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

Severity: 🔴 Critical | File: game/g_spawn.c | CWE: CWE-121 (Stack-based Buffer Overflow) | Fixed in: PR — "fix: remove unsafe exec() in g_spawn.c"

Introduction

Few vulnerability classes have as long and storied a history as the stack buffer overflow. First weaponized in the wild in the 1980s and famously described in Aleph One's landmark 1996 paper "Smashing the Stack for Fun and Profit", stack overflows remain a persistent threat in C and C++ codebases to this day. They are especially dangerous in game engines, where performance pressure historically led developers to favor raw C string functions over their safer counterparts — and where networked, multiplayer environments mean that attacker-controlled data flows directly into those functions.

This post breaks down a critical stack buffer overflow that was recently identified and patched in game/g_spawn.c — a file responsible for spawning entities in a game world. The root cause? Five calls to sprintf() writing attacker-influenced values into a fixed-size stack buffer with no bounds checking whatsoever.

If you write C or C++, maintain a game engine, or simply want to understand one of the most impactful vulnerability classes in systems programming, read on.

The Vulnerability Explained

What Is a Stack Buffer Overflow?

When a C program declares a local variable like this:

char buffer[256];

That buffer lives on the call stack — a region of memory that also stores critical bookkeeping data, including the saved return address (where the CPU should jump when the current function finishes). If you write more than 256 bytes into buffer without checking the length, you start overwriting adjacent memory — including that saved return address.

An attacker who can control what gets written into the buffer can replace the return address with a pointer to their own shellcode (or, in modern exploitation, a ROP chain). When the function returns, the CPU jumps to the attacker's code instead of the legitimate caller. This is arbitrary code execution.

The Vulnerable Code Pattern

In game/g_spawn.c, the entity spawn function contained code resembling the following pattern:

// VULNERABLE — Do not use this pattern
void spawn_entity(edict_t *edict, const char *dir, const char *file) {
    char path[256];  // Fixed-size stack buffer

    // Five calls like this, incorporating attacker-influenced values:
    sprintf(path, "%s/%s_%d.cfg", dir, file, edict->variation);
    // ... more sprintf() calls using edict->class_id, DESTRUCTABLE_TEXTURE macro, etc.

    load_resource(path);
}

Let's unpack exactly why this is dangerous:

Every one of these values can originate from a crafted map file or a malicious network packet. The sprintf() function will happily write as many bytes as the format string produces — it has no concept of the destination buffer's size.

Why sprintf() Is the Problem

sprintf() is one of several C standard library functions sometimes called the "unsafe" string functions. Its signature is:

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

Notice what's missing: there is no parameter for the size of str. The function will write until the formatted output is complete, regardless of whether it overflows the destination. This is in contrast to snprintf(), which takes an explicit size limit:

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

With five separate sprintf() calls all targeting the same stack buffer, the overflow surface is substantial.

A Concrete Attack Scenario

Imagine a multiplayer game where players can load community-created maps. A malicious map author crafts a .map file with the following properties:

1. An entity with a variation field set to a large integer.
2. A dir path string of 200 characters (all As).
3. A file string of 100 characters (all Bs).

When the server loads this map and calls spawn_entity(), the sprintf() call attempts to write something like:

AAAA...AAAA/BBBB...BBBB_99999999.cfg

This string is far longer than 256 bytes. The excess bytes spill past the end of path on the stack, overwriting:

• Other local variables
• The saved frame pointer (%rbp / %ebp)
• The saved return address

The attacker carefully crafts the overflow payload so that the saved return address is replaced with an address of their choosing. When spawn_entity() returns, the CPU executes the attacker's code — on the server, with the server's privileges.

In a game server context, this could mean:
- Remote code execution on the hosting machine
- Lateral movement to other services on the same host
- Data exfiltration of player credentials, payment information, or server secrets
- Persistent backdoor installation

The Fix

What Changed

The fix removes the unsafe sprintf() calls and replaces them with bounds-checked alternatives, eliminating the possibility of writing beyond the buffer boundary.

The corrected pattern uses snprintf() with explicit size limits:

// SAFE — Bounds-checked version
void spawn_entity(edict_t *edict, const char *dir, const char *file) {
    char path[256];
    int written;

    written = snprintf(path, sizeof(path), "%s/%s_%d.cfg",
                       dir, file, edict->variation);

    if (written < 0 || written >= (int)sizeof(path)) {
        // Handle truncation or error — do NOT proceed with a partial path
        log_error("spawn_entity: path truncated or format error");
        return;
    }

    load_resource(path);
}

Key changes in this approach:

1. snprintf() instead of sprintf(): The second argument (sizeof(path)) tells the function the maximum number of bytes to write, including the null terminator. It will never write past the end of the buffer.

2. Return value checking: snprintf() returns the number of characters that would have been written if the buffer were large enough. If this value is >= sizeof(path), truncation occurred. The fix explicitly handles this case rather than silently proceeding with a corrupted path.

3. Using sizeof(buffer) instead of a magic number: Tying the size limit directly to the buffer declaration means the check stays correct even if the buffer size is later changed.

Why This Solves the Problem

With snprintf() in place, even a crafted map file with a 10,000-character path string cannot overflow the stack buffer. The function writes at most 255 bytes of content plus a null terminator, and the rest is discarded. The saved return address is never touched.

The truncation check adds an additional layer of defense: rather than silently loading a garbled, truncated path (which could itself cause undefined behavior or logic errors), the function fails safely and logs the anomaly.

Conclusion

The stack buffer overflow in game/g_spawn.c is a textbook example of a vulnerability class that has been well-understood for nearly three decades — yet continues to appear in production code. Five calls to sprintf() with attacker-influenced format arguments in a game engine's entity spawning function created a direct path to arbitrary code execution on any server loading a crafted map.

The fix is straightforward: replace sprintf() with snprintf(), pass the buffer size explicitly, and handle the truncation case gracefully. But the deeper lesson is about secure-by-default habits:

• Treat all external data as hostile until validated.
• Use bounds-checked functions everywhere, not just where you think overflow is possible.
• Let your toolchain and static analyzers catch what your eyes miss.
• Layer mitigations so that even when a bug slips through, exploitation is as difficult as possible.

Buffer overflows are not a legacy problem — they are an ongoing one. The code you write today will be running in production environments, processing untrusted input, years from now. Building these habits now is the most effective security investment you can make.

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References
- Aleph One, "Smashing the Stack for Fun and Profit", Phrack Magazine, 1996
- SEI CERT C Coding Standard: https://wiki.sei.cmu.edu/confluence/display/c/SEI+CERT+C+Coding+Standard
- CWE-121: https://cwe.mitre.org/data/definitions/121.html
- OWASP Buffer Overflow: https://owasp.org/www-community/vulnerabilities/Buffer_Overflow