Stack Buffer Overflow in count.c: How sprintf() Can Lead to Arbitrary Code Execution

Introduction

Buffer overflows are one of the oldest vulnerabilities in software security — and yet they continue to appear in production code today. When a C program writes more data into a fixed-size buffer than the buffer can hold, the excess data spills into adjacent memory regions. On the stack, that adjacent memory often includes the function's return address: the pointer that tells the CPU where to jump when the function finishes executing.

If an attacker can control what gets written past the end of that buffer, they can redirect execution to code of their choosing. This is the essence of a stack buffer overflow, and it's exactly what was lurking in count.c.

This post breaks down the vulnerability, explains how it could be exploited, and walks through the fix — along with practical guidance for avoiding this class of bug in your own code.

The Vulnerability Explained

What Went Wrong

The vulnerability (tracked as V-001, severity: CRITICAL) existed in three sprintf() calls within count.c. The problematic pattern looked something like this:

char aux[SOME_FIXED_SIZE];
char aux2[SOME_FIXED_SIZE];

// At line 248 — building a format string using MAX_NAME_LEN
sprintf(aux, "%-*s", MAX_NAME_LEN, some_name);

// At lines 445-446 — same pattern with MAX_FS_LEN
sprintf(aux2, "%-*s", MAX_FS_LEN, filesystem_name);

The core issue: sprintf() writes into the destination buffer with no knowledge of how large that buffer actually is. It will happily write 10 bytes, 100 bytes, or 10,000 bytes — stopping only when it hits a null terminator in the source data, not when it hits the end of the destination buffer.

Breaking It Down

Let's look at what makes this dangerous:

1. Fixed-size stack buffers: aux and aux2 are declared with a compile-time constant size. They live on the stack, right next to the function's saved return address and other critical bookkeeping data.

2. Unbounded format string construction: The format string uses MAX_NAME_LEN and MAX_FS_LEN as field-width specifiers. If these constants are large — or if the actual string data exceeds them — the output can easily overflow the destination buffer.

3. No size validation: There is no check before the sprintf() call to confirm that the formatted output will fit within aux or aux2. The code assumes the constants are "safe enough," which is a dangerous assumption.

The Stack Layout: Why This Is So Dangerous

To understand the impact, picture the stack frame for the vulnerable function:

High addresses
┌─────────────────────────┐
│   Return Address        │  ← Attacker wants to overwrite this
├─────────────────────────┤
│   Saved Frame Pointer   │
├─────────────────────────┤
│   Other local variables │
├─────────────────────────┤
│   aux2[SOME_FIXED_SIZE] │
├─────────────────────────┤
│   aux[SOME_FIXED_SIZE]  │  ← sprintf() writes here
└─────────────────────────┘
Low addresses

When sprintf() overflows aux, it writes past the end of the buffer, potentially overwriting aux2, other local variables, the saved frame pointer, and finally the return address. When the function returns, the CPU jumps to whatever address is now in that slot — which an attacker can set to point to malicious shellcode or to a ROP (Return-Oriented Programming) gadget chain.

Real-World Attack Scenario

Consider a scenario where count.c is part of a system utility that processes filesystem or file names supplied by users or read from external input:

1. An attacker crafts a filename or filesystem label that is unusually long.
2. The input is passed into the vulnerable sprintf() call.
3. The formatted output overflows aux, smashing the stack.
4. The return address is overwritten with the address of attacker-controlled code.
5. When the function returns, the CPU executes the attacker's payload — with the privileges of the running process.

If this utility runs as root (common for filesystem-related tools), the result is full system compromise.

Even without achieving code execution directly, an overflow can cause crashes, corrupt adjacent data structures, or be chained with other vulnerabilities for privilege escalation.

The Fix

What Changed

The fix replaces the unsafe sprintf() calls with their bounds-checking counterpart, snprintf(). The snprintf() function accepts an explicit size parameter, ensuring it will never write more bytes than the destination buffer can hold.

Before (vulnerable):

char aux[BUFFER_SIZE];
char aux2[BUFFER_SIZE];

// Line 248 — no bounds checking
sprintf(aux, "%-*s", MAX_NAME_LEN, name);

// Lines 445-446 — same unsafe pattern
sprintf(aux2, "%s", fs_name);
sprintf(aux,  "%-*s", MAX_FS_LEN, aux2);

After (fixed):

char aux[BUFFER_SIZE];
char aux2[BUFFER_SIZE];

// Bounded write — will not exceed BUFFER_SIZE
snprintf(aux, sizeof(aux), "%-*s", MAX_NAME_LEN, name);

// Bounded writes
snprintf(aux2, sizeof(aux2), "%s", fs_name);
snprintf(aux,  sizeof(aux),  "%-*s", MAX_FS_LEN, aux2);

Why snprintf() Solves the Problem

The snprintf(buf, size, format, ...) function guarantees:

• At most size - 1 characters are written to buf.
• The output is always null-terminated (as long as size > 0).
• If the output would have exceeded size, it is silently truncated rather than overflowing.

