Stack Buffer Overflow in MapScale: How Five Unsafe sprintf Calls Created a Critical Vulnerability

Introduction

Buffer overflows are one of the oldest vulnerabilities in software security — they predate the web, predate modern operating systems, and yet they continue to appear in production code today. This week, a critical stack-based buffer overflow was discovered and patched in src/mapscale.c, a C source file responsible for rendering scale bar labels in a mapping application.

The vulnerability, tracked as V-001 (CWE-120), involved five separate calls to sprintf that wrote formatted strings into a fixed-size stack buffer named label without ever specifying how much data could be written. In C, that's the kind of oversight that can turn a misconfigured unit label into a full remote code execution primitive.

If you write C or C++ code — or maintain any codebase that does — this post is for you.

The Vulnerability Explained

What Is a Stack-Based Buffer Overflow?

When a C program declares a local variable like this:

char label[64];

It's reserving exactly 64 bytes on the call stack — a contiguous region of memory that also stores other critical data, including the function's return address (the memory location the CPU jumps to when the function finishes executing).

If you write more than 64 bytes into label, you don't get an error. In C, you just keep writing — right over the return address, over saved registers, over whatever happens to be adjacent on the stack. This is a stack-based buffer overflow.

The Vulnerable Code Pattern

In mapscale.c, the problematic pattern looked like this:

char label[64]; /* Fixed-size stack buffer */

/* Lines ~325 and ~369 — unbounded sprintf calls */
sprintf(label, "%g %s", scale_value, unitText[map->scalebar.units]);

There are five instances of this pattern across the file. The format string "%g %s" combines a floating-point number with a string pulled from unitText[map->scalebar.units] — a value that can be influenced by user input or external configuration.

Here's why this is dangerous:

• sprintf has no idea how large label is
• It will happily write as many bytes as the format string produces
• If unitText[map->scalebar.units] contains a long string (say, 200 characters), sprintf will write well beyond the 64-byte boundary of label
• Adjacent stack data — including the return address — gets overwritten

How Could This Be Exploited?

An attacker who can control the content of unitText values (via a malicious map configuration file, a crafted API request, or a compromised data source) could:

1. Craft a unit label string that is long enough to overflow the label buffer
2. Overwrite the return address on the stack with an address of their choosing
3. Redirect execution to attacker-controlled code (shellcode, a ROP chain, or an existing function)

Attack Scenario

Imagine a web-based mapping service that allows users to upload custom map configuration files. The configuration includes scale bar settings, including unit labels. A malicious user uploads a config like this:

{
  "scalebar": {
    "units": "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA[CRAFTED_RETURN_ADDRESS]"
  }
}

When the server renders the scale bar for this map, sprintf dutifully copies that string into the 64-byte label buffer, overflowing it. With the right payload, the attacker controls where the function returns — and potentially what code runs next.

In practice, modern systems have mitigations like stack canaries, ASLR, and NX bits that make exploitation harder. But "harder" is not "impossible," and these mitigations can sometimes be bypassed or may not be present in all deployment environments.

Real-World Impact

• Arbitrary code execution on the server or client running the mapping software
• Privilege escalation if the process runs with elevated permissions
• Denial of service (crash) even if full code execution is not achieved
• Data exfiltration if an attacker can redirect execution to leak memory contents

This is why CWE-120 ("Buffer Copy without Checking Size of Input") is consistently listed among the most dangerous software weaknesses by MITRE and SANS.

The Fix

What Changed

The fix replaces all five unbounded sprintf calls with snprintf, which requires the caller to specify the maximum number of bytes to write — including the null terminator.

Before (vulnerable):

char label[64];

/* No bounds checking — will overflow if unitText is long */
sprintf(label, "%g %s", scale_value, unitText[map->scalebar.units]);

After (fixed):

char label[64];

/* Bounds-checked — will never write more than sizeof(label) bytes */
snprintf(label, sizeof(label), "%g %s", scale_value, unitText[map->scalebar.units]);

Why This Works

snprintf accepts a second argument — the maximum number of characters to write (including the null terminator). If the formatted output would exceed that limit, snprintf truncates the output rather than overflowing the buffer.

