CVE-2026-78221 in OpenVPN
Summary
by MITRE • 09/07/2026
An incorrect buffer size calculation in the Windows Interactive Service in OpenVPN 2.7_alpha1 through 2.7.6 allows local authenticated users to cause memory corruption or disclose sensitive information via crafted NRPT inputs.
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Analysis
by VulDB Data Team • 09/07/2026
The vulnerability identified involves a critical logic error within the Windows Interactive Service component of OpenVPN versions ranging from 2.7 alpha 1 through 2.7.6, specifically concerning the handling of Next Rule Policy Table (NRPT) configurations. This flaw stems from an incorrect calculation of buffer sizes during the processing of NRPT inputs provided by local authenticated users. In software development, accurate memory allocation is paramount to preventing overflow conditions and ensuring data integrity. When a system fails to correctly calculate the required size for a buffer before copying or manipulating input data, it creates a scenario where subsequent operations may write beyond the allocated boundaries or read from uninitialized memory regions. This specific implementation defect allows an attacker who has already gained local authentication on the affected Windows host to exploit this miscalculation during the processing of network policy rules.
From a technical perspective, the core issue lies in how the Interactive Service parses and allocates memory for NRPT data structures. The service is responsible for managing VPN-related policies that interact with the operating system's networking stack. When an authenticated user submits crafted input intended to define routing or filtering rules, the application fails to account for all necessary bytes required by the underlying Windows API calls or internal data structures. This discrepancy leads to a buffer overflow condition if the calculated size is smaller than the actual input length, potentially allowing arbitrary code execution through heap corruption. Alternatively, if the logic results in reading beyond valid memory bounds without writing, it can lead to information disclosure vulnerabilities where sensitive kernel or application memory contents are exposed to the user space process handling the VPN connection.
The operational impact of this vulnerability is significant for environments relying on OpenVPN for secure remote access with strict policy enforcement. Since the attack vector requires local authentication, a compromised workstation or an insider threat actor can leverage this flaw to escalate privileges or extract sensitive data from the system memory. The disclosure of sensitive information could include encryption keys, session tokens, or other credentials stored in adjacent memory blocks due to the lack of proper isolation enforced by the buffer size error. Furthermore, if the memory corruption leads to a crash, it results in a denial of service for the VPN client and potentially disrupts network connectivity for all users relying on that endpoint. The ability to cause memory corruption also opens pathways for further exploitation chains, where an attacker might use the initial vulnerability as a foothold to execute arbitrary code with higher privileges within the context of the Interactive Service process.
This flaw aligns closely with Common Weakness Enumeration (CWE) identifiers such as CWE-120 Buffer Copy without Checking Size of Input and CWE-787 Out-of-bounds Write, depending on whether the primary effect is overwriting memory or reading it. In terms of the MITRE ATT&CK framework, this vulnerability facilitates techniques associated with Privilege Escalation via Taint Analysis or Memory Corruption, allowing an attacker to move from a low-privileged user account toward system-level access. The exploitation relies heavily on local interaction, placing it within the scope of lateral movement and persistence strategies if combined with other post-exploitation tools.
To mitigate this risk, organizations running OpenVPN 2.7 alpha versions or up to version 2.7.6 must immediately apply vendor-provided patches that correct the buffer size calculation logic in the Windows Interactive Service. It is essential to verify that all endpoints are updated to a version where the NRPT input parsing routine properly validates and allocates memory based on actual input lengths rather than flawed estimates. Additionally, implementing strict local access controls can limit the number of users capable of triggering this vulnerability by restricting interactive service interactions to trusted administrative accounts only until patches are deployed. Regular auditing of VPN configuration files for malformed or excessively large NRPT entries can also serve as a defensive measure to detect potential exploitation attempts before they result in successful memory corruption or data leakage.