How buffer overflow from unsafe string copy functions happens in C network interface code and how to fix it

The Discovery: Unsafe String Operations in Ethernet Interface Code

In the generic/eth-impl.c file, we discovered two high-severity buffer overflow vulnerabilities caused by unsafe string copy operations. At line 786, the code used strncpy() to copy network interface names, and at line 903, sprintf() formatted device filenames without bounds checking. Both instances handle user-influenced data—network interface names from the system and device file paths—making them prime targets for exploitation.

The vulnerable code in tme_eth_ifaddrs_find() at line 786 looked like this:

switch(family) {
case AF_UNSPEC:
  strncpy(ifa_name, ifa->ifa_name, IFNAMSIZ);
  break;

And in tme_eth_alloc() at line 903:

sprintf(dev_filename, "%s%d", dev_fn, minor++);

These patterns represent a classic category of C programming errors: trusting that input data will fit within fixed-size buffers without verification. In network interface handling code, where interface names come from the operating system and could theoretically be manipulated through kernel modules or virtualization layers, this creates a real security risk.

The Vulnerability Explained

The strncpy() Problem at Line 786

The first vulnerability occurs when copying network interface names:

strncpy(ifa_name, ifa->ifa_name, IFNAMSIZ);

While strncpy() appears safer than strcpy() because it limits the copy to IFNAMSIZ bytes, it has a critical flaw: if the source string ifa->ifa_name is exactly IFNAMSIZ bytes or longer, strncpy() will NOT null-terminate the destination buffer. This means ifa_name could become a non-null-terminated character array.

Why is this dangerous? Any subsequent string operation on ifa_name—like strlen(), strcmp(), or printf()—will read past the buffer boundary looking for a null terminator that doesn't exist. This causes:

1. Information disclosure: Reading adjacent memory could leak sensitive data
2. Program crashes: Accessing unmapped memory triggers segmentation faults
3. Undefined behavior: The program state becomes unpredictable

Attack scenario: An attacker with the ability to create a virtual network interface (via kernel modules, container escapes, or virtualization APIs) could craft an interface name that's exactly IFNAMSIZ characters long. When tme_eth_ifaddrs_find() processes this interface, the strncpy() call fills ifa_name completely without null-termination. If the code then calls strlen(ifa_name) or passes it to another function expecting a C string, it reads into adjacent stack memory, potentially exposing cryptographic keys, authentication tokens, or other sensitive data stored nearby.

The sprintf() Problem at Line 903

The second vulnerability uses unbounded sprintf() to format device filenames:

sprintf(dev_filename, "%s%d", dev_fn, minor++);

This is even more dangerous than the strncpy() issue because sprintf() has no size limit whatsoever. It will write as many bytes as needed to format the string, regardless of the destination buffer size.

Attack scenario: If dev_fn contains a long path string (perhaps from an environment variable or configuration file), and minor increments to large values, the formatted string could easily exceed dev_filename's allocated size. For example, if dev_filename is a 256-byte buffer but dev_fn is "/very/long/path/to/network/devices/ethernet/interface" (52 chars) and minor reaches 999999 (6 digits), the total would be 58+ bytes. But more critically, if an attacker controls dev_fn through configuration injection, they could provide a 300-byte path that directly overflows dev_filename, overwriting the return address on the stack.

This enables arbitrary code execution: By carefully crafting the overflow data, an attacker could overwrite the function's return address to point to injected shellcode or existing code gadgets (ROP chains), gaining complete control of the process running with the application's privileges.

The Fix: Bounded String Operations with snprintf()

The security fix replaces both unsafe operations with snprintf(), which enforces buffer size limits and guarantees null-termination.

Fix #1: Replacing strncpy() at Line 786

Before:

strncpy(ifa_name, ifa->ifa_name, IFNAMSIZ);

After:

snprintf(ifa_name, sizeof(ifa_name), "%s", ifa->ifa_name);

This change provides three critical improvements:

1. Explicit buffer size: sizeof(ifa_name) tells snprintf() exactly how many bytes are available
2. Guaranteed null-termination: snprintf() always writes a null terminator within the specified size
3. Truncation safety: If ifa->ifa_name is too long, snprintf() truncates it safely and still null-terminates

The "%s" format string simply copies the string, making this functionally equivalent to a safe string copy. The key difference is that snprintf() will write at most sizeof(ifa_name) - 1 characters, then add a null terminator, ensuring ifa_name is always a valid C string.

Fix #2: Replacing sprintf() at Line 903

Before:

sprintf(dev_filename, "%s%d", dev_fn, minor++);

After:

snprintf(dev_filename, strlen(dev_fn) + sizeof(dev_minor), "%s%d", dev_fn, minor++);

This fix calculates the required buffer size as strlen(dev_fn) + sizeof(dev_minor), which accounts for:
- The length of the device filename prefix (dev_fn)
- The size needed for the minor device number (dev_minor)
- An implicit +1 for the null terminator (handled by snprintf())

By passing this calculated size to snprintf(), the code ensures that even if dev_fn is unexpectedly long, the write operation will truncate safely rather than overflow dev_filename. The function will write at most the specified number of bytes (minus one for the null terminator), preventing any memory corruption.

Why These Specific Changes Matter

Both fixes follow the same security principle: never trust that input data will fit in your buffer—enforce it programmatically.

