Introduction
At the start of 2025, on January 14th, Microsoft released over 20+ CVEs addressing Remote Code Execution (RCE) vulnerabilities in Microsoft Telephony Services, primarily caused by heap-based buffer overflows. Seeing more than 20 RCE vulnerabilities in one service is normal when it comes to Microsoft because you could find the same patterns for the vulnerability everywhere in the code. The following table lists these CVEs:
| CVE Number | Vulnerability Type | Affected Component |
| CVE-2025-21417 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21413 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21411 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21409 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21339 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21306 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21305 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21303 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21302 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21286 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21282 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21273 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21266 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21252 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21250 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21248 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21246 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21245 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21244 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21243 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21241 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21240 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21239 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21238 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21237 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21236 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21233 | Remote Code Execution | Windows Telephony Service |
| CVE-2025-21223 | Remote Code Execution | Windows Telephony Service |
According to the Microsoft Security Response Center (MSRC), nearly all of these vulnerabilities are heap-based buffer overflows. This insight guides us in deciding what to focus on during patch diffing and analysis.
Before, we dive into the technical details of patch analysis, let’s take a moment to understand the fundamentals of Microsoft Telephony Services and how they operate.
Microsoft Telephony
Microsoft Telephony, in this context, refers to Microsoft’s Telephony Application Programming Interface (TAPI), a framework that allows developers to build applications capable of controlling telephony services in Windows. By using TAPI, an application can place, receive, and manage calls, regardless of whether these calls are routed over traditional phone lines, PBX systems, or VoIP services.
TAPI is available in different versions: TAPI 2.x uses a procedural C-style API, while TAPI 3.x is based on the Component Object Model (COM). Regardless of the version, the main goal is the same: to provide an abstraction layer between the application and the underlying telephony hardware or service. This is achieved through Telephony Service Providers (TSPs)—drivers or modules that interface directly with the physical or virtual telephony device. When an application wants to make a call, it will use TAPI functions (for TAPI 2.x) or COM objects (for TAPI 3.x) to initiate the request. TAPI then communicates with the TSP associated with the specific hardware or service, which handles the actual call setup. Throughout the call lifecycle, TAPI notifies the application of status changes—such as ringing, connected, on hold, or disconnected—using event callbacks or COM events. This allows developers to build rich telephony features, including call hold, transfer, conference, and more advanced media control were supported.
TAPI is particularly relevant in scenarios like call centers or customer relationship management (CRM) integrations, where “screen-pops” (customer information displayed when a call arrives) or automated outbound dialing can significantly improve productivity. It also supports legacy telephony environments where on-premises PBX systems remain critical to business operations. Although many modern telephony solutions now rely heavily on cloud-based APIs and SIP-based systems, TAPI is still an important solution for those who need tight integration with Windows applications and existing telephony infrastructures. In practical terms, getting started involves installing and configuring the appropriate TSP for the hardware or service you plan to use. From there, developers can reference TAPI’s functions and data structures (for 2.x) or COM interfaces (for 3.x) in their code to open a “line,” place a call, handle events, and tear down the connection when done. Comprehensive testing is essential to ensure call quality, handle potential errors, and provide a seamless user experience.
Overall, Microsoft Telephony through TAPI remains a powerful means of integrating voice communication capabilities directly into Windows-based applications. While modern cloud telephony solutions may dominate new deployments, TAPI’s established architecture continues to be a reliable choice for on-premises and hybrid environments that require close control over telephony hardware and legacy systems.
From a high-level perspective, the diagram shows how TAPI 2.x and TAPI 3.x fit into the overall Microsoft Telephony architecture by connecting various telephony service providers (TSPs) and media service providers (MSPs) to underlying hardware and network protocols. At the top, you see TAPI 2.x and TAPI 3.x, each handling call control, call center control, and media control features in Windows. Both TAPI versions communicate with a central component called the TAPI Server, typically via RPC (Remote Procedure Call).
