CVE-2026-62816: RMCAST Buffer Overflow Vulnerability

CVE-2026-62816 Overview

CVE-2026-62816 is a heap-based buffer overflow in the Windows Reliable Multicast Transport Driver (RMCAST). An unauthorized attacker on an adjacent network can trigger the flaw to execute arbitrary code on the target system. The vulnerability is tracked as CWE-122: Heap-based Buffer Overflow and carries a CVSS v3.1 score of 8.8. No authentication or user interaction is required, and successful exploitation compromises confidentiality, integrity, and availability. Microsoft published the advisory on August 11, 2026. See the Microsoft CVE-2026-62816 Advisory for vendor guidance.

Critical Impact
An adjacent, unauthenticated attacker can achieve remote code execution in a privileged kernel driver context by sending crafted multicast traffic.

Affected Products

Windows systems that ship the Reliable Multicast Transport Driver (RMCAST.sys)
Refer to the Microsoft CVE-2026-62816 Advisory for the authoritative list of affected builds

Discovery Timeline

2026-08-11 - CVE-2026-62816 published to NVD
2026-08-12 - Last updated in NVD database

Technical Details for CVE-2026-62816

Vulnerability Analysis

The Reliable Multicast Transport Driver implements the Pragmatic General Multicast (PGM) protocol on Windows. RMCAST processes multicast datagrams in kernel mode, which places any parsing defect directly in a privileged execution path. CVE-2026-62816 is a heap-based buffer overflow in this driver, meaning the driver writes past the bounds of a dynamically allocated kernel pool buffer while processing attacker-supplied packet data.

Because exploitation happens through the network stack, the attacker does not need credentials, local access, or user interaction. The attack vector is limited to an adjacent network, so the attacker must be able to send PGM multicast traffic that reaches the vulnerable host.

Root Cause

The root cause is missing or incorrect bounds validation on data parsed from PGM packets before it is copied into a heap-allocated buffer inside RMCAST. When a crafted packet declares or produces a length larger than the destination allocation, the copy operation overwrites adjacent kernel pool memory. Microsoft has not published implementation-level details, and no verified exploit code is available.

Attack Vector

An attacker on the same broadcast domain or a reachable multicast segment transmits malformed PGM datagrams to a system running RMCAST. The driver parses the traffic and performs an out-of-bounds heap write. A tuned overflow can corrupt kernel pool metadata or adjacent objects, yielding arbitrary code execution in kernel context. The Microsoft advisory is the authoritative technical reference: Microsoft CVE-2026-62816 Advisory. No public proof-of-concept has been observed.

Detection Methods for CVE-2026-62816

Indicators of Compromise

Unexpected PGM (IP protocol 113) or multicast traffic directed at hosts that do not normally consume multicast
Kernel crashes, bugchecks, or RMCAST.sys faults in the System event log correlated with inbound multicast bursts
New or unusual listeners bound to multicast groups on servers

Detection Strategies

Alert on inbound PGM traffic from untrusted VLANs or subnets where multicast is not an operational requirement
Monitor Windows kernel crash telemetry for stack frames involving RMCAST.sys and correlate with network capture data
Track patch state for Windows systems and flag hosts missing the Microsoft update that addresses CVE-2026-62816

Monitoring Recommendations

Capture and retain network flows for multicast and PGM traffic at network boundaries and between VLANs
Enable Windows Error Reporting collection to identify driver crashes indicative of exploitation attempts
Baseline which servers legitimately use Message Queuing (MSMQ) or PGM so anomalous multicast consumers stand out

How to Mitigate CVE-2026-62816

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-62816 Advisory to all Windows hosts
Inventory systems where the RMCAST driver or MSMQ multicast features are installed and prioritize them for patching
Restrict PGM and multicast traffic at network boundaries until patches are deployed

Patch Information

Microsoft has issued a security update addressing CVE-2026-62816. Consult the Microsoft CVE-2026-62816 Advisory for the specific KB articles, build numbers, and update channels applicable to each supported Windows release.

Workarounds

Disable the Message Queuing multicast feature on hosts where it is not required, which removes the RMCAST attack surface
Block IP protocol 113 (PGM) and unnecessary multicast group traffic at switches, routers, and host firewalls
Segment servers that must use multicast into dedicated VLANs with strict ingress filtering from user networks

bash

# Example: block inbound PGM (IP protocol 113) with Windows Firewall
netsh advfirewall firewall add rule name="Block PGM Inbound (CVE-2026-62816)" \
    dir=in action=block protocol=113

# Example: verify whether the Message Queuing multicast component is installed
Get-WindowsFeature -Name MSMQ-Multicasting