Stack Smashing via sprintf: How Unbounded Writes Broke a C Simulation Engine

Severity: Critical | File: universe/command.c | CVE Class: CWE-121 (Stack-Based Buffer Overflow)

Introduction

In an era of memory-safe languages and hardened runtimes, it can be tempting to assume that classic C memory corruption bugs are a solved problem — a relic of the 1990s. They are not. Buffer overflows remain one of the most consistently exploited vulnerability classes in production software, and this week's patch in universe/command.c is a textbook reminder of why.

Four lines of code. Four calls to sprintf. Four opportunities for an attacker to seize control of the program's execution. This post breaks down exactly what went wrong, how it could be exploited, and what every developer writing C (or reviewing C) should take away from this fix.

The Vulnerability Explained

What Is a Buffer Overflow?

A buffer overflow occurs when a program writes more data into a fixed-size memory region than it was designed to hold. The excess bytes spill over into adjacent memory, corrupting whatever lives there — return addresses, function pointers, local variables, or heap metadata.

In C, the standard library function sprintf is a notorious enabler of this class of bug. Its signature looks innocent:

int sprintf(char *str, const char *format, ...);

The problem? sprintf has absolutely no knowledge of how large str is. It will write as many bytes as the format string and its arguments demand, regardless of whether the destination buffer can hold them.

The Vulnerable Code

In universe/command.c, four calls to sprintf (at lines 394, 399, 408, and 410) wrote attacker-influenced string values into a single fixed-size buffer called result_str:

// VULNERABLE — Before the fix
char result_str[256]; // Fixed-size buffer on the stack

// Line 394 — met_being_name is attacker-controlled
sprintf(result_str, "You have met %s", met_being_name);

// Line 399 — relationship_str2 is attacker-controlled
sprintf(result_str, "Your relationship with %s", relationship_str2);

// Line 408 — four_characters is attacker-controlled
sprintf(result_str, "Character trait: %s", four_characters);

// Line 410 — another four_characters write
sprintf(result_str, "Additional trait: %s", four_characters);

The variables met_being_name, relationship_str2, and four_characters are populated from simulation save file data — content that is entirely under the attacker's control.

Why Is This Critical?

result_str lives on the stack. On the stack, just above (or below, depending on architecture and compiler) local variables sits the saved return address — the memory address the CPU will jump to when the current function returns.

If an attacker can overflow result_str with a carefully crafted string, they can overwrite that return address with an address of their choosing. When the function returns, instead of going back to its caller, the CPU jumps to attacker-controlled code.

This is the essence of a stack smashing attack, and it is the most direct path to arbitrary code execution.

Attack Scenario: Crafting a Malicious Save File

Here is how a real-world exploit of this vulnerability would unfold:

Step 1 — Identify the Buffer Size

An attacker reverse-engineers or reads the source of universe/command.c and determines that result_str is 256 bytes. They know the format string prefix (e.g., "You have met " — 13 characters), so the actual payload space before overflow begins is roughly 256 - 13 = 243 bytes.

Step 2 — Craft the Save File

The attacker creates a simulation save file where a being's name is set to a string of 300+ bytes:

being_name = "AAAAAAAAAA...AAA[shellcode_or_ROP_chain]"

The first ~243 A characters fill result_str to capacity. The remaining bytes overflow into adjacent stack memory, overwriting the saved return address with a chosen value.

Step 3 — Trigger the Code Path

The attacker loads the save file in the simulation. When the game processes the being's name and calls the vulnerable sprintf, the overflow occurs silently. When the surrounding function returns, execution redirects to the attacker's payload.

Step 4 — Code Execution

Depending on the platform's mitigations (ASLR, stack canaries, NX bit), the attacker may achieve direct shellcode execution, a return-oriented programming (ROP) chain, or at minimum a reliable crash (denial of service).

Key insight: The attacker never needs to interact with the running process directly. A single malicious save file shared on a modding forum, sent via a multiplayer session, or distributed as a "scenario pack" is sufficient to trigger the exploit on any machine that opens it.

The Fix

The fix replaces all four unbounded sprintf calls with snprintf, the bounds-checking variant that accepts an explicit maximum byte count:

// FIXED — After the patch
char result_str[256];

// Line 394
snprintf(result_str, sizeof(result_str), "You have met %s", met_being_name);

// Line 399
snprintf(result_str, sizeof(result_str), "Your relationship with %s", relationship_str2);

// Line 408
snprintf(result_str, sizeof(result_str), "Character trait: %s", four_characters);

// Line 410
snprintf(result_str, sizeof(result_str), "Additional trait: %s", four_characters);

Why snprintf Solves the Problem

snprintf's signature adds one critical parameter:

int snprintf(char *str, size_t size, const char *format, ...);

The size argument tells snprintf the maximum number of bytes (including the null terminator) it is permitted to write. No matter how long the source strings are, snprintf will never write beyond result_str's boundary. The output is simply truncated to fit.

