CVE-2026-85943 in Windows
Summary
by MITRE • 10/14/2026
Time-of-check time-of-use (toctou) race condition in Reliable Multicast Transport Driver (RMCAST) allows an authorized attacker to elevate privileges locally.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability identified as a Time-of-Check-Time-of-Use, or TOCTOU, race condition within the Reliable Multicast Transport driver presents a significant security risk by allowing local privilege escalation. This class of flaw arises from a fundamental timing discrepancy in how the operating system handles resource access and validation. Specifically, there is a gap between the moment the kernel verifies that an authorized user has permission to perform an action and the actual execution of that action. During this narrow window, an attacker with valid authentication credentials can manipulate the underlying state or resources referenced by the driver. By exploiting this race condition, the attacker forces the system to operate on stale or modified data, effectively bypassing the initial security checks performed during the validation phase.
In the context of the Reliable Multicast Transport Driver, which is responsible for managing network communications using multicast protocols, such a flaw can have severe consequences. The driver likely interacts with kernel-mode structures that control memory allocation, file access, or device I/O operations without sufficient synchronization mechanisms to prevent concurrent modification by multiple processes. An attacker leveraging this vulnerability can alter these shared resources between the check and use phases. For instance, if the driver checks for write permissions on a specific configuration file before modifying it, an attacker might replace that file with a malicious script or binary during the interval. When the driver subsequently performs the write operation based on its earlier positive verification, it inadvertently executes code under elevated privileges granted to the service account running the driver.
The operational impact of this vulnerability is primarily centered around unauthorized privilege escalation from a standard user context to SYSTEM or kernel-level access. Once an attacker achieves local system privileges, they gain unrestricted control over the affected machine. This includes the ability to install persistent backdoors, steal sensitive data such as credentials and encryption keys, disable security logging mechanisms, and pivot further into other systems within the network infrastructure. Because the attack requires only authorized user status, it is particularly dangerous in environments where multiple users share a system or where service accounts have broad permissions but are not strictly isolated from untrusted inputs.
From an industry standards perspective, this vulnerability aligns with CWE-367, which defines Time-of-check Time-of-use (TOCTOU) race conditions as flaws resulting from the use of information to decide on actions before those actions occur, where that information may have changed in the meantime. Furthermore, the exploitation technique maps directly to MITRE ATT&CK tactic T1059, specifically sub-techniques related to command and script interpreters or system services abuse, depending on how the driver is invoked. The attacker utilizes legitimate administrative tools or scripts to trigger the race condition, thereby blending malicious activity with normal operational behavior to evade detection by standard security monitoring solutions.
Mitigation strategies for this vulnerability must address both the immediate technical flaw and broader architectural weaknesses. The primary remediation involves applying vendor-provided patches that update the Reliable Multicast Transport Driver to versions where the TOCTOU gap has been closed through proper synchronization primitives such as mutexes, semaphores, or atomic operations. These mechanisms ensure that the check for permissions and the subsequent use of resources occur within a single, uninterruptible transaction. Additionally, organizations should enforce strict least-privilege principles by ensuring that service accounts running network drivers have only the minimum necessary permissions required for their function. Regular vulnerability scanning focused on kernel-mode components and driver integrity checks can also help identify instances where such race conditions might be present before they are exploited in a live environment.