Showing posts with the label Uninitialized Memory Vulnerability

Integer Overflow Reference: Min & Max Values

A reference for when working with integers, and looking for integer overflows and underflows.

When an integer type, such as an int or unsigned short, overflows (the variable is given a value greater than the maximum value it can hold), the integer "wraps around" and becomes the minimum value the type can store.
Similarly, when an integer type underflows (the variable is given a value smaller than the maximum value it can hold), the integer "wraps around" and becomes the maximum value the type can store.

Use the chart below to find the minimum and maximum values each type can hold.
Size Chart
TypeSize In BytesMinimum ValueMaximum Valuechar1 byte-128+127unsigned char1 byte0+255short2 bytes-32,768+32,767unsigned short2 bytes0+65,535int4 bytes-2,147,483,648+2,147,483,647long4 bytes-2,147,483,648+2,147,483,647unsigned int4 bytes0+4,294,967,295unsigned long4 bytes0+4,294,967,295long long8 bytes-9,223,372,036,854,775,808+9,223,372,036,854,775,807unsigned long long8 bytes0…

What Value Is Stored In Uninitialized Variables?

The value in an uninitialized variable is one of: zero, a compiler dependent value (such as 0xCC's in visual studio), or data previously stored in that memory location (old data).
Types of Uninitialized Variables And Their Values Classic C/C++ Uninitialized Stack Variables The classic type of uninitialized variables are local function variables written in a low level language (such as C/C++). You would think when these variables are left uninitalized they would simply save the last value they were give. However, there is a catch: when code is compiled in debug mode, the compiler may inject its own code that initializes empty variables to a default value.
This is done to protect against vulnerabilities (more on this later), and to more easily detect bugs by giving the variable a bogus value that can be easily identified as uninitialized if it is for example printed to the screen.

Below, a  program compiled with Visual Studio in debug mode prints an uninitialized variable. Code c…