Below the TAPI Server, you find different TSPs (Telephony Service Providers). These TSPs are specialized drivers or modules that handle specific types of telephony connections. Examples include:
- Unimodem TSP for traditional modems connected to PSTN/ISDN lines.
- Remote TSP for remote telephony connections over a network.
- H.323 TSP for H.323-based VoIP communications.
- IP Conference TSP for IP-based conferencing.
- KMDD TSP (Kernel-mode device driver TSP) for lower-level device access.
Each TSP can communicate with relevant network or hardware interfaces, such as Windows Sockets 2 (Winsock) for IP traffic, NDIS (Network Driver Interface Specification) for network adapters, or a direct PSTN/ISDN connection for analog or digital telephony.
On the TAPI 3.x side, there is an additional layer for MSPs (Media Service Providers). These MSPs work in tandem with the TSPs to handle media streaming. Common MSPs in the diagram include:
- Wave MSP for traditional audio streams.
- H.323 MSP for handling media in H.323-based VoIP.
- IP Conference MSP for conference-specific media control.
- RCA MSP (Resource Connection and Addressing MSP) for call center scenarios.
- Third Party MSP for vendor-specific or custom media services.
These MSPs rely on the DirectShow Streaming Filter Graph to process the incoming and outgoing audio/video data. The diagram shows links to RTP (Real-time Transport Protocol), codecs for audio/video compression, and even more specialized components if needed.
On the far right, you see references to LDAP (Lightweight Directory Access Protocol) and Windows 2000 Active Directory, indicating how directory services can be integrated for user lookup, authentication, or enterprise telephony management. This integration is particularly relevant for enterprise deployments where user information and call routing might be centrally managed.
Overall, the highlights are about how TAPI 2.x provides a primarily call-control focus (especially for legacy and standard telephony scenarios), while TAPI 3.x layers in media management, conferencing, and richer call center controls. Both rely on the TAPI Server to interface with various TSPs, which in turn connect to either hardware devices like modems and NICs or software-based protocols like TCP/IP and H.323.
TAPI Files
In this diagram, It shows how different versions of the Telephony Application Programming Interface (TAPI) coexist and interact within a Windows environment. On the far left are various TAPI-based applications (in both 16-bit and 32-bit forms), in the center are core TAPI libraries (the DLL files), and on the right are the components that manage telephony services system-wide (TAPISRV.EXE, Telephony Service Providers, and kernel-mode drivers).
Applications and Their TAPI Versions
On the left, you can see three distinct application types. The first is a 16-bit TAPI 1 application using TAPI.DLL and a 16-bit “thunk” layer to communicate with the 32-bit TAPI32.DLL. This indicates backward compatibility mechanisms for older software. Next, you have a 32-bit TAPI 2 application that directly accesses TAPI32.DLL. Finally, there’s a TAPI 3 application, which uses both TAPI3.DLL for call control logic and an MSP (Media Service Provider) for managing audio and video streams. This arrangement highlights that Windows supports older and newer TAPI applications running simultaneously, each using the libraries appropriate for its generation.
Core TAPI Libraries (DLLs)
The middle portion shows how the applications connect to the relevant TAPI libraries.
- TAPI32.DLL: Handles the functionality for both TAPI 1.x and TAPI 2.x applications. It includes the fundamental procedures for initializing telephony services, enumerating devices, and handling call states (e.g., call setup, ring, connect).
- TAPI3.DLL: Powers the COM-based TAPI 3.x model, allowing more advanced scenarios such as IP telephony, media-rich conferencing, and more sophisticated call handling.
- Media Service Provider (MSP): Works alongside TAPI3.DLL to manage media streams—this can involve capturing and playing audio/video or integrating with DirectShow for advanced processing and conferencing.
TAPISRV.EXE and Telephony Service Providers (TSPs)
On the right, you see TAPISRV.EXE, commonly referred to as the TAPI Service/Server. It communicates with each TAPI DLL via RPC (Remote Procedure Call). Its role is to centralize and manage telephony operations for the entire system. Applications may request actions like placing a call or querying device status, but TAPISRV.EXE determines which Telephony Service Provider (TSP) to use and orchestrates those low-level interactions.
