Stack Buffer Overflow in C: How Unbounded sprintf() Calls Create Critical Vulnerabilities

Vulnerability: CWE-120 Stack Buffer Overflow
Severity: Critical
File: doc/src/docedit.c
Fixed in: PR — fix: add buffer-length check in docedit.c

Introduction

Few vulnerability classes have a longer history — or a more devastating track record — than the humble stack buffer overflow. From the Morris Worm of 1988 to modern exploit chains targeting embedded systems, unbounded memory writes into fixed-size stack buffers remain a root cause of critical security failures.

This post examines a real-world stack buffer overflow discovered in doc/src/docedit.c, a documentation build utility, where two separate sprintf() calls wrote attacker-influenced data into fixed-size stack buffers without any length validation. The result? A classic, highly exploitable CWE-120: Buffer Copy Without Checking Size of Input vulnerability.

If you write C or C++ code — or work on systems that include any C components — this one's for you.

The Vulnerability Explained

What Is a Stack Buffer Overflow?

When a C program declares a local variable like this:

char filename[256];

It's reserving 256 bytes of space on the call stack — a region of memory that also stores critical bookkeeping data, including the saved return address (where the program should jump when the current function returns).

If you write more than 256 bytes into filename, you don't just corrupt the buffer — you start overwriting adjacent stack data, including that saved return address. An attacker who controls the overflowing input can replace the return address with a location of their choosing, effectively hijacking program execution.

The Vulnerable Code

Two locations in doc/src/docedit.c exhibited this pattern:

Location 1 — Line 29: Path construction

char filename[256];
// ...
sprintf(filename, "%s" PATH_SEP "%s", path, name);

Here, path and name are concatenated into a fixed 256-byte buffer using sprintf(). The sprintf() function performs no bounds checking — it will happily write as many bytes as the format string produces, regardless of the destination buffer's size.

If the combined length of path + separator + name exceeds 255 characters (plus the null terminator), the write overflows the buffer and begins corrupting the stack frame.

Location 2 — Line 104: Line formatting

char line[128];
// ...
sprintf(line, "__%s__\n\n", type);

Similarly, type is embedded into a fixed 128-byte line buffer. If type is longer than approximately 122 characters, the buffer overflows.

How Could This Be Exploited?

The exploitability depends on how path, name, and type are sourced. In a documentation build tool, these values might come from:

• Filenames or directory paths passed as command-line arguments
• Content parsed from documentation source files
• Environment variables

Consider this attack scenario:

A malicious documentation project includes a file with an extremely long name — say, 400 characters. When the build utility processes this file, it calls sprintf(filename, "%s" PATH_SEP "%s", path, name) with a combined length of 400+ characters. The 256-byte filename buffer overflows, corrupting the saved return address on the stack. On a system without modern mitigations (or with a bypass), the attacker's controlled value redirects execution to shellcode or a ROP chain.

Even in environments with stack canaries and ASLR, buffer overflows can lead to:

• Crashes and denial of service (reliable, even with mitigations)
• Information disclosure (leaking stack/heap addresses to defeat ASLR)
• Full code execution (with sufficient exploit sophistication)

The Fix

The fix for this class of vulnerability is straightforward: replace unbounded sprintf() with bounded alternatives that respect the destination buffer's size.

The Right Tools for the Job

Before and After

Before (vulnerable):

char filename[256];
sprintf(filename, "%s" PATH_SEP "%s", path, name);

After (safe):

char filename[256];
snprintf(filename, sizeof(filename), "%s" PATH_SEP "%s", path, name);

Before (vulnerable):

char line[128];
sprintf(line, "__%s__\n\n", type);

After (safe):

char line[128];
snprintf(line, sizeof(line), "__%s__\n\n", type);

Why snprintf() Solves the Problem

snprintf(buf, n, fmt, ...) guarantees that at most n-1 bytes are written to buf, always followed by a null terminator (as long as n > 0). The function also returns the number of bytes that would have been written if the buffer were large enough — allowing callers to detect truncation:

int written = snprintf(filename, sizeof(filename), "%s" PATH_SEP "%s", path, name);
if (written < 0 || (size_t)written >= sizeof(filename)) {
    // Handle truncation or encoding error
    fprintf(stderr, "Error: path too long\n");
    return -1;
}

This pattern — check the return value and handle truncation explicitly — is the gold standard for safe string formatting in C.

