libpcap is one of the most widely used packet capturing libraries for networking and security tools. In early 2025, a critical vulnerability was found in the Windows version of libpcap—CVE-2025-11964. This issue is serious because it can lead to buffer overflows when converting error messages containing Unicode characters into UTF-8.
This article breaks down how the bug works, possible ways to exploit it, a code snippet to show the problem, and references to the original advisory.
What is libpcap?
libpcap is an open-source C/C++ library for packet capturing. Popular tools like Wireshark, tcpdump, Nmap, and many others use it. While it’s UNIX-focused, there’s a Windows port widely used with the WinPcap/Npcap libraries.
Description of the Vulnerability
On Windows, when libpcap needs to show an error message, sometimes that message contains Unicode (UTF-16LE) characters. To display it, the library converts those messages to UTF-8.
The function utf_16le_to_utf_8_truncated() is supposed to do this conversion safely. But, if the input UTF-16 message contains characters that need four bytes in UTF-8 (for example, emojis or certain language scripts), the code miscalculates how much space is left in the output buffer. It can write more bytes than the buffer allows—causing a buffer overflow.
That means an attacker can send or cause the system to display specially-crafted error messages that could lead to memory corruption, crashes, or even code execution.
Affected versions:
All Windows builds of libpcap before [check the libpcap version with patch]
- Only the Windows code path is affected; UNIX/Linux platforms are not.
How the Bug Happens: Simplified
Let’s illustrate the problem with a simplified code snippet inspired by the actual vulnerable code (for demo purposes):
// Vulnerable code snippet (simplified):
int utf_16le_to_utf_8_truncated(
char *buf, int buf_len, const uint16_t *src, int src_len
) {
int out_pos = ;
for (int i = ; i < src_len; i++) {
uint16_t ch = src[i];
if (ch < x80) {
if (out_pos + 1 > buf_len) break;
buf[out_pos++] = (char)ch;
} else if (ch < x800) {
if (out_pos + 2 > buf_len) break;
buf[out_pos++] = xC | (ch >> 6);
buf[out_pos++] = x80 | (ch & x3F);
} else if (ch >= xD800 && ch <= xDBFF) { // High Surrogate
// Surrogate pair: double-check boundary!
if (i + 1 < src_len) {
uint16_t ch2 = src[i+1];
if (ch2 >= xDC00 && ch2 <= xDFFF) { // Low Surrogate
uint32_t codepoint =
(((ch - xD800) << 10) | (ch2 - xDC00)) + x10000;
// Needs 4 bytes in UTF-8!
if (out_pos + 4 > buf_len) break;
buf[out_pos++] = xF | ((codepoint >> 18) & x07);
buf[out_pos++] = x80 | ((codepoint >> 12) & x3F);
buf[out_pos++] = x80 | ((codepoint >> 6) & x3F);
buf[out_pos++] = x80 | (codepoint & x3F);
i++; // Skip the next one too
}
}
} else {
if (out_pos + 3 > buf_len) break;
buf[out_pos++] = xE | ((ch >> 12) & xF);
buf[out_pos++] = x80 | ((ch >> 6) & x3F);
buf[out_pos++] = x80 | (ch & x3F);
}
}
return out_pos;
}
The bug: The calculation for multi-byte characters doesn't always check if there’s enough space *before* writing, allowing accidental writing past the intended buffer.
How is this Exploitable?
On Windows, system error messages are sometimes built using external inputs or can be triggered via device or network driver failures. If a malicious actor can cause a service to process or display a Unicode error message with a 4-byte UTF-8 character (like x1F600, the 😀 emoji, as a surrogate pair), libpcap will overflow its buffer.
If the application is running with high privileges (as Wireshark sometimes does), this overflow could be used to:
Corrupt critical memory, possibly leading to code execution
Attack example:
A specially crafted device driver error or network response induces a Windows error string with a surrogate pair, triggering the vulnerable code when processed by libpcap under an administrator context.
Example Exploit Scenario
Suppose you have a network service running on Windows that calls libpcap and reports errors. An attacker sends malformed packets or manipulates a device to fail, causing a Windows error message with a 4-byte Unicode char.
When the error routine tries to convert the message, the out-of-bounds write occurs, possibly allowing the attacker to overwrite adjacent memory (function pointers, return addresses, etc.).
Since the conversion runs in-process, the feasibility of turning this into code execution depends on heap layout and overwritten data. At the very least, reliable denial-of-service is possible.
## How to Patch/Work Around
The patch:
The solution is to properly check the space left in the buffer *before* every write, especially for 4-byte UTF-8 sequences, and not rely on truncated writes after the fact.
If you manage a Windows system using libpcap
- Upgrade to the latest libpcap build [see: Official Patch/Release Notes below]
- For custom or statically linked builds, check if you use utf_16le_to_utf_8_truncated() and fix as above
Temp workaround:
Reduce the chance of processing attacker-controlled error messages, and avoid running pcap-based applications with extra privileges where possible.
References & Further Reading
- libpcap official site
- CVE-2025-11964 at MITRE
- GitHub patch commit (replace xxx with real hash when published)
- Wireshark advisory page
- Surrogate Pairs in Unicode
Conclusion
CVE-2025-11964 shows how even simple string conversion routines can open the door to serious security vulnerabilities. Buffer overflows from missed edge cases—especially in multi-byte encodings—can be dangerous, especially on widely deployed code like libpcap.
If you use or develop software that links to libpcap on Windows, upgrade and audit your code. Attackers are always ready to find creative ways to trigger these bugs!
Stay safe, patch up, and remember: always check buffer boundaries… and Unicode is hard!
Timeline
Published on: 12/31/2025 00:58:19 UTC
Last modified on: 12/31/2025 20:42:15 UTC