Critical Buffer Overflow in IPv6 Parsing: How a Wrong Array Size Could Crash Your App

Introduction

Off-by-one errors and hardcoded buffer sizes are among the oldest and most dangerous bugs in systems programming. They don't look scary — a single number in an array declaration — but they can open the door to memory corruption, crashes, and in the worst case, full remote code execution.

This post breaks down a critical buffer overflow vulnerability (CWE-120) found in src/uv-common.c, a file responsible for parsing network addresses. The root cause? A buffer declared as 40 bytes to hold an IPv6 address string that can legally be up to 46 bytes long. That 6-byte gap is all an attacker needs.

Whether you're a seasoned C developer or newer to systems programming, this vulnerability is a textbook example of why magic numbers are dangerous and why platform-defined constants exist for a reason.

The Vulnerability Explained

What Is a Buffer Overflow?

A buffer overflow occurs when a program writes more data into a fixed-size memory region than it was allocated to hold. In C, there's no automatic bounds checking — if you declare char buf[40] and write 46 bytes into it, you'll silently overwrite adjacent memory. Depending on what lives in that adjacent memory (return addresses, function pointers, other variables), the consequences range from a crash to arbitrary code execution.

This class of vulnerability is catalogued as CWE-120: Buffer Copy without Checking Size of Input ("Classic Buffer Overflow") and has been responsible for some of the most famous exploits in computing history.

The Vulnerable Code

The vulnerability lives in the uv_ip6_addr function, which parses an IPv6 address string and populates a sockaddr_in6 structure:

// BEFORE (vulnerable)
int uv_ip6_addr(const char* ip, int port, struct sockaddr_in6* addr) {
  char address_part[40];   // ← Magic number: 40 bytes
  size_t address_part_size;
  const char* zone_index;
  // ...
  memcpy(address_part, ip, address_part_size);
  // ...
}

The problem is deceptively simple:

• IPv6 addresses can be up to 45 characters long (e.g., fe80::1%eth0 with zone IDs, or a full IPv4-mapped address like ::ffff:192.168.100.200).
• Adding the required null terminator brings the maximum to 46 bytes.
• The POSIX standard defines INET6_ADDRSTRLEN = 46 for exactly this reason.
• The buffer here is declared as 40 bytes — 6 bytes short.
• On Windows, the situation is even worse: optional formatting marks can push the required size to 65 bytes.

The memcpy call copies address_part_size bytes from the ip pointer directly into this undersized buffer. If address_part_size is derived from the input string's length without independent validation against the destination buffer's capacity, a long IP string will overflow the buffer.

How Could It Be Exploited?

The ip parameter originates from network input in applications that parse external addresses. This means an attacker who can supply a crafted network address string — through a server connection, a configuration file parsed from user input, or a network packet — can potentially:

1. Trigger a crash (Denial of Service): Overwriting stack memory corrupts the stack frame, causing an immediate segmentation fault or access violation.
2. Overwrite the return address (Remote Code Execution): On systems without stack canaries or ASLR, a carefully crafted overflow can redirect execution to attacker-controlled shellcode.
3. Corrupt adjacent variables: Even without controlling the exact overflow content, corrupting nearby stack variables can alter program logic in unpredictable ways.

Attack Scenario

Imagine a server application using libuv to accept incoming connections. The server calls uv_ip6_addr() to parse the connecting client's IP address:

Normal IPv6:   "2001:0db8:85a3:0000:0000:8a2e:0370:7334"  → 39 chars ✓
Max valid:     "fe80::1:2:3:4:5:6:7%eth0interface123456"    → 45 chars ✗ OVERFLOW
Malicious:     "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" → 46 chars ✗ OVERFLOW

An attacker on the network sends a connection with a crafted source address or triggers parsing of a malicious address string. The 40-byte buffer overflows, and depending on the platform and compiler settings, the attacker may gain control of the instruction pointer.