By passing sizeof(aux) as the size argument, the code ties the write limit directly to the actual buffer size. Even if MAX_NAME_LEN is enormous, or if the input string is unexpectedly long, the stack is protected.

A Note on Truncation

One subtlety worth understanding: snprintf() prevents memory corruption by truncating output, but truncation itself can sometimes introduce logic bugs (e.g., a truncated filename might match a different file). The return value of snprintf() tells you how many characters would have been written if the buffer were large enough:

int written = snprintf(aux, sizeof(aux), "%-*s", MAX_NAME_LEN, name);
if (written >= (int)sizeof(aux)) {
    // Output was truncated — handle this case explicitly
    fprintf(stderr, "Warning: name truncated\n");
}

Checking this return value is a best practice that transforms a silent failure into an observable, handleable condition.

Prevention & Best Practices

1. Ban sprintf() in New Code

There is almost no legitimate reason to use sprintf() in modern C code. Configure your compiler or linter to warn on its use:

CFLAGS += -Wformat -Wformat-overflow

With GCC 7+ and Clang, -Wformat-overflow can detect some (not all) cases where sprintf() will overflow a buffer at compile time.

2. Use Safe Alternatives Consistently

3. Validate Constants Against Buffer Sizes

If your buffer size is derived from a constant like MAX_NAME_LEN, add a static assertion to catch mismatches at compile time:

#define MAX_NAME_LEN  256
#define BUFFER_SIZE   128

// This will cause a compile-time error if MAX_NAME_LEN >= BUFFER_SIZE
_Static_assert(MAX_NAME_LEN < BUFFER_SIZE,
    "BUFFER_SIZE must be larger than MAX_NAME_LEN");

4. Enable Compiler and OS Mitigations

Modern compilers and operating systems include several mitigations that make buffer overflows harder to exploit (though not impossible):

• Stack Canaries (-fstack-protector-strong): Places a random value between local variables and the return address; checked before function return.
• ASLR (Address Space Layout Randomization): Randomizes the memory layout, making it harder to predict where shellcode or gadgets live.
• NX/DEP (No-Execute / Data Execution Prevention): Marks the stack as non-executable, blocking simple shellcode injection.
• FORTIFY_SOURCE (-D_FORTIFY_SOURCE=2): Adds compile-time and runtime checks for common string functions.

CFLAGS += -fstack-protector-strong -D_FORTIFY_SOURCE=2 -pie -fPIE
LDFLAGS += -Wl,-z,relro,-z,now

These are defense-in-depth measures — they reduce exploitability but do not fix the underlying bug. Always fix the root cause.

5. Use Static Analysis Tools

Several tools can catch buffer overflows before they reach production:

• Coverity — Industry-standard static analyzer with excellent C/C++ support
• cppcheck — Free, open-source static analyzer for C/C++
• AddressSanitizer (ASan) — Runtime memory error detector; add -fsanitize=address during testing
• Valgrind — Runtime analysis for memory errors

6. Security Standards and References

This vulnerability maps to well-established security standards:

• CWE-121: Stack-based Buffer Overflow — The canonical classification for this bug class
• CWE-676: Use of Potentially Dangerous Function — Covers the use of sprintf() and similar unsafe functions
• OWASP: Buffer Overflow — High-level overview and prevention guidance
• SEI CERT C Coding Standard: STR07-C — Use bounds-checking interfaces for string manipulation
• NIST NVD — For tracking CVEs related to buffer overflows in third-party libraries

Conclusion

The buffer overflow fixed in count.c is a textbook example of a vulnerability class that has been well-understood for over 30 years — and yet continues to appear in real-world code. The root cause is simple: sprintf() trusts that the caller has allocated enough space, and when that trust is misplaced, the consequences can be catastrophic.

The fix is equally simple: replace sprintf() with snprintf() and pass the actual buffer size. One function, one parameter, zero stack smashing.

Key takeaways:

• ✅ Never use sprintf() in C code — always use snprintf() with an explicit size
• ✅ Check snprintf()'s return value to detect and handle truncation
• ✅ Add compiler warnings (-Wformat-overflow) to catch these issues early
• ✅ Enable stack protections (-fstack-protector-strong) as a defense-in-depth measure
• ✅ Run static analysis (cppcheck, Coverity) and dynamic analysis (ASan, Valgrind) regularly
• ✅ Validate buffer size assumptions with _Static_assert at compile time

Memory safety vulnerabilities are preventable. The tools, techniques, and language features to avoid them have existed for decades. The key is building secure coding habits — and making unsafe functions like sprintf() simply off-limits in your codebase.

Stay safe, write bounds-checked code, and keep shipping secure software. 🔒

This vulnerability was identified and fixed by OrbisAI Security. Automated security scanning helps catch critical issues like this before they reach production.