Buffer Overflow in tmpnam.c: Why strcpy Still Haunts Us in 2024

Severity: 🔴 HIGH | CWE: CWE-120 | File: patches/musl/zig/lib/libc/musl/src/stdio/tmpnam.c

Introduction

There's a reason security educators have been warning developers about strcpy() for over three decades — and yet, it keeps showing up in production code, quietly waiting to cause serious damage. This week, we're examining a high-severity buffer overflow vulnerability discovered in a patched copy of the musl libc tmpnam() implementation embedded within a custom Zig toolchain tree.

The vulnerability is deceptively simple: a call to strcpy() with no bounds checking. But the consequences — memory corruption, stack smashing, potential arbitrary code execution — are anything but simple.

Whether you're a systems programmer, a toolchain maintainer, or a developer who occasionally dips into C, this vulnerability is a perfect case study in why memory-safe coding practices are non-negotiable, especially when you're maintaining patched forks of upstream libraries.

The Vulnerability Explained

What Is tmpnam() and Why Does It Exist?

The tmpnam() function is a C standard library function that generates a string representing a valid temporary file name — one that doesn't conflict with any existing file at the time of the call. It's been part of the C standard since C89, and its signature looks like this:

char *tmpnam(char *buf);

If buf is non-NULL, the generated name is stored there. If buf is NULL, the name is stored in a static internal buffer. Simple enough — but the devil is in the implementation details.

The Vulnerable Code

The vulnerable implementation in patches/musl/zig/lib/libc/musl/src/stdio/tmpnam.c (around line 28) looked something like this:

// VULNERABLE: Before the fix
char *tmpnam(char *buf)
{
    static char internal[L_tmpnam];
    char s[L_tmpnam];

    // ... name generation logic that builds 's' ...

    // ⚠️ DANGER: No bounds checking!
    strcpy(buf ? buf : internal, s);

    return buf ? buf : internal;
}

Do you see the problem? Let's break it down:

1. strcpy() copies bytes from source to destination until it hits a null terminator (\0).
2. It does not check whether the destination buffer is large enough to hold the source string.
3. If the source string s is longer than the destination buffer, strcpy() will happily write past the end of the buffer — corrupting adjacent memory.

Technical Deep Dive: Why This Is Dangerous

The C standard defines L_tmpnam as the minimum buffer size needed to hold a temporary name. In most implementations, this is a small constant (often 20 bytes). The code assumes that s will always fit within L_tmpnam bytes — but that assumption breaks in several real scenarios:

Scenario 1: Developer Modifies the Name-Generation Logic

Since this is a patched fork of musl libc living inside a custom Zig toolchain tree, it's far more likely to be modified than upstream-managed code. A developer might extend the naming scheme (e.g., adding a longer prefix, a hostname, or a UUID) without realizing they've just made s larger than L_tmpnam. Now every call to tmpnam() silently overflows.

Scenario 2: Caller Passes an Undersized Buffer

The C standard says the caller should provide a buffer of at least L_tmpnam bytes — but there's nothing in the code to enforce this. A caller who passes a smaller buffer (perhaps due to a copy-paste error or a misunderstanding of the API) will trigger an overflow.

// Caller code that triggers the overflow
char small_buf[8]; // Way too small!
tmpnam(small_buf); // 💥 Buffer overflow — strcpy writes past small_buf

Scenario 3: Stack Smashing and Heap Corruption

Depending on where buf lives (stack or heap), the overflow corrupts different things:

• Stack overflow: Overwrites return addresses, saved frame pointers, or local variables — the classic stack smashing attack vector.
• Heap overflow: Corrupts heap metadata or adjacent heap allocations, potentially enabling heap exploitation techniques.

CWE-120: Buffer Copy Without Checking Size of Input

This vulnerability is classified under CWE-120 — "Buffer Copy Without Checking Size of Input ('Classic Buffer Overflow')". It's one of the most well-known and long-standing vulnerability classes in software security, consistently appearing in the CWE Top 25 Most Dangerous Software Weaknesses.

Real-World Impact

In the context of a toolchain library, the impact is significant:

• Arbitrary code execution: A carefully crafted overflow can overwrite a return address, redirecting execution to attacker-controlled code.
• Privilege escalation: If the vulnerable code runs in a privileged context (e.g., a build system running as root), an exploit could escalate privileges.
• Denial of service: Even without full exploitation, a buffer overflow typically causes a crash — disrupting build pipelines and development workflows.
• Supply chain risk: Vulnerabilities in toolchain libraries can propagate to every project built with that toolchain, amplifying the blast radius.

The Fix

What Changed?

The fix replaces the unbounded strcpy() call with a size-bounded alternative that respects the destination buffer's capacity. The corrected implementation uses snprintf() or strncpy() with an explicit size limit:

// FIXED: After the patch
char *tmpnam(char *buf)
{
    static char internal[L_tmpnam];
    char s[L_tmpnam];
    char *dest = buf ? buf : internal;

    // ... name generation logic that builds 's' ...

    // ✅ SAFE: Size-bounded copy with explicit limit
    snprintf(dest, L_tmpnam, "%s", s);

    return dest;
}

Or alternatively with strncpy() (with proper null-termination):

// Also valid fix using strncpy
strncpy(dest, s, L_tmpnam - 1);
dest[L_tmpnam - 1] = '\0'; // Always explicitly null-terminate!

Why snprintf() Is the Preferred Choice

While both strncpy() and snprintf() are safer than strcpy(), snprintf() is generally preferred for string copying because:

⚠️ strncpy() does not null-terminate the destination if the source is longer than n. You must manually add dest[n-1] = '\0'.

