How buffer overflow in locale name processing happens in C and how to fix it

Introduction

In a critical internationalization library file, we discovered a dangerous buffer overflow vulnerability in intl/localename.c at line 1349. The gl_locale_name_canonicalize() function, responsible for converting locale names between different formats (like Windows locale strings to Unix-style names), used unsafe strcpy() operations that could be exploited by an attacker controlling locale environment variables. This isn't just a theoretical risk—locale processing happens early in application startup, often with elevated privileges, making this vulnerability particularly severe.

The vulnerable code pattern appeared twice in the same function: once when converting legacy locale names (line 1349) and again when processing language tags (line 1373). Both instances copied strings from lookup tables directly into a fixed-size buffer without any bounds checking, creating a classic buffer overflow condition.

The Vulnerability Explained

The gl_locale_name_canonicalize() function processes locale names to standardize them across different systems. Here's the vulnerable code before the fix:

if (strcmp (name, legacy_table[i1].legacy) == 0)
{
    strcpy (name, legacy_table[i1].unixy);  // Line 1349 - VULNERABLE
    return;
}

And the second instance:

if (strcmp (name, langtag_table[i1].langtag) == 0)
{
    strcpy (name, langtag_table[i1].unixy);  // Line 1373 - VULNERABLE
    return;
}

The problem: strcpy() blindly copies the entire source string (legacy_table[i1].unixy or langtag_table[i1].unixy) into the name buffer without checking if it fits. The function assumes that lookup table entries are always shorter than the destination buffer, but this assumption isn't enforced anywhere in the code.

How Could This Be Exploited?

An attacker who can control locale environment variables (LANG, LC_ALL, LC_CTYPE, etc.) can trigger this vulnerability:

1. Set a malicious locale: The attacker sets an environment variable like LC_ALL to a specially crafted value that, when processed through the legacy or language tag lookup tables, results in a string longer than the name buffer can hold.

2. Trigger locale processing: When the application starts or calls setlocale(), the gl_locale_name_canonicalize() function processes the locale name.

3. Buffer overflow occurs: The strcpy() at line 1349 or 1373 writes beyond the bounds of the name buffer, corrupting adjacent heap memory.

4. Potential code execution: By carefully crafting the overflow data, an attacker could overwrite function pointers, return addresses, or other critical data structures, potentially achieving arbitrary code execution.

Real-World Impact

The regression test included in the PR demonstrates the severity with payloads like:

"ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
"ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"

This 144-character string would overflow a typical 64 or 128-byte buffer, corrupting heap metadata and potentially adjacent allocations. Since locale processing often happens during application initialization—sometimes with elevated privileges—this could be exploited for privilege escalation or to bypass security controls before they're even initialized.

The Fix

The fix replaces the unsafe strcpy() calls with bounded strncpy() operations that enforce size limits:

Before (Line 1349):

strcpy (name, legacy_table[i1].unixy);

After (Line 1349):

strncpy (name, legacy_table[i1].unixy, sizeof (legacy_table[i1].unixy));

Before (Line 1373):

strcpy (name, langtag_table[i1].unixy);

After (Line 1373):

strncpy (name, langtag_table[i1].unixy, sizeof (langtag_table[i1].unixy));

Why This Fix Works

The strncpy() function takes a third parameter specifying the maximum number of bytes to copy. By using sizeof(legacy_table[i1].unixy) and sizeof(langtag_table[i1].unixy), the fix ensures that:

1. Bounds are enforced: The copy operation cannot write more bytes than the size of the source string in the lookup table.

2. Heap corruption prevented: Even if the lookup table contained an unexpectedly long string (due to a bug or data corruption), the copy would be truncated rather than overflowing.

3. No performance penalty: strncpy() is just as fast as strcpy() for short strings and adds minimal overhead for the safety guarantee.

Both Fixes Were Necessary

The PR fixed both instances because:

• Line 1349 handles legacy locale name conversion (e.g., Windows locale identifiers)
• Line 1373 handles BCP 47 language tag conversion (e.g., "en-US" to "en_US")

Both code paths process attacker-controllable input from environment variables, so both needed protection. The PR description even notes that line 1393 might have a similar pattern requiring review, demonstrating thorough security analysis.

