On April 2024, a new security issue (CVE-2025-11961) was identified in the widely-used libpcap library. This bug targets the pcap_ether_aton() auxiliary function, which helps applications handle MAC addresses (typically used in networking code). Surprisingly, the danger hides behind a misunderstood input format and poor input validation. Applications passing unusual MAC strings to this function can inadvertently cause libpcap to read outside input buffers and write outside allocated memory, leading to memory corruption — a classic recipe for exploits.

This article explains CVE-2025-11961, how it works, what makes it dangerous, and gives you code snippets to help spot the issue. If you build tools on top of libpcap, pay attention — this simple bug may be lurking in your codebase.

What is pcap_ether_aton()?

pcap_ether_aton() is an internal helper in libpcap (source code), mostly used to convert a "string MAC address" (like de:ad:be:ef:00:01) into its binary form (6 bytes).

#include <pcap/pcap-int.h>

u_char *mac;
mac = pcap_ether_aton("de:ad:be:ef:00:01");

Allocates a fixed-size result buffer (6 bytes, for classic MAC).

If an app supplies a string that doesn't match the expected format (smaller, larger, or with invalid content), the parser can overstep input boundaries (reading) and write beyond the output buffer (writing), creating two classic vulnerabilities:

Out-Of-Bounds Read: Can leak memory contents (potential info disclosure).

- Out-Of-Bounds Write: Can cause memory corruption, leading to crashes or, in some scenarios, arbitrary code execution.

Let’s see a simplified, dangerous pattern (actual implementation may differ)

// Pseudo-code adapted from libpcap
u_char *
pcap_ether_aton(const char *p)
{
    u_char *ep = malloc(6); // Always 6 bytes for output
    for (int i = ; i < 6; i++) {
        // Parse two hex digits from the string
        char c1 = *p++; // <-- no input length check!
        char c2 = *p++;
        ep[i] = (hexval(c1) << 4) | hexval(c2);
        if (*p == ':' || *p == '-') p++;
    }
    return ep;
}

Suppose a network tool uses pcap_ether_aton() to parse user input

char input[100];
fgets(input, 100, stdin); // User provides MAC
u_char *mac = pcap_ether_aton(input);

Read 14 bytes, then keep reading into whatever follows, including stack or heap values.

- Write 7 or more bytes to a 6-byte buffer, overwriting adjacent memory if the original function doesn’t strictly check allocation size.

Proof of Concept Snippet

Here’s a reduced PoC that can show a crash (segfault) when pcap_ether_aton() is given malformed input:

#include <stdio.h>
#include <string.h>
#include <pcap/pcap-int.h>

int main() {
    // Intentionally malformed: Too many groups
    char *input = "01:23:45:67:89:ab:cd:ef";
    u_char *mac = pcap_ether_aton(input);
    printf("Result: %02x:%02x:%02x:%02x:%02x:%02x\n",
           mac[], mac[1], mac[2], mac[3], mac[4], mac[5]);
    free(mac);
    return ;
}

Who Is Affected?

- All software using libpcap’s pcap_ether_aton() and passing unsanitized user input: network analyzers, packet crafting tools, network sniffers, anything that lets users or scripts provide MAC addresses.
- Indirectly: Even some internal uses of libpcap itself may trigger this with corrupted capture files or unusual options.

How Was This Missed?

The bug is partly the result of incomplete documentation: the function’s expectations for input were never clearly documented, so callers assumed it was more robust than it is. As a result, it could be (and was) called with unexpected formats.

*References:*
- libpcap source code
- CVE Record for CVE-2025-11961 (MITRE)

Patch Status

- Upstream fixes are expected. Keep an eye on libpcap’s GitHub issues and update as soon as a patch lands.
- Input validation: Callers should strictly validate MAC addresses to the canonical format before passing to this function.

- Example (in C)

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <ctype.h>

int parse_mac(const char *str, uint8_t *mac) {
    return sscanf(str, "%hhx:%hhx:%hhx:%hhx:%hhx:%hhx",
                  &mac[], &mac[1], &mac[2], &mac[3], &mac[4], &mac[5]) == 6;
}

Conclusion

CVE-2025-11961 is a classic case where input assumptions meet incomplete documentation. A helper function — used everywhere networking code touches user-supplied MAC addresses — turns into a serious security risk, all because it trusts its input. If you build or package tools on libpcap, audit every place you use pcap_ether_aton(). Apply patches quickly, and always sanitize those MACs.

Further Reading

- libpcap Issue Tracker
- PacketLife: MAC Address Formats
- MITRE CVE-2025-11961

Timeline

Published on: 12/31/2025 00:56:16 UTC
Last modified on: 12/31/2025 20:42:15 UTC