If we don’t rigorously validate file uploads to a sensitive server-side program, threat actors can upload specially crafted files designed to exploit subtle flaws in that program's memory allocation processes. Successfully exploiting these flaws (i.e., vulnerabilities) can result in high-severity attacks like denial of service or arbitrary code execution.

In this article, we’ll review how stack memory works and discuss how stack-based buffer overflows occur.

What is Stack Memory?

Stack memory is the section of our computer’s RAM used to store local variables and function calls. The physical region of memory allocated to the stack is what we call the stack memory buffer.

Data added to a stack memory buffer follows a last-in, first-out (LIFO) structure. Given that RAM memory is always limited to some degree, LIFO structure literally ensures the last data item added to the stack will be the first item removed to free up memory for subsequent actions.

In stack memory, functions executed for any given program are added to the top of the stack to store things like local variables, function parameters, and the return address (which tells the program where to resume execution when the function call is complete). Memory associated with a local variable is allocated right when the function is called and subsequently deallocated when the function exits the stack.

How does a Stack-Based Buffer Overflow Occur?

Stack-based buffer overflows occur when more memory is written to a stack memory buffer than it is designed to accommodate.

Once a threat actor has identified a stack-based buffer overflow vulnerability in an application, they can craft a malicious input (i.e., a specially designed file) with properties intended to trigger excessive memory allocation and/or deliver a concealed payload of malicious code. Depending on the circumstances, they can submit this file through a client-side file upload portal themselves, or they can employ social engineering techniques to submit this file to an application user (often one with escalated permissions within the application) and trick them into opening the file within the vulnerable application instead. Either way, by successfully overloading a stack memory buffer for any given application workflow, they can potentially overwrite adjacent stack memory buffers and initiate various attacks.

A sudden memory buffer overwrite can cause the application to crash, resulting in denial of service. If the attacker manages to overwrite the return address stored on the stack, they can force the vulnerable program to begin executing malicious code of their choosing instead of executing subsequent stack functions via normal LIFO structure.

Sophisticated threat actors could, for example, use this process to inject payloads into a program containing shell code, which could enable them to create backdoors into a system and/or perform a wide range of other malicious actions.

How can Stack-Based Buffer Overflows be Prevented?

Vulnerabilities in server-side processes need to be monitored and patched through regular software updates.

Unfortunately, however, we won’t always have the benefit of knowing about these vulnerabilities before they’re exploited in one of our own applications. Zero-day threats can take us off guard completely, and the best way to combat such threats is to forward deploy our own security policies ahead of sensitive server entry points.

Preventing Stack-Based Buffer Overflow with Cloudmersive

The Cloudmersive Advanced Virus Scan API provides 360-degree content protection for file upload processes by scanning files for viruses, malware, and non-malware content threats. Invalid or malformed files - such as those used to exploit Stack-Based Buffer Overflow vulnerabilities - can be detected and blocked by setting custom threat rules in the API request body.

For more information on the Cloudmersive Virus Scan API, please do not hesitate to reach out to a member of our sales team.