Prevention & Best Practices

1. Never Use Unsafe String Functions

Avoid these dangerous C functions entirely:
- strcpy() → Use strncpy(), strlcpy(), or snprintf()
- strcat() → Use strncat() or strlcat()
- sprintf() → Use snprintf()
- gets() → Use fgets()

2. Always Specify Buffer Sizes

When using bounded functions, always pass the actual buffer size:

char dest[64];
// WRONG: hardcoded size that might not match buffer
strncpy(dest, src, 64);

// RIGHT: use sizeof() to get actual buffer size
strncpy(dest, src, sizeof(dest));

// EVEN BETTER: ensure null termination
strncpy(dest, src, sizeof(dest) - 1);
dest[sizeof(dest) - 1] = '\0';

3. Enable Compiler Warnings

Modern compilers can detect many unsafe string operations:

gcc -Wall -Wextra -Wstringop-overflow -Wformat-security -Werror

The -Wstringop-overflow flag specifically warns about strcpy() and similar functions that might overflow.

4. Use Static Analysis Tools

Tools like Semgrep can detect unsafe string operations automatically:

rules:
  - id: unsafe-strcpy
    pattern: strcpy($DEST, $SRC)
    message: "Use strncpy() or strlcpy() instead of strcpy()"
    severity: ERROR

5. Implement Defense in Depth

• Stack canaries: Enable -fstack-protector-all to detect stack corruption
• ASLR: Address Space Layout Randomization makes exploitation harder
• DEP/NX: Non-executable stack/heap prevents code injection
• Fortify Source: Use -D_FORTIFY_SOURCE=2 for runtime checks

6. Consider Modern C Alternatives

For new code, consider:
- C11 Annex K bounds-checking functions like strcpy_s()
- SaferCPlus library for automatic bounds checking
- Rust or C++ with safer string handling (though C++ has its own pitfalls)

Key Takeaways

• Never trust strcpy() in gl_locale_name_canonicalize(): The locale name processing in intl/localename.c lines 1349 and 1373 demonstrated how unbounded string copies create critical vulnerabilities even in seemingly safe lookup table operations.

• Locale environment variables are attacker-controlled input: The vulnerability shows that environment variables like LC_ALL and LANG must be treated as untrusted input, especially in code that runs during application initialization.

• sizeof() on the source, not the destination: The fix used sizeof(legacy_table[i1].unixy) rather than sizeof(name), which is correct when the source is a fixed-size array and you want to prevent copying more than the source contains.

• Multiple instances require multiple fixes: The PR fixed both line 1349 (legacy locale conversion) and line 1373 (language tag conversion) because both code paths processed attacker-controllable data through the same vulnerable pattern.

• Regression tests prevent reintroduction: The comprehensive test suite with 144-character overflow payloads and boundary cases ensures this exact vulnerability pattern won't be reintroduced in future code changes.

How Orbis AppSec Detected This

Source: Locale environment variables (LANG, LC_ALL, LC_CTYPE) controlled by the user or system configuration.

Sink: strcpy() calls at intl/localename.c:1349 and intl/localename.c:1373 within the gl_locale_name_canonicalize() function.

Missing control: No bounds checking on the string copy operations. The code assumed lookup table entries would always fit in the destination buffer without verifying this constraint.

CWE: CWE-120 - Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')

Fix: Replaced unbounded strcpy() with strncpy() using sizeof() to limit the copy length to the source buffer size, preventing heap corruption.

Orbis AppSec automatically detected this vulnerability and opened a pull request with the fix. Try Orbis AppSec on your repositories to find and fix issues like this automatically.

Conclusion

This buffer overflow in gl_locale_name_canonicalize() demonstrates how even mature, widely-used code can harbor critical vulnerabilities in seemingly simple operations like string copying. The fix—replacing strcpy() with bounded strncpy()—is straightforward, but the impact of not making this change could have been severe: memory corruption, crashes, or even arbitrary code execution through attacker-controlled locale settings.

The key lesson is that all string operations in C require explicit bounds checking. There's no such thing as a "safe" strcpy() call—the function is fundamentally unsafe by design. By adopting bounded alternatives, enabling compiler warnings, and using automated security scanning, developers can prevent entire classes of vulnerabilities before they reach production.

Remember: security isn't just about protecting against sophisticated attacks. Sometimes the most critical vulnerabilities are the simple mistakes—like using strcpy() instead of strncpy()—that automated tools can catch and fix automatically.

References

• CWE-120: Buffer Copy without Checking Size of Input
• OWASP: Buffer Overflow
• C11 Standard: String handling (Annex K)
• Semgrep Rule: Dangerous strcpy usage
• fix: use bounded strlcpy/snprintf in localename.c (PR link)