The transition from strncpy() to snprintf() might seem like a minor syntax change, but it represents a fundamental shift in how the code handles string boundaries:

• strncpy(): "Copy up to N bytes, but don't guarantee null-termination"
• snprintf(): "Format into exactly N bytes, always null-terminate, truncate if needed"

For the sprintf() replacement, the improvement is even more dramatic—going from no bounds checking to strict size enforcement.

In both cases, if the input data exceeds the buffer size, the code now truncates safely rather than corrupting memory. This might cause functional issues (truncated interface names or device paths), but functional bugs are infinitely preferable to security vulnerabilities that enable arbitrary code execution.

Prevention & Best Practices

1. Ban Unsafe String Functions

Establish a coding standard that prohibits:
- strcpy() and strcat() (no bounds checking)
- sprintf() and vsprintf() (no bounds checking)
- gets() (fundamentally unsafe, removed in C11)

Use these safe alternatives:
- snprintf() and vsnprintf() (bounded, null-terminated)
- strlcpy() and strlcat() (BSD-style, size-aware)
- strcpy_s() and strcat_s() (C11 Annex K, optional)

2. Always Use sizeof() for Buffer Sizes

When calling bounded string functions, pass sizeof(buffer) rather than hardcoded constants:

// Good: size automatically tracks buffer changes
char name[64];
snprintf(name, sizeof(name), "%s", input);

// Bad: if buffer size changes, this breaks
char name[64];
snprintf(name, 64, "%s", input);

3. Validate Return Values

snprintf() returns the number of characters that would have been written (excluding null terminator). Check if truncation occurred:

int written = snprintf(buffer, sizeof(buffer), "%s", input);
if (written >= sizeof(buffer)) {
    // Truncation occurred - handle the error
    log_error("Input too long, truncated to fit buffer");
}

4. Use Static Analysis Tools

Configure static analysis tools like Semgrep, Clang Static Analyzer, or Coverity to flag unsafe string operations:

# Semgrep rule to detect unsafe string functions
rules:
  - id: ban-unsafe-string-functions
    pattern-either:
      - pattern: strcpy(...)
      - pattern: strcat(...)
      - pattern: sprintf(...)
      - pattern: gets(...)
    message: "Unsafe string function detected - use bounded alternative"
    severity: ERROR

5. Enable Compiler Security Features

Use compiler flags that add runtime bounds checking:
- -D_FORTIFY_SOURCE=2 (GCC/Clang): Adds runtime checks for buffer overflows
- -fstack-protector-strong: Protects against stack-based buffer overflows
- -Wformat-security: Warns about potentially unsafe format strings

6. Consider Memory-Safe Languages for New Code

For new projects or components, consider memory-safe languages like Rust, Go, or modern C++ with smart pointers and bounds-checked containers. These languages prevent buffer overflows through type system guarantees rather than programmer discipline.

Key Takeaways

• Never use strncpy() for network interface names: It doesn't guarantee null-termination, creating vulnerabilities in tme_eth_ifaddrs_find() and similar interface enumeration code
• sprintf() is never acceptable in production code: The unbounded write in tme_eth_alloc() line 903 could enable arbitrary code execution through device filename formatting
• snprintf() with sizeof() is the safe pattern: Using snprintf(ifa_name, sizeof(ifa_name), "%s", ...) prevents both overflow and null-termination issues
• Calculate buffer requirements explicitly: The fix at line 903 uses strlen(dev_fn) + sizeof(dev_minor) to compute the exact size needed for safe formatting
• Static analysis catches these patterns reliably: The Semgrep rule c.lang.security.insecure-use-string-copy-fn successfully identified both vulnerable call sites for remediation

How Orbis AppSec Detected This

• Source: Network interface names from system calls (ifa->ifa_name in getifaddrs() results) and device filename prefixes (dev_fn parameter)
• Sink: Unsafe string copy operations at strncpy() in generic/eth-impl.c:786 and sprintf() in generic/eth-impl.c:903
• Missing control: No buffer size validation or bounds checking before copying potentially oversized strings into fixed-size destination buffers
• CWE: CWE-120 (Buffer Copy without Checking Size of Input)
• Fix: Replaced strncpy() and sprintf() with snprintf() calls that enforce explicit buffer size limits using sizeof() and calculated lengths

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

The buffer overflow vulnerabilities in generic/eth-impl.c demonstrate why unsafe string functions remain one of the most dangerous categories of C programming errors. Despite decades of awareness, strcpy(), strncpy(), and sprintf() continue to appear in production code, creating opportunities for memory corruption and arbitrary code execution.

The fix—replacing these functions with bounded snprintf() calls—is straightforward, but the real lesson is about development culture. Security-critical code handling network interfaces and device files must undergo rigorous review for unsafe string operations. Static analysis tools should be configured to automatically reject code using these dangerous functions, and development teams should establish clear guidelines requiring bounded alternatives.

By treating buffer safety as a non-negotiable requirement rather than a best practice, we can eliminate an entire class of vulnerabilities that have plagued C programs since the language's inception. The changes in this PR show that retrofitting safety into existing code is achievable—and essential for maintaining secure systems.

References

• CWE-120: Buffer Copy without Checking Size of Input
• CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer
• OWASP: Buffer Overflow
• C11 Standard - Annex K: Bounds-checking interfaces
• snprintf() Manual Page
• Semgrep Rule: Insecure String Copy Functions
• fix: use bounded strlcpy/snprintf in eth-impl.c...