A buffer overflow happens when a program tries to write more data into a block of memory than that block was designed to hold. The excess data spills into neighboring memory locations, corrupting whatever was stored there. Depending on what gets overwritten, this can crash the program, corrupt data, or let an attacker take control of the system entirely.
How Memory Buffers Work
When a program runs, the operating system assigns it chunks of memory called buffers to temporarily hold data. Think of a buffer like a row of mailboxes, each one holding a single piece of information. If a program is told to expect a username of up to 8 characters, it reserves 8 slots of memory for that input. The problem starts when someone (or something) sends 20 characters instead.
Those extra 12 characters don’t just vanish. They write over whatever sits in the adjacent memory slots. Those neighboring slots might contain other variables the program needs, or they might contain something far more critical: the return address that tells the program where to go next after finishing its current task. Overwriting that return address is exactly how attackers hijack a program’s execution.
Why Buffer Overflows Are Dangerous
In a classic exploit, an attacker sends carefully crafted input to a vulnerable program. The input is too large for the buffer, so it overflows and overwrites the return pointer on the program’s call stack. The attacker sets that pointer to redirect execution to malicious code they’ve embedded within the overflow data itself. When the program finishes its current function and follows the return pointer, it jumps straight into the attacker’s code.
At that point, the attacker can execute arbitrary commands on the system, often with the same privileges as the vulnerable program. If that program runs with administrator or root access, the attacker now controls the machine. As OWASP notes, when the consequence is arbitrary code execution, it can be used to subvert every other security measure in place. This is why buffer overflows have been one of the most exploited vulnerability classes for decades.
The very first major internet attack used this technique. On November 2, 1988, the Morris Worm exploited a buffer overflow in the Unix “finger” program (among other vulnerabilities) and spread across the early internet. Within 24 hours, roughly 6,000 of the 60,000 computers connected to the internet at the time were infected.
Stack Overflows vs. Heap Overflows
Buffer overflows fall into two main categories based on where in memory they occur. Stack-based overflows target the call stack, the region of memory that tracks function calls, local variables, and return addresses. Because the stack has a predictable structure, these overflows are generally easier to exploit. An attacker knows roughly how far past the buffer they need to write to reach the return pointer and redirect execution.
Heap-based overflows target the heap, a more loosely organized region of memory used for data that persists beyond a single function call. Exploiting heap overflows is more complex because the memory layout is less predictable, but they can still corrupt critical data structures and alter program behavior. Both types can lead to full system compromise in the right conditions.
Languages That Allow Buffer Overflows
Buffer overflows are primarily a problem in languages like C and C++ that give programmers direct access to memory without automatic safety checks. Several standard C functions are notorious for enabling overflows because they copy data without verifying the destination is large enough. The function gets(), which reads input from the user, has no way to limit how much data it accepts. Similarly, strcpy() copies one string into another with no size check, and sprintf() formats data into a buffer without confirming it fits.
Safer alternatives exist for each of these. The function fgets() replaces gets() by requiring the programmer to specify a maximum input size. Functions like strcpy_s() and snprintf() add size parameters that prevent writing past the buffer’s boundary. Using these alternatives is one of the simplest ways to eliminate overflow vulnerabilities in C code.
A joint advisory from the NSA and CISA recommends a more fundamental solution: using memory-safe languages. Languages like Rust, Go, Java, Python, C#, and Swift include built-in protections that prevent entire classes of memory vulnerabilities, including buffer overflows. These languages enforce bounds checking, which means every memory access is automatically verified to fall within allocated boundaries. Rust takes a particularly strict approach, using ownership and borrowing rules that ensure only one part of the code can modify a piece of memory at a time, and that memory is automatically freed when it’s no longer in use.
How Modern Systems Defend Against Overflows
Even when code is written in an unsafe language, modern operating systems and compilers add layers of protection that make buffer overflows harder to exploit.
Data Execution Prevention (DEP) marks certain areas of memory as non-executable. Even if an attacker manages to inject malicious code into a buffer, the processor will refuse to run it because that memory region is flagged as “data only.” This neutralizes the classic attack of embedding executable instructions inside overflow data.
Address Space Layout Randomization (ASLR) shuffles the memory layout of a program each time it runs. Critical components like the stack, heap, and system libraries load at unpredictable addresses. This makes it far harder for an attacker to know where to redirect execution, since the target address changes every time.
Stack canaries are small, randomly generated values that compilers insert between the buffer and the return pointer on the stack. Before a function returns, the program checks whether the canary value is still intact. If an overflow has overwritten the buffer and reached the return pointer, it will have corrupted the canary along the way. When the program detects a modified canary, it terminates immediately rather than following a potentially hijacked return address.
None of these defenses is perfect on its own. Attackers have developed techniques to bypass each one individually. But combined, they raise the difficulty of exploitation significantly. A successful exploit against a modern system typically needs to defeat ASLR, DEP, and stack canaries all at once.
Finding Overflows Before Attackers Do
Developers use two broad approaches to catch buffer overflows during development. Static analysis tools scan source code without running it, looking for patterns that indicate potential overflows, like calls to unsafe functions or array accesses that could exceed their bounds. Dynamic analysis tools test the program while it’s actually running, feeding it unexpected inputs and monitoring memory behavior in real time.
Research from NIST evaluated five static analysis tools against 291 test cases designed to cover different types of C buffer overflows. Each tool had different strengths: some excelled at tracking how data flows between functions, while others focused specifically on catching misuse of string manipulation functions. No single tool caught everything, which is why security teams typically combine multiple tools and approaches. Pairing automated analysis with manual code review remains the most reliable way to catch overflow vulnerabilities before software ships.
Current Relevance
Buffer overflows have been a top software vulnerability for over three decades, though their ranking has shifted in recent years. In MITRE’s 2025 CWE Top 25 list of the most dangerous software weaknesses, the general category of “improper restriction of operations within the bounds of a memory buffer” dropped from position 20 to 39. This decline reflects real progress: wider adoption of memory-safe languages, better compiler protections, and improved coding practices have collectively reduced the prevalence of these bugs in new software.
That said, the world still runs on enormous amounts of C and C++ code in operating systems, embedded devices, network infrastructure, and legacy applications. Buffer overflows in this existing codebase continue to surface regularly, and they remain among the most severe vulnerabilities when they do appear, because they so often lead to full system compromise.