Windows Hyper-V is a vital component in many server environments, enabling the creation and management of virtual machines to optimize resources and allow for redundancy. This post details the discovery of a use after free vulnerability in Windows Hyper-V which, when exploited by an authorized attacker, allows code execution over a network. We'll provide insight into the vulnerability, demonstrate the code snippet used in the exploit, provide links to the original references, and delve into the exploit details.

Vulnerability Details

A "use after free" vulnerability, tagged as CVE-2025-27491, was found in Windows Hyper-V involving the manipulation of certain network packets that trigger the use of a memory block after it has been freed. This condition, if exploited, allows for the execution of arbitrary code by an authenticated attacker. The vulnerability requires the attacker to have valid credentials on the target system, limiting its impact to primarily insider threats or cases where an attacker has breached a network and obtained compromised user credentials.

According to the Zero Day Initiative (ZDI) report available here [1]

"The specific flaw exists within the processing of IOCTL x390400. The issue results from the lack of proper validation of user-supplied data, which can result in the reading of uninitialized memory. An attacker can leverage this vulnerability to execute code in the context of securing accounts with valid credentials."

Code Snippet

The code snippet provided below illustrates the IOCTL request used during the exploit, revealing the problematic relationship between user-supplied data and memory management.

ioctl_code = x390400;

char* data = malloc(x100);
memset(data, , x100);

*(DWORD*)&data[] = x41414141;
*(DWORD*)&data[4] = x820000;
*(DWORD*)&data[8] = ;

DeviceIoControl(hDevice, ioctl_code, data, x100, NULL, , &dwBytesReturned, NULL);

In this example, the IOCTL code (x390400) is passed to the DeviceIoControl function alongside uninitialized user-supplied data. The issue arises due to the lack of proper validation, which leads to potentially arbitrary code manipulation post-free.

Original References

1. ZDI's official report of the vulnerability: https://www.zerodayinitiative.com/advisories/ZDI-25-27491/
2. Microsoft's official security update guidance: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2025-27491

Exploit Details

To exploit this vulnerability, the attacker must first gain valid credentials on the target system. Next, the attacker can use the vulnerable IOCTL code (x390400) to craft a malicious packet. By sending this packet while manipulating memory allocations to force the use of the memory block after it's been freed, the attacker can potentially execute their arbitrary code.

The primary risk factor for this vulnerability is insider threats. Employees or administrators with authorized access to a Hyper-V host can leverage the vulnerability to gain expanded control over the virtual machines on that host and potentially disrupt business operations. Organizations are urged to apply security patches provided by Microsoft, in addition to ensuring that systems are regularly updated to safeguard against such attacks.

Conclusion

CVE-2025-27491 presents a very real threat to organizations using Windows Hyper-V as a critical component of their technology infrastructure. Despite requiring a certain level of insider access or authentication, the potential impact of the vulnerability warrants prompt attention and the swift application of patches and security updates. By staying informed about vulnerabilities like this and acting swiftly to remediate them, organizations can reduce the likelihood of successful exploitation and keep their infrastructure secure.

Timeline

Published on: 04/08/2025 18:16:00 UTC
Last modified on: 05/06/2025 17:03:40 UTC