Threats

Overflow vulnerabilities [CWE-119] are well-recognized vulnerabilities and have historically been a significant concern in software application security.

In the 2021 CWE Top 25 Most Dangerous Software Weaknesses list, CWE-119 was ranked 17th, highlighting its ongoing relevance and the potential risk it poses​. Furthermore, overflow vulnerabilities can be exploited to achieve many different attack tactics including code execution, privilege escalation, lateral movement, denial of service (DoS), and sensitive data theft.  However, there are many different types of "overflow" attacks: Buffer, Heap, Stack, and Integer are the most common.

In today's article, we demystify the differences between the various types of overflow vulnerabilities to help defenders better understand them and also review the broad process that stack-based buffer overflow attacks follow to execute attacker supplied commands. 

What Are Overflow Vulnerabilities?

The fundamental cause of overflow vulnerabilities [CWE-119] is the failure to properly validate or control the amount of data being written into a fixed memory space or memory buffer. Buffers (temporary allocated memory used for data transfer) have fixed size determining the maximum amount of data they can hold. This helps manage memory usage and ensure efficient data processing within a program. Also, in C programming, variables have fixed sizes, with an ASCII character (char) taking 1 byte (in UTF-8 encoding, a character in C can vary in size from 1 to 4 bytes). An integer (int) uses 4 bytes, a short int occupying 2 bytes, a long int usually requiring 4 bytes on 32-bit systems and 8 bytes on 64-bit systems. A float comprising 4 bytes for single precision floating-point numbers, a double consisting of 8 bytes for double precision floating-point numbers, and a long double often using 8 bytes on 32-bit systems and 16 bytes on 64-bit systems. However, these exact sizes can vary based on the compiler and machine architecture.

Exploiting an overflow vulnerability can lead to various consequences depending on the particular case. Executable RAM refers to regions of Random Access Memory (RAM) from which code can be directly executed by the CPU. If the data being written outside of the intended scope falls into executable RAM, and contains executable commands it can allow an attacker to execute arbitrary code potentially under the permission context of the exploited application, potentially as the root user. Other consequences of an overflow vulnerability may be to alter the intended flow of the application to call unintended functions, read sensitive information, or cause the system to crash.

How to Mitigate Overflow Vulnerabilities?

In the context of security, modern operating systems and hardware often use techniques like NX (No Execute), Address Space Layout Randomization (ASLR) to allocate unpredictable memory locations, or DEP (Data Execution Prevention) to mark certain areas of RAM as non-executable to enhance security and prevent overflow vulnerabilities from being exploitable. 

For software developers, the most effective ways to mitigate overflow vulnerabilities include employing strict input validation for all untrusted or user supplied inputs, using memory safe programming languages or memory safe functions that automatically manage memory usage, ensuring to only use software libraries from trusted sources and updating software dependencies frequently. Also, having software applications security tested to verify they use good coding practices is an important step for verifying the security of critical applications.

Understanding Different Types of Overflow Attacks

Overflow vulnerabilities are a common type of security issue in computer systems where the boundaries of a data buffer are exceeded, leading to unintended consequences such as crashing the program, corrupting data, or even allowing an attacker to execute arbitrary code. Here are some common types of overflow vulnerabilities:

• Buffer Overflow [CWE-119]: "Buffer overflow" is a general term that applies to any overflow vulnerability that exploits a buffer. It refers to the situation where a program writes more data to a buffer than it can hold, leading to memory corruption. A "buffer" in software is a temporary storage area in memory used to hold data while it is being transferred from one place to another within a computer program. This includes both the stack and heap constructs.

• Stack Overflow [CWE-121]: This occurs when a program writes more data to a stack buffer than it can hold, potentially overwriting other data on the stack, including the return address. A stack buffer is a temporary storage used for holding data during the execution of a function or process within a program and facilitating the execution flow of a program..

• Heap Overflow [CWE-122]: Heap overflow happens when a program writes more data to a dynamically allocated memory block (heap) than its allocated size. The purpose of the heap is to provide a flexible memory area for dynamically allocated memory blocks, allowing for the allocation and deallocation of memory at runtime as needed by the program. A heap overflow can lead to corruption of adjacent heap data structures and can result in arbitrary code execution on behalf of the attacker.

• Integer Overflow [CWE-190] [CWE-680]: Integer overflow occurs when the result of an arithmetic operation exceeds the range that can be represented with a given integer data type. This can lead to unexpected behavior or security vulnerabilities if the overflow is not properly handled.