The Fix

What Changed

The fix is elegant in its simplicity — replace the hardcoded magic number 40 with the platform-defined constant INET6_ADDRSTRLEN:

// AFTER (fixed)
int uv_ip6_addr(const char* ip, int port, struct sockaddr_in6* addr) {
  /* INET6_ADDRSTRLEN is needed for a full IPv4-mapped IPv6 address
   * for the given platform plus a NUL byte. In posix this is defined to be 46.
   * On Windows this buffer needs 65 bytes for additional optional
   * formatting marks that may be present. */
  char address_part[INET6_ADDRSTRLEN];
  size_t address_part_size;
  const char* zone_index;
  // ...
  memcpy(address_part, ip, address_part_size);
  // ...
}

Why This Fix Works

The constant INET6_ADDRSTRLEN is defined in <netinet/in.h> (POSIX) and <ws2tcpip.h> (Windows). It is guaranteed by the platform to be large enough to hold any valid IPv6 address string including the null terminator. Using it instead of a hardcoded number means:

1. The buffer is always correctly sized for the current platform.
2. Future-proofing: If the standard ever changes, the constant updates automatically.
3. Self-documenting code: The constant name makes the intent crystal clear to any reader.

The Diff at a Glance

- char address_part[40];
+ /* INET6_ADDRSTRLEN is needed for a full IPv4-mapped IPv6 address
+  * for the given platform plus a NUL byte. In posix this is defined to be 46.
+  * On Windows this buffer needs 65 bytes for additional optional
+  * formatting marks that may be present. */
+ char address_part[INET6_ADDRSTRLEN];

One line changed. One constant substituted. A critical vulnerability closed.

Prevention & Best Practices

This vulnerability is a perfect teaching moment for several secure coding principles that every C/C++ developer should internalize.

1. Never Use Magic Numbers for Buffer Sizes

// ❌ Dangerous: magic numbers hide intent and invite errors
char ip_buf[16];
char ipv6_buf[40];

// ✅ Safe: use named constants
char ip_buf[INET_ADDRSTRLEN];    // 16 on POSIX
char ipv6_buf[INET6_ADDRSTRLEN]; // 46 on POSIX, 65 on Windows

Named constants communicate intent, adapt to platforms, and are harder to get wrong.

2. Validate Input Length Before Copying

Even with a correctly sized buffer, always validate that the source data fits before copying:

// ✅ Always bounds-check before memcpy
if (address_part_size > sizeof(address_part) - 1) {
    return UV_EINVAL; // Reject oversized input
}
memcpy(address_part, ip, address_part_size);
address_part[address_part_size] = '\0';

3. Prefer Safer String Functions

Where possible, use length-bounded alternatives:

// ❌ Unsafe
strcpy(dest, src);

// ✅ Safer alternatives
strncpy(dest, src, sizeof(dest) - 1);
dest[sizeof(dest) - 1] = '\0';

// ✅ Even better (where available)
strlcpy(dest, src, sizeof(dest));  // BSD/macOS
snprintf(dest, sizeof(dest), "%s", src);  // Portable

4. Enable Compiler Protections

Modern compilers offer several hardening features that can catch or mitigate buffer overflows:

# GCC/Clang: Enable stack canaries, FORTIFY_SOURCE, and ASLR
gcc -fstack-protector-strong \
    -D_FORTIFY_SOURCE=2 \
    -fPIE -pie \
    -Wformat -Wformat-security \
    -o myapp myapp.c

5. Use Static Analysis Tools

Several tools can catch this class of bug automatically:

6. Know Your Security Standards

This vulnerability maps to well-known security standards:

• CWE-120: Buffer Copy without Checking Size of Input
• CWE-121: Stack-based Buffer Overflow
• OWASP: A03:2021 – Injection (memory corruption variants)
• CERT C: STR31-C — Guarantee that storage for strings has sufficient space for character data and the null terminator
• SANS Top 25: #2 — Out-of-bounds Write

Conclusion

A 6-byte discrepancy between a hardcoded buffer size and the actual maximum length of an IPv6 address string created a critical, network-reachable buffer overflow. The fix — swapping 40 for INET6_ADDRSTRLEN — is a single line change, but it closes a vulnerability that could have enabled denial-of-service attacks or remote code execution in any application using this network parsing code.

The key takeaways:

🔴 Magic numbers in buffer declarations are a red flag. Always use platform-defined constants for sizes tied to external standards.

🟡 Network input is attacker-controlled input. Any code path that processes data from the network must be held to the highest scrutiny.

🟢 The fix was simple because the right tools existed. INET6_ADDRSTRLEN was always there — it just wasn't being used. Know your standard library.

🔵 Automated scanning catches what human review misses. This vulnerability was identified through automated security scanning, demonstrating the value of integrating security tooling into your CI/CD pipeline.

Buffer overflows have been killing software security for over 40 years. We have the constants, the compiler flags, the sanitizers, and the static analysis tools to eliminate them. Use them.

This vulnerability was automatically detected and fixed by OrbisAI Security. Automated security scanning helps teams catch critical issues before they reach production.