This means:
- The label buffer can never be overrun, regardless of how long unitText is
- The stack layout remains intact
- The function return address is never touched

The sizeof(label) Pattern

Notice the fix uses sizeof(label) rather than a hardcoded 64. This is a best practice because:

• If the buffer size changes in the future, the snprintf limit updates automatically
• It eliminates the risk of the size argument getting out of sync with the actual buffer declaration
• It's self-documenting — the code clearly says "don't write more than this buffer can hold"

All Five Instances

It's worth emphasizing that all five occurrences were fixed, not just one. This is critical. A partial fix that leaves even one unbounded sprintf call in place would leave the vulnerability open. Security fixes need to be comprehensive — finding the pattern in one place should always prompt a search for the same pattern elsewhere in the codebase.

Prevention & Best Practices

1. Never Use sprintf for String Formatting in C

Make it a team rule: sprintf is banned. Use snprintf everywhere, always with sizeof(buffer) as the size argument.

/* ❌ Never do this */
sprintf(buf, "Hello, %s!", username);

/* ✅ Always do this */
snprintf(buf, sizeof(buf), "Hello, %s!", username);

2. Use Static Analysis Tools

Several tools can catch unbounded sprintf calls automatically:

• Coverity — Detects CWE-120 and related buffer issues
• Clang Static Analyzer — Free, integrates with build systems
• cppcheck — Lightweight, easy to add to CI/CD
• Flawfinder — Specifically flags dangerous C functions like sprintf, strcpy, gets
• CodeQL — GitHub's semantic code analysis engine

Add these to your CI pipeline so buffer overflow patterns are caught before they reach production.

3. Enable Compiler Warnings and Hardening Flags

Modern compilers can help detect and mitigate buffer overflows:

# Enable warnings
gcc -Wall -Wextra -Wformat-security

# Enable stack protection
gcc -fstack-protector-strong

# Enable FORTIFY_SOURCE (replaces sprintf with snprintf-like checks at runtime)
gcc -D_FORTIFY_SOURCE=2 -O2

_FORTIFY_SOURCE=2 is particularly powerful — it replaces calls to sprintf, strcpy, and similar functions with bounds-checked versions at compile time, providing a runtime safety net even if developers forget.

4. Consider Safer Alternatives to C String Functions

If you're writing new code, consider these safer patterns:

5. Validate Input Length Before Processing

Even with snprintf, it's good practice to validate that input strings are within expected bounds before using them:

#define MAX_UNIT_TEXT_LEN 32

if (strlen(unitText[map->scalebar.units]) > MAX_UNIT_TEXT_LEN) {
    /* Log error and return safely */
    msSetError(MS_MISCERR, "Unit text too long", "renderScalebar()");
    return MS_FAILURE;
}

snprintf(label, sizeof(label), "%g %s", scale_value, unitText[map->scalebar.units]);

This provides defense-in-depth: even if snprintf truncates silently, you've already caught the anomaly and can log or reject it.

6. Security Standards and References

• CWE-120: Buffer Copy without Checking Size of Input
• CWE-121: Stack-based Buffer Overflow
• OWASP: Buffer Overflow: Overview and prevention guidance
• SEI CERT C Coding Standard: STR07-C: Use bounds-checking interfaces for string manipulation
• SANS CWE Top 25: Buffer overflows consistently rank in the top 5 most dangerous software errors

Conclusion

Five lines of code. Five missing size arguments. That's all it took to introduce a critical, potentially exploitable vulnerability into a C codebase. This is not a criticism of the original developer — sprintf is a deeply familiar function, and the mistake is easy to make, especially in code that has evolved over years or decades.

The key takeaways from this vulnerability and its fix:

1. sprintf is unsafe — replace it with snprintf everywhere, without exception
2. Use sizeof(buffer) as the size argument to snprintf to keep the limit in sync with the buffer declaration
3. Search for patterns, not instances — when you find one unsafe call, find all of them
4. Automate detection — add static analysis tools to your CI/CD pipeline so these patterns are caught before code review
5. Defense in depth — combine snprintf, input validation, compiler hardening flags, and runtime mitigations for the strongest protection

Buffer overflows have been killing software security since the 1980s. They don't have to. With the right habits, the right tools, and a culture of security-conscious code review, they're entirely preventable.

Write safe code. Review for patterns, not just logic. And when in doubt, check the bounds.

This vulnerability was identified and fixed as part of an automated security scanning and remediation workflow. The fix was verified by both automated re-scanning and LLM-assisted code review.

Have questions about buffer overflow prevention or C security best practices? Drop them in the comments below.