A TSP is a driver or component specifically designed to interface with a particular type of telephony hardware or protocol. It could be an analog modem TSP, an H.323 (VoIP) TSP, or something more specialized. Each TSP interacts with the operating system at a deeper level—often using a kernel-mode driver for direct hardware access or protocol support. This layered approach lets TAPI be device-agnostic: from the application’s standpoint, placing a call is the same whether the underlying mechanism is a traditional telephone line or a VoIP channel.
Registry and Telephony Control Panel
Also connected to TAPISRV.EXE is the Windows Registry, which holds telephony-related settings such as dialing rules, location properties, and user preferences. The Telephony Control Panel or Dialing Properties interfaces let users configure these settings in a GUI rather than editing the Registry directly. TAPISRV.EXE reads and writes these configurations, ensuring all TAPI applications and TSPs share a consistent set of telephony rules.
Putting It All Together
When a TAPI-based application performs an action (like making a call), it calls into the corresponding TAPI library (e.g., TAPI32.DLL or TAPI3.DLL). That library uses RPC to communicate the request to TAPISRV.EXE, which then delegates the request to the appropriate TSP. The TSP, in turn, works with any required kernel-mode drivers to interact with the physical (or virtual) telephony hardware. Throughout this process, status changes—like ringing, connected, or disconnected—are sent back up to the application via the same chain of communication. This modular design ensures multiple applications, TAPI versions, and telephony technologies can coexist while relying on a unified underlying service to manage all call-related activities.
Incoming Calls Flow
Let’s discuss the high-level flow of an incoming call within the TAPI 3 architecture on Windows. Moving from bottom to top, you can see how a call enters through telephony hardware or a network interface (cloud icon), is detected by the Telephony Service Provider (TSP), and then proceeds upward through TAPISRV, TAPI3.DLL, and ultimately to the application. The diagram also shows how the Media Service Provider (MSP) integrates with TAPI3.DLL to handle audio and video streaming.
1. Call Arrival and TSP Notification
An incoming call is first received by the TSP (Telephony Service Provider). The TSP is responsible for interfacing with the actual telephony hardware or VoIP stack. When a new call arrives, the TSP sends a LINE_NEWCALL message to TAPISRV, signifying that a call is being offered. TAPISRV is the Telephony API service that coordinates all telephony requests system-wide, managing communication between TSPs and higher-level TAPI clients.
2. TAPISRV Informs TAPI 3
Upon receiving the LINE_NEWCALL message, TAPISRV notifies any interested clients or TAPI layers (in this case, TAPI 3). This allows TAPI 3 to become aware of the new, incoming call. TAPI 3 then creates its own high-level Call object in the application’s context.
3. Creating the Call Object and MSP Call
After TAPI 3 learns of the new call, it calls the method ITMSPAddress::CreateMSPCall on the MSP (Media Service Provider). The MSP is in charge of actual audio/video media streaming for a call. During this step, the MSP:
- Creates an MSP Call object, which encapsulates the logic needed to handle media streams (e.g., setting up audio, video, or other data channels).
- Creates default streams based on the media types required for the incoming call.
The MSP then returns an Unknown pointer to the newly created MSP Call object back to TAPI 3.
4. Aggregation and Application Notification
TAPI 3 aggregates the MSP Call object into its own TAPI Call object, effectively merging telephony control and media handling into one cohesive interface. This aggregation is what enables the application to use interfaces like ITStreamControl to manage audio/video streams. Once the call object is ready, TAPI 3 notifies the application of the new call, typically by raising an event or callback indicating that a call is offering.
5. Application Prepares the Call
At this point, the application can perform any pre-call setup needed before answering. For example, it might:
- Select a particular audio or video device (using ITStream::SelectTerminal).
- Configure codecs or other parameters.
- Update the user interface so an agent or user can see there is an incoming call.