Using sizeof — The Right Way

Notice that the fix uses sizeof(result_str) rather than a hardcoded 256. This is deliberate and important. If a future developer changes the buffer size, sizeof automatically tracks the new value. A hardcoded constant could become stale and introduce a new vulnerability.

// Fragile — hardcoded size can drift from reality
snprintf(result_str, 256, ...);

// Robust — always matches the actual buffer
snprintf(result_str, sizeof(result_str), ...);

What About Truncation?

A reasonable question: if snprintf silently truncates long strings, could that cause logic bugs? In this case, result_str is used for display purposes — showing the player information about a being or relationship. Truncating a very long name to fit the buffer is far preferable to allowing arbitrary code execution. For security-critical data (keys, hashes, identifiers), truncation must be handled explicitly, but for UI strings, it is the correct and safe behavior.

Prevention & Best Practices

This vulnerability is entirely preventable. Here is how to avoid it in your own C code and how to catch it when reviewing others'.

1. Banish sprintf from Your Codebase

Treat sprintf as deprecated. Modern C development should use snprintf everywhere a string is being written to a fixed-size buffer. Many organizations enforce this via compiler warnings or static analysis rules.

# GCC/Clang: enable format and bounds warnings
gcc -Wall -Wextra -Wformat -Wformat-overflow ...

2. Always Use sizeof with Buffer Functions

When calling snprintf, strncpy, strncat, or any other bounds-aware function, pass sizeof(buffer) rather than a literal number. This keeps the size in sync with the declaration automatically.

3. Validate Input at the Boundary

Before attacker-controlled data ever reaches a sprintf (or snprintf) call, validate and sanitize it at the point of ingestion — when the save file is parsed. Enforce maximum lengths on string fields:

#define MAX_BEING_NAME_LEN 64

if (strlen(raw_name) > MAX_BEING_NAME_LEN) {
    // Reject or truncate at parse time, log a warning
    return PARSE_ERROR_NAME_TOO_LONG;
}

4. Enable Compiler and OS Mitigations

While mitigations are not a substitute for fixing the root cause, they raise the bar for exploitation significantly:

5. Use Static Analysis Tools

Integrate static analysis into your CI pipeline to catch unbounded writes automatically:

• Coverity — Industry-standard static analyzer with excellent C/C++ support
• CodeQL — GitHub's semantic code analysis engine; has built-in queries for buffer overflows
• Flawfinder — Lightweight, specifically designed to flag dangerous C functions like sprintf, strcpy, gets
• clang-tidy — Modular linter with security-focused checks

Running Flawfinder against the vulnerable code would have flagged these sprintf calls immediately:

universe/command.c:394: [4] (buffer) sprintf:
  Does not check for buffer overflows. Use snprintf or vsnprintf.

6. Know Your CWEs

Understanding the Common Weakness Enumeration taxonomy helps you recognize and communicate about vulnerability classes:

• CWE-121 — Stack-Based Buffer Overflow
• CWE-120 — Buffer Copy without Checking Size of Input ("Classic Buffer Overflow")
• CWE-134 — Use of Externally-Controlled Format String

This vulnerability maps primarily to CWE-121 and is referenced in the OWASP Top 10 under A03:2021 – Injection and in the SANS/CWE Top 25 Most Dangerous Software Weaknesses.

A Note on Save File Trust

This vulnerability highlights an important security principle that is frequently overlooked in game and simulation development: save files are untrusted input.

Developers often treat save files as internal data — something the application wrote and will read back. But save files are stored on disk, can be edited by users, and in multiplayer or modding contexts, can be created entirely by third parties. Any data read from a save file must be treated with the same skepticism as data received from a network socket or a web form.

This means:
- Validate all field lengths before processing
- Sanitize string content for unexpected characters
- Consider signing or checksumming save files if integrity matters
- Fuzz your save file parser with tools like AFL++ or libFuzzer

Conclusion

Four lines. Four missing bounds checks. A critical severity vulnerability that could hand an attacker full control of the host machine — delivered via nothing more exotic than a crafted save file.

The fix is elegantly simple: replace sprintf with snprintf and pass sizeof(result_str). But the lesson is broader than any single function call. It is about cultivating a security mindset in every line of C you write:

• Assume all external input is hostile.
• Know the size of every buffer you write to.
• Use the bounds-checking variant of every string function.
• Let your tools (compilers, linters, sanitizers) catch what your eyes miss.

Buffer overflows have been on the OWASP and SANS Top 25 lists for decades. They will remain there as long as C code is written without discipline. The good news is that with the right habits and tooling, they are entirely preventable.

Secure code is not an accident. It is a practice.

This vulnerability was identified and patched by OrbisAI Security. If you'd like to learn more about automated security scanning for your codebase, visit their site.

Further Reading:
- OWASP Buffer Overflow Attack
- CWE-121: Stack-Based Buffer Overflow
- Smashing The Stack For Fun And Profit — Aleph One (Phrack, 1996)
- SEI CERT C Coding Standard: STR07-C
- GCC Stack Protector Documentation