How the Fix Eliminates the Vulnerability

By passing L_tmpnam as the maximum number of bytes to write, snprintf() ensures that:

1. No more than L_tmpnam - 1 characters are written to the destination buffer (leaving room for the null terminator).
2. The destination is always null-terminated, regardless of the source string's length.
3. If s is somehow longer than L_tmpnam, the string is safely truncated rather than causing memory corruption.

The fix is minimal, targeted, and doesn't change the function's external behavior for well-formed inputs — it simply adds a safety net for malformed or unexpected ones.

Prevention & Best Practices

1. Ban strcpy(), strcat(), and gets() From Your Codebase

These functions are inherently unsafe. Many organizations enforce this through compiler warnings or static analysis rules:

// ❌ Never use these:
strcpy(dest, src);
strcat(dest, src);
gets(buf);
sprintf(buf, fmt, ...);

// ✅ Use these instead:
strncpy(dest, src, sizeof(dest) - 1); dest[sizeof(dest)-1] = '\0';
strncat(dest, src, sizeof(dest) - strlen(dest) - 1);
fgets(buf, sizeof(buf), stdin);
snprintf(buf, sizeof(buf), fmt, ...);

2. Use sizeof() or Explicit Constants — Never Magic Numbers

When using size-bounded functions, always derive the size from the actual buffer:

// ❌ Bad: Magic number that might not match the buffer
strncpy(dest, src, 64);

// ✅ Good: Size derived from the actual buffer
strncpy(dest, src, sizeof(dest) - 1);
dest[sizeof(dest) - 1] = '\0';

3. Enable Compiler Hardening Flags

Modern compilers offer flags that detect or prevent buffer overflows at compile time and runtime:

# GCC / Clang
-Wall -Wextra           # Enable warnings (catches some unsafe function usage)
-Wformat-security       # Warn on format string vulnerabilities
-fstack-protector-all   # Add stack canaries to detect overflow at runtime
-D_FORTIFY_SOURCE=2     # Enable glibc buffer overflow detection
-fsanitize=address      # AddressSanitizer: detect overflows at runtime (dev/test)

4. Use Static Analysis Tools

Integrate static analysis into your CI/CD pipeline to catch these issues before they reach production:

• Coverity — Industry-standard static analyzer, excellent at finding buffer overflows
• CodeQL — GitHub's semantic code analysis engine
• Clang Static Analyzer — Built into the LLVM toolchain
• Flawfinder — Lightweight scanner specifically for C/C++ dangerous functions
• Semgrep — Fast, customizable static analysis with rules for unsafe C functions

5. Be Extra Vigilant With Patched Forks

This vulnerability highlights a specific risk pattern: patched forks of upstream libraries. When you maintain a fork, you take on the responsibility of:

• Tracking upstream security fixes and backporting them
• Ensuring local modifications don't introduce new vulnerabilities
• Running additional scrutiny on any changes to security-sensitive code (memory management, cryptography, I/O)

Consider adding a comment in forked files to flag them for extra review:

/* PATCHED FORK: This file is a modified copy of musl libc's tmpnam.c.
 * Any modifications must be reviewed for security implications.
 * Upstream reference: https://git.musl-libc.org/...
 * Last synced with upstream: 2024-01-15
 */

6. Consider Memory-Safe Languages for New Code

Where feasible, new systems code should be written in memory-safe languages like Rust, Zig (with its safety features), or Go. Ironically, this vulnerability lives in a Zig toolchain — a reminder that even modern toolchains carry legacy C code that needs careful attention.

Relevant Security Standards

• CWE-120: Buffer Copy Without Checking Size of Input
• CWE-121: Stack-based Buffer Overflow
• CWE-122: Heap-based Buffer Overflow
• OWASP: Buffer Overflow: OWASP's guidance on buffer overflow prevention
• SEI CERT C Coding Standard: STR31-C: Guarantee sufficient storage for strings

Conclusion

This vulnerability is a perfect reminder that there is no such thing as "safe enough" legacy code — especially in patched forks that live outside the scrutiny of upstream maintainers. A single strcpy() call, hiding in a rarely-examined corner of a toolchain library, carries the potential for memory corruption, code execution, and supply chain compromise.

The fix is simple — swap strcpy() for snprintf() with an explicit size limit. But the lesson is broader:

Every unbounded memory operation is a vulnerability waiting for the right conditions to trigger it.

Key takeaways from this vulnerability:

• ✅ Never use strcpy(), strcat(), or gets() in new or maintained C code
• ✅ Always use size-bounded alternatives: snprintf(), strncpy() (with explicit null termination), strncat()
• ✅ Patched forks need extra security scrutiny — they don't benefit from upstream security processes
• ✅ Enable compiler hardening flags and integrate static analysis into your pipeline
• ✅ Treat toolchain security as application security — vulnerabilities in build tools affect everything built with them

The fact that this was caught and fixed through automated security scanning is a win. The goal is to build systems where these vulnerabilities are found by tools before they're found by attackers.

Stay safe, and keep your buffers bounded. 🔐

This vulnerability was identified and patched by OrbisAI Security. Automated security scanning and AI-assisted code review helped surface this issue before it could be exploited.

References:
- CWE-120: Buffer Copy Without Checking Size of Input
- OWASP Buffer Overflow Attack
- SEI CERT C Coding Standard
- musl libc source
- GCC Security Hardening Options