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Buffer Overflow

Posted on Apr 2, 2024 by
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What Is Buffer Overflow?

Buffer overflow is a distinctive computing exception, arising when a program tries to insert data into a buffer beyond its designated capacity. This overflow leads to data spilling into neighboring memory spaces, potentially overwriting vital information. Exploiting buffer overflow not only allows attackers to manipulate computer memory but also empowers them to disrupt or take control of program execution. The consequences range from data corruption and program failures to the execution of malicious code, presenting a serious security risk.

Types of Buffer Overflow Attacks

A buffer overflow attack exploits vulnerabilities in system buffers, occurring when the data volume surpasses the storage capacity. This can lead to program stack damage, compromise program integrity, system shutdown, or unauthorized execution of instructions. Buffer overflow attacks manifest in various types:

  • Stack Overflow: This type arises when the memory allocated on the stack exceeds its capacity during program execution. Stack overflow, common and disruptive, halts program execution immediately, often displaying an error message. Successful attacks require uncontrolled data size when writing to the stack.

  • Heap Overflow: Dynamic memory allocation surpassing the heap size leads to a heap overflow. Although immediate execution may not halt, the program can become unstable or crash. Maliciously, attackers exploit a heap buffer overflow to execute arbitrary code or gain sensitive information.

  • Format String Overflow: Utilizing user-customized format strings in programming languages can enable attackers to manipulate the *printf() series functions, snooping on stack content. Ultra-long inputs can trigger a conventional buffer overflow, or %n can be used to overwrite pointers or return addresses.

  • Integer Overflow: Arising when arithmetic operations exceed the value range of an integer type, either exceeding the maximum (overflow) or falling below the minimum representable value (underflow). Although many integer overflows may not directly exploit vulnerabilities, indirect exploitation is possible if integer variables determine operations like memory allocation.

  • Unicode Overflow: Creating buffer overflow by inserting Unicode characters into an input expecting ASCII characters. Unicode characters, being larger than ASCII characters, can alter program behavior, posing additional security risks. Vigilance is crucial to prevent such vulnerabilities and their potential exploitation.

How Do Attackers Exploit Buffer Overflows?

An intruder can input meticulously crafted data into a program, prompting the program to attempt storing this input in a buffer. This input has the potential to overwrite segments of memory linked to the buffer space. When the program's memory layout is clearly defined, the attacker can deliberately overwrite regions containing executable code, substituting it with their own executable instructions, thereby altering the program's intended functionality. Buffer overflow attacks typically unfold in the following sequence:

  • 1. Inject Attack Code.

  • 2. Jump to the Attack Code.

  • 3. Execute the Attack Code.

Utilizing stack overflow attacks poses a threat to the integrity of stack data.

The attacker can capitalize on vulnerabilities in buffer overflow, aiming to compromise various objects, including ARG (actual parameter during function call), RETADDR (memory's next operation instruction address), EBP (stack frame status value before function call), and LOCVAR (local variable within the function).

A common strategy in stack overflow exploits involves manipulating the RETADDR value, storing either the address of the injected attack code or privileged system function addresses in the code area to replace RETADDR. When RETADDR undergoes modification, following the function call, the program redirects to the attacker-designated address, executing instructions as intended by the attacker. This grants the attacker system control permissions, with potentially severe consequences. EBP is another frequent target, where the attacker constructs a virtual stack frame with RETADDR pointing to the attack code. Overflowing with the EBP value of the current stack frame, the overflowed EBP value becomes the address of the constructed virtual stack frame. Subsequently, the virtual stack is activated. Following the execution of the current stack frame, the program transitions to the location indicated by the RETADDR value of the virtual stack frame, eventually reaching the attacker-designed address to execute the intended attack instructions.

Heap overflow attacks pose a threat to the integrity of heap data.

Dynamic and non-contiguous memory allocation in the heap complicates address prediction for attackers. While launching heap overflow attacks is more challenging than stack overflow attacks, attackers can employ specific techniques to execute them successfully.

  • Dword Shoot Attack: Dword Shoot enables the writing of any data to any memory position. With one word equaling four bytes, this attack executes a program to write four bytes of data to a 4-byte address, facilitating malicious operations. Both Linux and Windows manage heaps using doubly linked lists, where each allocated memory block comprises head and tail pointers along with memory data. During heap memory release, the operation M→head→tail=M→tail is performed. If an attacker overflows into adjacent memory space to M, modifying the head pointer of M to direct to a specially designed virtual node, and altering the tail pointer of M to point to a designated location (e.g., shellcode), the tail pointer of the virtual node will then point to the shellcode post M→head→tail=M→tail operation. Consequently, the tail pointer calling the virtual node transforms into a shellcode. Another operation, M→tail→head=M→head, during removal from the linked list, can also be exploited to implement attacks.字

  • Heap Spraying Attack: Heap spraying is an attacker's technique in exploits that facilitates the execution of arbitrary code. By adding a substantial amount of slide code (meaningless instructions occupying memory space for executing attack instructions) in front of the shellcode to form an injection code segment, the attacker requests a significant amount of memory from the system. Repeated injection of code segments fills up the memory, allowing the attacker to control the program flow using other vulnerability attack techniques. Consequently, the program jumps to the heap and executes the shellcode, enabling the execution of core attack instructions within the shellcode to obtain system control permissions.

How Can Buffer Overflow Attacks Be Prevented?

Buffer overflow attacks represent the most prevalent form of remote network assaults, allowing anonymous internet users to potentially seize partial or complete control over a host. Effectively mitigating buffer overflow vulnerabilities would address a significant portion of security threats. Various fundamental methods exist to protect buffers from such attacks and their repercussions:

  • Checking Integrity: Conduct checks before the program pointer becomes invalid, ensuring the integrity of data and preventing buffer overflow.

  • Randomizing Address Space: Randomly arrange address space positions in key data areas. This approach disrupts the predictability of buffer overflow attacks, making it nearly impossible to determine the locations of executable code once the address space is randomized.

  • Preventing Data Execution: Designate specific areas of memory as either executable or non-executable to thwart attacks attempting to run code in non-executable regions.

  • Writing Secure Code: Utilize compilers capable of identifying unsafe functions or errors, employing bounds functionality checking to safeguard buffers. Avoid insecure functions, such as replacing gets() with fgets() in the C language. Employ protected languages or incorporate specialized security programs within the code to proactively prevent buffer overflow vulnerabilities.

Despite the implementation of these preventative measures, the emergence of new buffer overflow vulnerabilities remains a possibility. In such instances, engineers must promptly address and rectify the affected software, ensuring that users can obtain patches to fortify the system against potential exploits in time.

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