• Format String Overflow [CWE-134]: This occurs when a program uses unvalidated user input as the format string parameter for certain functions like printf(), leading to potential memory corruption or disclosure of sensitive information.

• Off-by-One Error [CWE-193]: While not always categorized as an overflow vulnerability, off-by-one errors can lead to overflow exploitation consequences. It happens when a program reads or writes one byte beyond the bounds of an allocated memory buffer. While off-by-one errors will generally lead to system crashes, they can sometimes trigger buffer overflows that allow arbitrary code execution.

• Environment Variable Overflow [CAPEC-10]: In environments where environment variables are used extensively, overflowing them can lead to various security vulnerabilities, such as altering program behavior or executing arbitrary code.

What Programming Languages Are Most Impacted By Overflow Attacks?

Overflow vulnerabilities can potentially impact all programming languages, but they are more prevalent in languages with manual memory management and less stringent runtime checks, such as C and C++. However, modern "memory safe" programming languages provide features that mitigate the risk of overflow vulnerabilities. 

For example:

• Memory-Safe Languages: Memory safe languages like Rust, GoLang, Java, C#, JavaScript, PHP, and Python have automatic memory management (garbage collection) and built-in array bounds checking, which significantly reduces the risk of buffer overflows. However, it's important to note that while memory safe languages aim to prevent certain classes of memory vulnerabilities, they can still encounter memory leaks and performance issues when used in unsafe ways.

• Safe Standard Libraries: Some non memory safe languages provide standard libraries that include functions designed to prevent buffer overflows, such as bounds-checked array types and safe string manipulation functions.

How Do Stack-Based Buffer Overflow Attacks Work?

Stack-based buffer overflow attacks are a type of security exploit where a program writes more data to a buffer located on the call stack than it was intended to hold. This can occur when the program does not properly validate input data size or fails to perform proper bounds checking. Stack-based buffer overflow attacks can be devastating if successfully exploited, as they often lead to arbitrary code execution. To mitigate these attacks, developers should adopt secure coding practices such as bounds checking, input validation, and the use of safer functions like fgets() instead of gets() in C/C++. Additionally, techniques like stack canaries and Address Space Layout Randomization (ASLR) are commonly employed to make buffer overflow attacks more difficult to execute successfully.

Here's how a typical stack-based buffer overflow attack works:

• Vulnerable Code: The attacker identifies a vulnerable piece of code in the target program that accepts user input without proper validation or bounds checking. This often involves functions like strcpy(), strcat(), or gets() in C/C++, which do not verify the size of input data before copying it into a buffer.

• Crafting Malicious Input: The attacker crafts input data that is larger than the buffer size allocated by the program. This input data typically includes malicious code that the attacker wants to execute.

• Overflowing the Buffer: The attacker sends the crafted input to the target program. When the program copies this input into the buffer, it overflows the buffer and corrupts adjacent memory locations on the stack.

• Controlled Overwrite: By carefully crafting the input data, the attacker can overwrite critical data stored on the stack, such as function return addresses or local variables' values. Overwriting the return address is particularly common, as it allows the attacker to redirect the program's execution flow to the malicious code they supplied.

• Execution of Malicious Code: When the vulnerable function returns and attempts to jump to the corrupted return address, it instead jumps to the attacker's malicious code. This code is then executed with the privileges of the compromised process, potentially allowing the attacker to gain control of the system, escalate privileges, or perform other malicious actions.

Conclusion

Overflow attacks, including buffer, heap, stack, and integer overflows, represent significant vulnerabilities in software security, with CWE-119 still ranking as a notable concern in the CWE Top 25 Most Dangerous Software Weaknesses list of 2021. These vulnerabilities stem from improper handling of data sizes, leading to potential unauthorized code execution, data theft, and system compromise. Buffer overflows, particularly, involve exceeding a buffer's fixed memory allocation, commonly seen in stack and heap structures, while integer overflows arise from arithmetic operations exceeding the representable range of a data type.

Modern programming practices and languages have evolved to mitigate these risks through memory-safe languages and security features like NX (No Execute), ASLR (Address Space Layout Randomization), and DEP (Data Execution Prevention). Despite these measures, overflow vulnerabilities persist, especially in languages like C and C++ that lack automatic memory management, highlighting the ongoing need for secure coding practices, input validation, and the use of safer library functions to protect against such attacks.

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