6. Answering the Incoming Call
When the application is ready to connect, it calls ITBasicCallControl::Answer on the TAPI Call object. Internally, TAPI 3 executes the following steps:
- Calls lineAnswer, which is the TAPI 2–style command to answer a call.
- TAPISRV then calls TSPI_lineAnswer, passing the instruction to the TSP.
- The TSP initiates call streaming, often instructing the MSP to start sending and receiving media. In some implementations, the TSP itself may trigger the media start without going back to the MSP if they are tightly integrated.
7. Ongoing Communication Between TSP and MSP
Once the call is active, the TSP and MSP continue to communicate for tasks such as updating call state, exchanging in-call events, or modifying media streams. This communication flows via opaque data buffers through TAPISRV and TAPI 3:
- The TSP sends data to the MSP by issuing LINE_SENDMSPDATA messages to TAPISRV, which are then received by the MSP through ITMSPAddress::ReceiveTSPData.
- The MSP sends data or instructions back to the TSP through a MSP_TSP_DATA event, which TAPI 3 forwards to the TSP via TSPI_lineReceiveMSPData.
This mechanism allows both service providers (the TSP and MSP) to stay in sync about call progress and handle any mid-call adjustments (e.g., hold, transfer, codec changes).
The following diagram summarize the flaw:
- Incoming Call (TSP → TAPISRV):
The TSP detects an incoming call (from a phone line, VoIP gateway, etc.) and sends a LINE_NEWCALL message to TAPISRV to indicate a call is being offered. - TAPISRV Notifies TAPI 3 (TAPISRV → TAPI3):
TAPISRV broadcasts the event to any subscribed TAPI layers. TAPI3 receives this notification, prompting it to set up a TAPI 3 Call object. - MSP Call Creation (TAPI3 → MSP):
TAPI3 calls ITMSPAddress::CreateMSPCall on the Media Service Provider (MSP) to create the underlying MSP Call object. This object will manage the audio/video streams for the call. - Aggregating MSP Call (MSP → TAPI3):
The MSP returns an IUnknown pointer to its newly created MSP Call object. TAPI3 merges (aggregates) this with its own TAPI Call object, then notifies the application that a new call has arrived. - Application Prepares and Answers (App → TAPI3):
The application can inspect call parameters, configure terminals (e.g., microphones, speakers), and once ready, calls ITBasicCallControl::Answer to accept the call. - Call Answer Lifecycle (TAPI3 → TAPISRV → TSP):
TAPI3 invokes lineAnswer (a TAPI 2–style function) through TAPISRV, which instructs the TSP (TSPI_lineAnswer) to officially answer the call on the telephony device or network endpoint. - Starting Media Streaming (TSP → MSP):
After the call is answered, the TSP initiates media streaming. In many implementations, the TSP signals the MSP (directly or indirectly) to begin sending/receiving audio or video. Post-answer, the TSP and MSP continue to exchange updates and control data (like hold, transfer, or codec changes) using opaque data buffers via TAPISRV and TAPI3.
Conclusion
The significant number of RCE vulnerabilities discovered in Microsoft Telephony Services, largely caused by heap-based buffer overflows, highlights the importance of thoroughly understanding TAPI and its underlying architecture. While TAPI offers a robust framework for integrating telephony features into Windows-based applications, any flaw in its components can have far-reaching implications. By exploring how TSPs, MSPs, and TAPISRV interact, researchers can better identify potential exploitation paths and implement effective mitigation strategies. Going forward, it is crucial for organizations to apply Microsoft’s patches promptly to protect against these RCE threats. For expert guidance on vulnerability management, penetration testing, or securing telephony systems, contact SecureLayer7 to leverage tailored solutions and stay ahead of evolving security risks.
References
- https://learn.microsoft.com/en-us/windows/win32/tapi/telephony-application-programming-interfaces
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21417
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21413
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21411
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21409
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21339
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21306
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21305
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21303
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21302
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21286
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21282
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21273
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21266
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21252
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21250
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21248
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21246
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21245
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21244
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21243
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21241
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21240
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21239
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21238
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21237
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21236
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21233
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21223
