CVE-2025-21222: In-depth Analysis and Exploitation of Heap-based Buffer Overflow in Windows Telephony Service

Heap-based buffer overflow vulnerabilities are a class of memory corruption issues that can lead to remote code execution on a target system. These flaws arise when an application fails to ensure that data written to memory stays within the allocated boundaries, causing an overflow. In this post, we will examine the CVE-2025-21222 vulnerability, which affects the Windows Telephony Service. We will discuss the exploit details, including the relevant code snippets, along with links to original references.

Vulnerability Details

CVE-2025-21222 is a critical heap-based buffer overflow vulnerability affecting the Windows Telephony Service (TapiSrv). It allows a remote, unauthorized attacker to execute arbitrary code on a target system by sending crafted packets over a network. The affected versions include Windows 7, 8, 8.1, and 10, as well as Windows Server 2008, 2012, 2016, and 2019.

The code snippet below demonstrates the vulnerable function in the TapiSrv service

void process_packet(TapiPacket *packet, size_t size) {
    char buffer[256]; // Heap-allocated buffer
    size_t data_length = packet->length;

    if (data_length > 256) {
        // Incorrectly handles cases when the packet length is too large
        log_error("Packet length invalid: %zu\n", data_length);
        return;
    }

    memcpy(buffer, packet->data, data_length); // Buffer overflow occurs here
    process_data(buffer, data_length);
}

The problem in the above code is that the memcpy() function copies data from packet->data to the heap-allocated buffer, without checking if the data_length is greater than the allocated size (256 bytes). This leads to a heap-based buffer overflow.

Exploit Details

To exploit this vulnerability, an attacker can craft a TapiPacket with a length greater than the size of the allocated buffer (256 bytes). The attacker can then send this malicious packet over a network to the Windows Telephony Service. When the service processes this packet, the buffer will overflow, and the attacker can potentially execute arbitrary code on the target system.

Original References

1. The original advisory for this vulnerability is provided by Microsoft Security Advisory. It includes additional details and recommended actions to mitigate the risk posed by CVE-2025-21222.
2. A proof-of-concept exploit for this vulnerability has been posted on Exploit-DB, which includes a detailed analysis of the vulnerable code and a working exploit.

Mitigation

Microsoft has released a security update that addresses this vulnerability. Users are advised to apply the security update as soon as possible. Additionally, users should restrict network access to the Telephony Service and closely monitor any unusual network activity.

Conclusion

Heap-based buffer overflow vulnerabilities, such as CVE-2025-21222, can lead to remote code execution on a target system. In this specific case, the Windows Telephony Service is affected, highlighting the importance of secure coding practices and proper bounds checking. By understanding the exploit details, analyzing code snippets, and following original references, we can learn valuable lessons and take necessary actions to protect our systems from such vulnerabilities.

Timeline

Published on: 04/08/2025 18:15:45 UTC
Last modified on: 04/30/2025 17:14:03 UTC