Going Further: Dynamic Allocation

For cases where the output length is genuinely unbounded, consider using asprintf() (available on Linux/macOS) or manually allocating a buffer of the required size:

// asprintf allocates exactly as much memory as needed
char *filename = NULL;
int written = asprintf(&filename, "%s" PATH_SEP "%s", path, name);
if (written < 0 || filename == NULL) {
    // Handle allocation failure
    return -1;
}
// ... use filename ...
free(filename);

This eliminates the truncation risk entirely, at the cost of heap allocation and the need to free() the result.

Prevention & Best Practices

1. Ban sprintf() in Your Codebase

Add a linting rule or compiler warning to flag any use of sprintf(). Most modern C projects can enforce this via:

• -Wformat and -Wformat-overflow (GCC/Clang) — warn about format string issues and potential overflows
• -D_FORTIFY_SOURCE=2 — enables runtime checks for certain unsafe functions
• clang-tidy with the bugprone-unsafe-functions check
• cppcheck static analysis

2. Use Compiler Hardening Flags

CFLAGS += -Wall -Wextra -Wformat -Wformat-overflow
CFLAGS += -fstack-protector-strong   # Stack canaries
CFLAGS += -D_FORTIFY_SOURCE=2        # Runtime buffer checks
LDFLAGS += -z relro -z now           # Hardened memory mappings

These flags won't prevent all overflows, but they significantly raise the cost of exploitation.

3. Consider Memory-Safe Languages for New Code

If you're writing a new utility that processes untrusted filenames or document content, consider whether C is the right tool. Languages like Rust (which the broader project already uses, per the Cargo.lock in the repository) provide memory safety guarantees at the language level, making this entire class of vulnerability impossible by default.

4. Fuzz Your Build Tools

Documentation utilities and build tools often process untrusted input (filenames, content from external repositories, etc.) but are rarely subjected to the same security scrutiny as user-facing code. Tools like AFL++ or libFuzzer can automatically discover buffer overflows by generating large and malformed inputs:

# Example: fuzzing a build utility with AFL++
afl-fuzz -i corpus/ -o findings/ -- ./docedit @@

5. Know Your CWEs

This vulnerability maps to:

• CWE-120: Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
• CWE-121: Stack-based Buffer Overflow
• OWASP A03:2021: Injection (which includes memory injection via overflow)

Familiarizing yourself with these classifications helps when reviewing code and triaging security findings.

6. Code Review Checklist for C String Operations

When reviewing C code, flag any line containing:

• [ ] sprintf() — use snprintf() instead
• [ ] strcpy() — use strncpy() or strlcpy() instead
• [ ] strcat() — use strncat() or strlcat() instead
• [ ] gets() — never use; removed from C11
• [ ] Fixed-size buffers receiving external input without length validation

A Note on the Broader Security Context

It's worth noting that the repository also contains a separate, unrelated vulnerability involving OAuth tokens and API keys stored in plaintext on the filesystem (in plugins/auth-oauth2/src/store.ts). While that issue is distinct from the buffer overflow addressed here, it highlights an important principle: security vulnerabilities rarely exist in isolation.

A thorough security review should cover:
- Memory safety issues (like this buffer overflow)
- Cryptographic weaknesses (like plaintext credential storage)
- Authentication and authorization flaws
- Input validation across all trust boundaries

Addressing one class of vulnerability is a win — but it's the beginning of the conversation, not the end.

Conclusion

The stack buffer overflow in doc/src/docedit.c is a textbook example of a vulnerability that's been well-understood for decades yet continues to appear in real codebases. The root cause is simple: sprintf() was used where snprintf() should have been, and no one checked whether the inputs could exceed the buffer's capacity.

The fix is equally simple — but the lesson is broader:

In C, you are always one unbounded write away from a critical vulnerability. Treat every buffer write as a potential overflow until proven otherwise.

Use snprintf(). Check return values. Enable compiler warnings. Fuzz your tools. And when possible, consider whether a memory-safe language better fits the task at hand.

Security isn't a feature you add at the end — it's a discipline you practice at every line.

This post is part of our series on real-world vulnerability fixes. Automated security scanning and remediation powered by OrbisAI Security.