kind of vulnerability is to do a scanf without specifying format string? Ask Question. Asked 7 years, 9 months ago. Active 7 years, 9 months ago. Viewed 3k times. Active Oldest Votes. Hope this helps! Sign up or log in Sign up using Google.
Sign up using Facebook.Using some of the examples presented in the book, I thought it would be a good idea to explore how the theory on real code vulnerabilities stands true. When the indexes or cut conditions in iterative loops are badly programmed, it can result in more bytes being copied than was intended: either one byte off-by-one or several off-by-a-few.
This results in the null character being inserted one byte beyond the limit, generating an overflow. In line 20, there is a search for a double quotation mark within the string being parsed.
If it is found, the loop in line 22 will copy until a second double quotation mark is found. One thing that can often happen when attempting to avoid an excessive amount of data copying to a buffer — by checking the size to be copied — is that the number of bytes to be copied exceeds the largest number that can be represented by the data type, rotating toward small values.
The check ends up being completed only because the size is interpreted as a small number, but the copy is actually made up of a large number of elements. This results in an overflow of the destination buffer.
This type of vulnerability is still relevant and common nowadays. These are just a few types of vulnerabilities, and I would recommend looking at the book I mentioned earlier for more examples.
Furthermore, as already mentioned, when developers apply their own functions for parsing or handling strings, data frames, and so on, it increases the chance that they will make errors. Similar Articles.I believe scanf is one of the most common pitfalls that most novice C programmers encounter.
And it really does a good job, as long as the input given by the user is what was expected by the author of the program. Or else, things may get a little tricky especially for the newcomer. Here is an example, a simple program that prompts the user to enter a number, and then it print out whether it was a even or an odd number. The program will keep on asking until a valid number is entered:. Give me a number: abc You didn't type a number!
Give me a number: You didn't type a number! What did go wrong here? At first, the user is prompted to enter a number. Then scanf is called to read an integer number from standard input. The program blocks and waits for the user to type something and press Enter.
As a result, scanf puts back the character and bails out. The error message is displayed and we start all over again to ask the user for new input. It turns out that the problem here is that if some invalid input is entered, it never leaves the input buffer. What we need here is to find a solution, so that the user input is always taken off from the input buffer, either when it is valid or not.
This is how it will look like:. Give me a number: fasfdsfgsd You didn't type a number! Give me a number: 25 25 is odd. Another, less elegant, way to solve the same problem would be to still use scanf but also meticulously take care to remove the invalid line of data from the buffer.
Some examples of vulnerable code and how to find them
In C-terms, that would be:. But as I said before, I personally find this version kind of ugly, because, we are using fgets only to read and ignore a whole line from the input buffer, which is something we can avoid if we use sscanf. I believe most C programmers have been puzzled at their first steps, and I hope that these little headaches will not discourage new programmers from continueing their efforts. For these efforts will be greatly rewarded! Somebody should rewrite the scanf and include it in the newer compilers.
Should be better solution, aint it? Good article!! It should also be mentioned that scanf is often a security risk buffer overflow if it is used to read a string. Nice article about fgets and sscanf. Thank you… I tried all the way for validation….
Thank you so much! Even better, after you acquire the number, pack it into a single line code.
C library function - scanf()
Note that this final example of course leaves input unread, even from the same line, if there were more than 39 characters until the newline. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment.
It looks okay! But what happens if the user enters some invalid input? And it goes like this until we terminate the program with Ctrl-C. Not quite what expected, eh? This is how it will look like In C-terms, that would beIn a way, you could argue that scanf is the input version of the printf function. Because of that, scanf is quite particular about how text is input.
In this version, placeholder is a conversion character, and variable is a type of variable that matches the conversion character. The scanf function is prototyped in the stdio.
Format String Vulnerability and Prevention with Example
The preceding statement reads an integer value into the variable highscore. This assumes that highscore is an int variable. The preceding scanf statement waits for a floating-point value to be input, which is then stored in the temperature variable. In the preceding line, scanf accepts the first character input and stores it in the key variable.
So a space or a tab or the Enter key terminates the string. That sucks. One of the most common ways to put the scanf function to use is to read in a chunk of text from standard input. Exercise 1: Type the source code from scanf Swallows a String into a new project, ex, in Code::Blocks. Build and run. Line 5 declares a char array — a string variable — named firstname. The number in the brackets indicates the size of the array, or the total number of characters that can be stored there.
Basically, the statement at Line 5 sets aside storage for up to 15 characters. The scanf function in Line 8 reads a string from standard input and stores it in the firstname array. Prompt the user for their last name as well, and then display both names by using a single printf function. The number in the brackets refer to Line 5 gives the size of the char array, or the length of the string, plus one.
When you create a char array, or string variable, ensure that you create it of a size large enough to hold the text. That size should be the maximum number of characters plus one. The reason for increasing the char array size by one is that all strings in C end with a specific termination character. You must remember to add room for that character when you set aside storage for string input.
The scanf function can do more than read strings. It can read in any value specified by a conversion character. In scanf Eats an Integer, the scanf function reads in an integer value.
That character directs scanf to look for an int value for variable fav. Exercise 3 : Create a project, ex, using the source code shown in scanf Eats an Integer. Test the program by typing various integer values, positive and negative. The character is a C operator — specifically, the memory address operator. An ampersand must prefix any variable specified in the scanf function. The exception is an array, such as the firstname char array in scanf Eats an Integer. Try running the program again, but specify a decimal value, such as The reason you see incorrect output is that scanf is very specific.
It fetches only the variable type specified by the conversion character. Exercise 4 : Modify the source code from scanf Eats an Integer so that a floating-point number is requested, input, and displayed. The scanf function stops reading text input at the first white space character, space, tab, or Enter key.
Dan Gookin wrote the original For Dummies book in Now, with more than 11 million copies in print, his many books have been translated into 32 languages. Visit him at wambooli.There are several format strings that specify output in C and many other programming languages but our focus is on C.
Format string vulnerabilities are a class of bug that take advantage of an easily avoidable programmer error. If the programmer passes an attacker-controlled buffer as an argument to a printf or any of the related functions, including sprintf, fprintf, etcthe attacker can perform writes to arbitrary memory addresses.
The following program contains such an error:. Since printf has a variable number of arguments, it must use the format string to determine the number of arguments.C Programming Tutorial - 11 - Getting Input with scanf
It will naively print the next 15 addresses on the stack, thinking they are its arguments:. We start our string with AAAA to see this more explicitly:.
The 0x is the hex representation of AAAA. At this point we will take advantage of another format string feature: in a format specifier, we can also select a specific argument.
In our case, we see that 0x is the 10th argument to printf, so we can simplify our string This article is contributed by Akash Sharan. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
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